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Inheritance

1

Polygenic (multifactorial) liability HP:0010982

OUD is a complex trait, not Mendelian. Opioid addiction is moderately heritable, but until recently only the OPRM1 A118G variant had reached genome-wide significance with independent replication — an unusually sparse common-variant signal compared with other substance use disorders, which the largest multi-trait analyses have only recently begun to extend.

polygenic inheritance

Show evidence (2 references)

PMID:36207451 SUPPORT Human Clinical

"Opioid addiction (OA) is moderately heritable, yet only rs1799971, the A118G variant in OPRM1, has been identified as a genome-wide significant association with OA and independently replicated."

States both the moderate heritability and the sparseness of replicated common-variant signal that characterizes this trait.

PMID:36207451 SUPPORT Human Clinical

"Genetic correlations among the various OA phenotypes were uniformly high (rg > 0.9)."

Supports treating the differently ascertained opioid addiction phenotypes as one underlying liability.

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Discussions and Knowledge Gaps

4

What molecular processes actually account for the loss of mu-opioid receptor function after chronic opioid exposure, given that acute desensitization and internalization do not fully explain clinical tolerance?

KNOWLEDGE GAP oud_tolerance_molecular_basis

Attached to

pathophysiology#Mu-Opioid Receptor Desensitization and Tolerance

The authoritative review of MOR regulation states plainly that while some regulatory mechanisms contributing to tolerance are understood, there are large gaps in understanding the molecular processes responsible for loss of receptor function after chronic exposure. This matters beyond mechanism: tolerance to analgesia and to respiratory depression diverge, and that divergence is what makes the post-abstinence overdose spike so lethal.

Proposed experiments

Phosphorylation-site mutant models separating acute desensitization from chronic tolerance

exp_oud_phosphosite_tolerance

Use knock-in models with defined MOR phosphorylation-site mutations to test which receptor regulatory events are required for chronic analgesic tolerance versus acute desensitization, and whether the same events govern tolerance to respiratory depression.

Direct comparison of analgesic and respiratory tolerance time courses

exp_oud_respiratory_tolerance_divergence

Measure the development and loss of tolerance to analgesia and to respiratory depression in parallel within the same subjects, to characterize the divergence that underlies overdose risk after a period of abstinence.

Do the rodent operationalizations of hyperkatifeia — elevated reward thresholds, lowered pain thresholds, anxiety-like and dysphoric-like behaviour — correspond to the negative emotional state that drives relapse in people with opioid use disorder?

HUMAN MODEL MISMATCH oud_negative_affect_model_fidelity

Attached to

pathophysiology#Reward-System Downregulation and Hyperkatifeia

The evidence for the negative-affect stage is largely from extended-access animal models, where the state is inferred from reward thresholds and behavioural assays. Human hyperkatifeia is assessed by self-report of dysphoria and craving. The construct is plausible and clinically resonant, but the measurement bridge between the two species is an assumption rather than a demonstrated equivalence.

Proposed experiments

Human reward-sensitivity measurement across withdrawal and protracted abstinence

exp_oud_human_reward_threshold

Track objective reward sensitivity (effort-based decision making, monetary incentive delay fMRI) alongside self-reported dysphoria in people with OUD from acute withdrawal into protracted abstinence, to test whether a measurable reward deficit tracks the subjective state.

Prospective test of negative affect as a relapse predictor

exp_oud_relapse_prediction

Determine whether the magnitude of the negative-affect state during early abstinence prospectively predicts relapse, which is the claim the negative-reinforcement model makes and that animal work cannot establish.

Does the TLR4/microglial arm that opposes opioid analgesia and drives tolerance, hyperalgesia, and withdrawal in rodents operate at a clinically meaningful magnitude in people with opioid use disorder?

HUMAN MODEL MISMATCH oud_tlr4_glial_human_translation

Attached to

pathophysiology#Opioid-Induced TLR4 and Microglial Neuroimmune Activation

The rodent and in vitro evidence for this node is strong and convergent — non-stereoselective TLR4 activation in vitro, pharmacological blockade attenuating tolerance, hyperalgesia and withdrawal in vivo, and a dose-response shift in TLR4 knockout mice. The human evidence is not. The one human interventional test cited here, a placebo-controlled inpatient study of the glial modulator ibudilast in opioid-dependent volunteers, was null on its prespecified withdrawal-severity outcome and positive only in an exploratory pooled analysis of individual subscale items. A second constraint comes from the rodent work itself: TLR4 dependence was demonstrated for low-dose morphine, while high-dose morphine effects were neither TLR4- nor PKCepsilon-dependent, so the arm may not be engaged at the doses that characterize opioid use disorder. Until an adequately powered trial of a glial or TLR4-directed agent shows an effect on a prespecified clinical endpoint, this node should be read as a mechanism established in animals and only weakly corroborated in humans.

Proposed experiments

Adequately powered trial of a TLR4-directed or glial-modulating agent in OUD

exp_oud_tlr4_powered_trial

Test a TLR4 antagonist or glial modulator against placebo in people with opioid use disorder, powered on a single prespecified clinical endpoint (withdrawal severity, or opioid dose required for analgesia), so the result does not depend on exploratory subscale analysis.

Dose-ranging test of TLR4 dependence at OUD-relevant exposures

exp_oud_tlr4_dose_range

Establish whether the TLR4 arm is engaged across the full opioid dose range or only at low doses, since the demonstrated dose-dependence in rodents determines whether the mechanism is relevant to the high-exposure pattern of opioid use disorder at all.

Do chronic-opioid-associated DNA methylation changes represent a causal mechanism in opioid use disorder, a durable biomarker of exposure, or neither?

KNOWLEDGE GAP oud_epigenetic_mechanism

Attached to

pathophysiology#Polygenic Susceptibility to Opioid Addiction pathophysiology#Mu-Opioid Receptor Desensitization and Tolerance

An epigenetic arm is frequently asserted for opioid use disorder, and this entry deliberately does not model one, because the primary evidence does not currently support a mechanism node. The best-known positive finding is OPRM1 promoter hypermethylation at two CpG sites in *lymphocytes* of methadone-maintained former heroin addicts — a peripheral tissue, with the consequence for receptor expression stated by the authors only as a possibility. The one epigenome-wide study in the tissue that matters, postmortem dorsolateral prefrontal cortex, found no CpG site surviving false-discovery-rate correction. Direction of causation is also open: these are cross-sectional comparisons of people with long exposure histories, so a methylation difference is at least as easily read as a consequence of exposure as a cause of the disorder. Curating an epigenetic node from review-level synthesis would overstate all of this.

Proposed experiments

Adequately powered brain EWAS with cell-type deconvolution

exp_oud_ewas_powered_brain

Repeat the epigenome-wide analysis in postmortem brain at a sample size powered to survive multiple-testing correction, with cell-type deconvolution, so that a null result is informative rather than underpowered.

Longitudinal methylation sampling across exposure and abstinence

exp_oud_longitudinal_methylation

Sample methylation before, during, and after sustained opioid exposure in the same individuals to distinguish a pre-existing risk mark from an exposure-induced change, which no cross-sectional case-control design can do.

Test whether the OPRM1 CpG marks actually alter receptor expression

exp_oud_methylation_expression_link

Directly measure OPRM1 expression against methylation at the -18 and +84 CpG sites, in both lymphocytes and brain, to test the mechanism the original study could only propose.

Show evidence (4 references)

PMID:18650805 SUPPORT Human Clinical

"Direct sequencing of bisulfite-treated DNA showed that the percent methylation at two CpG sites was significantly associated with heroin addiction."

The strongest positive primary finding, but measured in lymphocytes rather than brain, so it supports an association and not a CNS mechanism.

PMID:18650805 SUPPORT Human Clinical

"Methylation of these CpG sites may lead to reduced OPRM1 expression in the lymphocytes of these former heroin addicts."

The authors state the functional consequence as a possibility, not a measurement, which is exactly the step this gap records as missing.

PMID:33667780 SUPPORT Human Clinical

"Although no CpG sites survived false-discovery rate correction for multiple testing, 13 sites surpassed a relaxed significance threshold"

The epigenome-wide result in brain tissue is null at the conventional threshold, which is the central reason no epigenetic mechanism node is curated in this entry.

+ 1 more reference

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Pathophysiology

13

Polygenic Susceptibility to Opioid Addiction

Inherited liability is polygenic and, relative to other substance use disorders, unusually concentrated on the opioid receptor gene itself. OPRM1 carries the only long-replicated genome-wide significant signal; gene-based analysis has added FURIN and PPP6C. Genetic loading modifies the probability that opioid exposure escalates to compulsive use; it does not cause the disorder without exposure.

OPRM1 hgnc:8156 FURIN hgnc:8568

Downstream

Exogenous Mu-Opioid Receptor Agonism OPRM1 variation acts on the receptor that exogenous opioids bind, modifying the signalling consequences of a given exposure.

Show evidence (3 references)

PMID:36207451 SUPPORT Human Clinical

"Gene-based analyses identified novel genome-wide significant associations with PPP6C and FURIN."

Documents the loci beyond OPRM1 that this susceptibility node represents.

PMID:35879402 SUPPORT Human Clinical

"The current study identified OUD variant associations at OPRM1, single variant associations with FURIN, and 18 GWS associations in the OUD-MTAG."

The largest OUD-specific GWAS independently replicates the two loci this node carries, and finds nothing else at genome-wide significance for diagnosed OUD without multi-trait augmentation.

PMID:35879402 SUPPORT Human Clinical

"The genetic architecture of OUD is likely influenced by both OUD-specific loci and loci shared across SUDs."

Bounds every additional locus curated in the `genetic` section: signals recovered by multi-trait analysis are partly shared across substance use disorders rather than opioid-specific.

Exogenous Mu-Opioid Receptor Agonism

Prescription opioids, heroin, and illicit synthetic opioids such as fentanyl act as agonists at the mu-opioid receptor, a Gi/Go-coupled GPCR. Receptor activation inhibits adenylyl cyclase and opens GIRK potassium channels, hyperpolarizing the target neuron. The same receptor event underlies analgesia, euphoria, tolerance, and — in the brainstem — respiratory depression, which is why the therapeutic and lethal effects of opioids cannot be pharmacologically separated at this node.

G protein-coupled opioid receptor signaling pathway GO:0038003 ↑ INCREASED adenylate cyclase-inhibiting signaling GO:0007193 ↑ INCREASED

Downstream

VTA Disinhibition and Mesolimbic Dopamine Release Receptor-mediated hyperpolarization of VTA GABAergic interneurons disinhibits dopaminergic projection neurons.

Mu-Opioid Receptor Desensitization and Tolerance Sustained agonist occupancy drives receptor phosphorylation, desensitization, and endocytosis.

Opioid-Induced TLR4 and Microglial Neuroimmune Activation Opioid exposure additionally engages the TLR4/MD-2 innate immune complex. This edge is drawn from the exposure node rather than from the receptor event it names because the activation is non-stereoselective and so is not mediated by the mu-opioid receptor; it is the same drug acting at a second, unrelated target.

Brainstem Respiratory Rhythm Suppression The same receptor is expressed on brainstem respiratory neurons, so agonism suppresses respiratory drive directly rather than as a downstream complication.

Show evidence (1 reference)

PMID:31400808 SUPPORT Other

"Opioids are powerful drugs that usurp and overpower the reward function of endogenous opioids and engage dramatic tolerance and withdrawal via molecular and neurocircuitry neuroadaptations within the same reward system."

Frames exogenous opioid action as hijacking the endogenous opioid reward system, the premise of this node.

VTA Disinhibition and Mesolimbic Dopamine Release

Mu-opioid receptor agonism on ventral tegmental GABAergic interneurons removes their inhibitory tone on dopaminergic projection neurons, increasing dopamine release in the nucleus accumbens. This disinhibition route is the positive reinforcement of early use, and it is mechanistically distinct from the direct dopamine-releasing action of stimulants.

dopaminergic neuron CL:0000700 GABAergic interneuron CL:0011005 medium spiny neuron CL:1001474

dopamine secretion GO:0014046 ↑ INCREASED

ventral tegmental area UBERON:0002691 nucleus accumbens UBERON:0001882

Downstream

Reward-System Downregulation and Hyperkatifeia Repeated supraphysiological reward-system activation drives the opponent adaptation that lowers hedonic set point.

Compulsive Opioid Seeking and Loss of Control Positive reinforcement sustains early, pre-dependence use.

Show evidence (1 reference)

PMID:31400808 SUPPORT Other

"Such negative emotional states that drive negative reinforcement are hypothesized to derive from the within-system dysregulation of key neurochemical circuits that mediate incentive-salience and/or reward systems (dopamine, opioid peptides) in the ventral striatum"

Identifies ventral striatal dopamine and opioid peptide circuits as the reward system that later dysregulates. Support is partial because this review describes the dysregulation rather than the acute disinhibition step itself.

Mu-Opioid Receptor Desensitization and Tolerance

Chronic agonist exposure produces receptor phosphorylation, desensitization, and internalization, so that progressively higher doses are needed for the same effect. Tolerance to analgesia and euphoria is what drives dose escalation; critically, tolerance to respiratory depression develops incompletely and is lost quickly during abstinence, which is the pharmacological basis of the post-detoxification and post-release overdose spike.

G protein-coupled opioid receptor signaling pathway GO:0038003 ↓ DECREASED

Downstream

cAMP-PKA Superactivation in Locus Coeruleus Noradrenergic Neurons Cellular counter-adaptation to sustained inhibitory signalling upregulates the cAMP cascade.

Opioid-Induced Hyperalgesia Chronic exposure sensitizes pronociceptive pathways in parallel with, and mechanistically distinct from, receptor-level tolerance.

Show evidence (2 references)

PMID:23321159 SUPPORT Other

"A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists."

Establishes tolerance as the central clinical consequence of the receptor regulation this node models.

PMID:23321159 SUPPORT Other

"While some functional MOR regulatory mechanisms contributing to tolerance are clearly understood, there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids."

The authors state explicitly that the molecular basis of chronic tolerance is incompletely understood, so this node's mechanism is only partially established.

Opioid-Induced TLR4 and Microglial Neuroimmune Activation

A non-classical, non-opioid-receptor arm of opioid action: opioids and some of their metabolites activate the TLR4/MD-2 innate immune complex on microglia and other myeloid cells, driving proinflammatory glial activation that opposes analgesia and contributes to analgesic tolerance, hyperalgesia, and withdrawal behaviour. The pharmacology is distinctive — activation is non-stereoselective, so it is not mediated by the mu-opioid receptor, and it is blocked by classical TLR4 antagonists. This node is deliberately kept parallel to, not upstream of, `Mu-Opioid Receptor Desensitization and Tolerance`: it is a second route to the same clinical endpoints rather than a cause of receptor-level desensitization. The arm is well evidenced in rodents and in vitro and only weakly corroborated in people; see the `oud_tlr4_glial_human_translation` discussion for that mismatch, and the bounding evidence below for its dose-dependence.

Microglia CL:0000129

TLR4/MD-2 innate immune signalling GO:0034142 ↑ INCREASED Proinflammatory microglial activation GO:0001774 ↑ INCREASED

Downstream

Opioid-Induced Hyperalgesia Blocking TLR4 signalling in vivo attenuates the development of opioid-induced hyperalgesia, which is the direction this edge asserts.

Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome TLR4 blockade attenuates opioid withdrawal behaviour, placing this arm upstream of withdrawal expression alongside — not instead of — the cAMP-PKA noradrenergic mechanism.

Show evidence (3 references)

PMID:19679181 SUPPORT In Vitro

"Morphine non-stereoselectively induced TLR4 signaling in vitro, blocked by a classical TLR4 antagonist and non-stereoselectively by naloxone."

Non-stereoselective activation blocked by a TLR4 antagonist is what establishes this as a TLR4 event rather than a mu-opioid receptor one.

PMID:19679181 SUPPORT Model Organism

"TLR4 opposition to opioid actions was supported by morphine treatment of TLR4 knockout mice, which revealed a significant threefold leftward shift in the analgesia dose response function, versus wildtype mice."

Genetic removal of TLR4 shifts the morphine dose-response, giving a loss-of-function line of evidence independent of the pharmacology.

PMID:25975386 SUPPORT Human Clinical

"Although overall SOWS scores did not differ between groups, exploratory analyses pooling the two ibudilast groups demonstrated that they had lower ratings of withdrawal symptoms"

The only human interventional test of this node available here. The prespecified comparison was null and the supportive result is an exploratory pooled subscale analysis, so it corroborates the node weakly and is recorded as PARTIAL rather than SUPPORT.

cAMP-PKA Superactivation in Locus Coeruleus Noradrenergic Neurons

Opioid agonism acutely inhibits adenylyl cyclase in locus coeruleus noradrenergic neurons; chronic exposure drives a compensatory upregulation of the cAMP-PKA cascade. While drug is present the two oppose each other and firing is near normal. This is the classic opponent-process substrate of opioid physical dependence.

noradrenergic neuron CL:0008025

adenylate cyclase-inhibiting signaling GO:0007193 ⚠ ABNORMAL

locus coeruleus UBERON:0002148

Downstream

Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome Removing the drug unmasks the compensatory cAMP-PKA upregulation as a surge of noradrenergic firing.

Show evidence (1 reference)

PMID:32002194 SUPPORT Human Clinical

"Lofexidine is an FDA-approved, alpha2-adrenergic receptor agonist for treatment of OWS."

That an alpha-2 adrenergic agonist is an approved treatment for opioid withdrawal is pharmacological support for a noradrenergic substrate; the trial does not itself demonstrate the cAMP-PKA mechanism, so support is partial.

Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome

On cessation or antagonist administration, unopposed noradrenergic hyperactivity produces the characteristic syndrome: rhinorrhea, lacrimation, mydriasis, piloerection, myalgia, nausea, vomiting, diarrhea, sweating, insomnia, and dysphoria. It is intensely aversive but, unlike alcohol or benzodiazepine withdrawal, rarely life-threatening in itself — its clinical danger is that it drives return to use, and that abstinence resets tolerance.

Downstream

Reward-System Downregulation and Hyperkatifeia Repeated withdrawal episodes recruit and sensitize the brain stress systems that generate the negative emotional state.

Compulsive Opioid Seeking and Loss of Control Using to relieve or avoid withdrawal is itself a DSM-5 pharmacological criterion and a direct route back into use.

Prefrontal Executive Control Deficit Repeated withdrawal episodes are part of the cycle that degrades prefrontal control, which is what makes the preoccupation/anticipation stage self-sustaining.

Show evidence (2 references)

PMID:32002194 SUPPORT Human Clinical

"Fear of opioid withdrawal syndrome (OWS) often dissuades opioid discontinuation."

Documents the aversiveness of the syndrome and its role as a barrier to cessation, which is the behavioural significance this node carries.

PMID:31400808 SUPPORT Other

"However, they also engage the brain systems for stress and pain (somatic and emotional) while producing hyperalgesia and hyperkatifeia, which drive pronounced drug-seeking behavior via processes of negative reinforcement."

Links the stress and pain systems engaged in withdrawal to negative-reinforcement drug seeking.

Reward-System Downregulation and Hyperkatifeia

The withdrawal/negative-affect stage combines a within-system reward deficit (reduced dopaminergic and opioid-peptide tone in ventral striatum) with between-system recruitment of extended-amygdala stress mediators — corticotropin-releasing factor, dynorphin, norepinephrine, hypocretin, glucocorticoids. The resulting dysphoria, anxiety, and anhedonia extend into protracted abstinence and drive relapse by negative reinforcement.

dopamine secretion GO:0014046 ↓ DECREASED

central amygdala UBERON:0002883 nucleus accumbens UBERON:0001882

Downstream

Compulsive Opioid Seeking and Loss of Control Negative reinforcement — using to remove an aversive internal state — sustains compulsive use after the euphoric effect has waned.

Show evidence (2 references)

PMID:31400808 SUPPORT Other

"Such negative emotional states that drive negative reinforcement are hypothesized to derive from the within-system dysregulation of key neurochemical circuits that mediate incentive-salience and/or reward systems (dopamine, opioid peptides) in the ventral striatum and from the between-system..."

Names both arms — within-system reward deficit and between-system stress recruitment — and their anatomy.

PMID:31400808 SUPPORT Model Organism

"In animal models, repeated extended access to drugs or opioids results in negative emotion-like states, reflected by the elevation of reward thresholds, lower pain thresholds, anxiety-like behavior, and dysphoric-like responses."

Provides the preclinical operationalization of the negative emotional state, including the elevated reward threshold that defines reward deficit.

Opioid-Induced Hyperalgesia

Paradoxical nociceptive sensitization caused by opioid exposure, in which a patient receiving opioids becomes more sensitive to painful stimuli. It is mechanistically distinct from tolerance — sensitization of pronociceptive pathways rather than loss of receptor responsiveness — and it matters clinically because it can be mistaken for undertreated pain and answered with a dose increase that worsens it.

sensory perception of pain GO:0019233 ↑ INCREASED

Downstream

Compulsive Opioid Seeking and Loss of Control Increased pain sensitivity motivates further opioid use, closing a self-reinforcing loop.

Show evidence (3 references)

PMID:21412369 SUPPORT Other

"Opioid-induced hyperalgesia (OIH) is defined as a state of nociceptive sensitization caused by exposure to opioids."

Defines the phenomenon this node models.

PMID:21412369 SUPPORT Other

"It is generally thought to result from neuroplastic changes in the peripheral and central nervous system (CNS) that lead to sensitization of pronociceptive pathways."

States the sensitization mechanism that distinguishes hyperalgesia from receptor-level tolerance.

PMID:21412369 SUPPORT Other

"Findings of the clinical prevalence of OIH are not available."

The review states that clinical prevalence is unknown, so no frequency claim can be attached to this node.

Brainstem Respiratory Rhythm Suppression

Mu-opioid receptors on brainstem respiratory neurons — the preBotzinger complex, which generates inspiratory rhythm, and the Kolliker-Fuse and parabrachial nuclei, which modulate breathing — mediate depression of respiratory drive. Combined with reduced consciousness and airway obstruction this produces ventilatory insufficiency. This is the proximate cause of opioid-induced death and the target of naloxone.

G protein-coupled opioid receptor signaling pathway GO:0038003 ↑ INCREASED

Downstream

Fatal Opioid Overdose Sustained ventilatory insufficiency produces hypoxaemia and death unless reversed.

Show evidence (2 references)

PMID:35471232 SUPPORT Other

"Opioid-induced respiratory depression (OIRD), the primary cause of opioid-induced death, is the neural depression of respiratory drive which, together with a decreased level of consciousness and obstructive sleep apnea, cause ventilatory insufficiency."

Establishes respiratory depression as the mechanism of opioid death and names its three components.

PMID:35471232 SUPPORT Model Organism

"Deletion of μ opioid receptors from neurons showed that the preBötC and KF/PBN contribute to OIRD with the KF as a respiratory modulator and the preBötC as inspiratory rhythm generator."

Receptor-deletion experiments localize the effect to specific brainstem nuclei, giving the anatomical claim causal support.

Fatal Opioid Overdose

Death from ventilatory insufficiency. Risk is not a simple function of disorder severity: it is dominated by tolerance state (sharply elevated after detoxification or incarceration), by co-ingested sedatives, and by the potency of the illicit supply. This is why overdose risk can rise at precisely the moment a person stops using.

Show evidence (1 reference)

PMID:35471232 SUPPORT Other

"Variability of responses to opioids and individual differences in physiological and neurological states (e.g., anesthesia, sleep-disordered breathing, concurrent drug administration) add to the risk."

Supports the claim that overdose risk is modified by physiological state and concurrent drugs rather than being a fixed property of the dose.

Prefrontal Executive Control Deficit

Impaired top-down inhibitory control over drug seeking, with altered prefrontal-limbic connectivity. This is the preoccupation/anticipation arm of the addiction cycle, and it removes the counterweight to both cue-driven and withdrawal-driven use.

prefrontal cortex UBERON:0000451

Downstream

Compulsive Opioid Seeking and Loss of Control Loss of executive control permits drug seeking that would otherwise be inhibited.

Show evidence (1 reference)

PMID:31400808 SUPPORT Other

"Compelling evidence indicates that plasticity in the brain pain emotional systems is triggered by acute excessive drug intake and becomes sensitized during the development of compulsive drug taking with repeated withdrawal."

Supports drug-induced plasticity in emotion-related systems during the development of compulsive use. This review does not itself demonstrate the prefrontal executive deficit, so support is partial and a prefrontal-specific citation is a curation gap.

Compulsive Opioid Seeking and Loss of Control

The convergent clinical endpoint: compulsive opioid seeking and use despite harm, with impaired control over intake. Three motivational routes feed it — positive reinforcement early, negative reinforcement from withdrawal and hyperkatifeia later, and escalating pain from hyperalgesia — while prefrontal executive deficit removes the brake.

Downstream

Brainstem Respiratory Rhythm Suppression Sustained use maintains repeated exposure to doses capable of suppressing respiratory drive, which is how the behavioural disorder becomes lethal.

Show evidence (1 reference)

PMID:31400808 SUPPORT Other

"However, they also engage the brain systems for stress and pain (somatic and emotional) while producing hyperalgesia and hyperkatifeia, which drive pronounced drug-seeking behavior via processes of negative reinforcement."

Identifies the negative-reinforcement drive toward the compulsive drug-seeking endpoint this node represents.

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Pathograph

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Phenotypes

21

Digestive 3

Diarrhea HP:0002014

Temporal: ACUTE

Show evidence (1 reference)

PMID:32002194 SUPPORT Human Clinical

"Fear of opioid withdrawal syndrome (OWS) often dissuades opioid discontinuation."

Supports the withdrawal syndrome as a whole; a sign-level citation is a curation gap.

Nausea and Vomiting HP:0002017

Temporal: ACUTE

Show evidence (1 reference)

PMID:32002194 SUPPORT Human Clinical

"Fear of opioid withdrawal syndrome (OWS) often dissuades opioid discontinuation."

Supports the withdrawal syndrome as a whole; a sign-level citation is a curation gap.

Constipation HP:0002019

Temporal: CHRONIC

Show evidence (2 references)

PMID:28034973 SUPPORT Human Clinical

"Constipation is a major untoward effect of opioids. Increasing prescription of opioids has correlated to increased incidence of opioid-induced constipation."

Evidence-based clinical guideline establishing constipation as a major opioid adverse effect and linking its incidence to opioid prescribing.

PMID:28034973 SUPPORT Human Clinical

"However, the inhibitory effects of opioids are not confined to the colon, but also affect higher segments of the gastrointestinal tract"

Supports the broader bowel-dysfunction framing used in this phenotype's description rather than constipation alone.

Endocrine 1

Hypogonadism HP:0000135

Temporal: CHRONIC

Show evidence (2 references)

PMID:22786453 SUPPORT Other

"Accompanying this upsurge in the use of long-term opioid therapy has been an increase in the occurrence of opioid associated endocrinopathy, most commonly manifested as an androgen deficiency and therefore referred to as opioid associated androgen deficiency (OPIAD)."

Establishes androgen deficiency as the characteristic endocrinopathy of long-term opioid therapy.

PMID:22786453 SUPPORT Other

"This syndrome is characterized by the presence of inappropriately low levels of gonadotropins (follicle stimulating hormone and luteinizing hormone) leading to inadequate production of sex hormones, particularly testosterone."

Specifies the hypogonadotropic mechanism, which is what makes this a central rather than a primary hypogonadism.

Immune 1

Opioid-Associated Immunosuppression Immunodeficiency HP:0002721

Scope. The human evidence here is for prescription opioid analgesic use, not for opioid use disorder, and the entry deliberately does not model injection-related infectious complications (see the entry `notes`). This phenotype is the pharmacological immunosuppression arm only; it is not a claim about the infectious burden of injection drug use, which has a different and much larger cause.

Show evidence (4 references)

PMID:29435555 SUPPORT Human Clinical

"Persons in the case group had greater odds than control participants of being current opioid users"

The primary human infection-risk result. The paper reports an adjusted odds ratio of 1.62 (95% CI 1.36 to 1.92), strongest for long-acting, high-potency, and high-dose opioids; the figures are given here rather than quoted because the reference validator strips the bracketed `[aOR]` abbreviation from a snippet, which would break an otherwise verbatim quote.

PMID:29435555 SUPPORT Human Clinical

"Opioid use is associated with an increased risk for IPD and represents a novel risk factor for these diseases."

The authors' own conclusion, stating the direction of the association this phenotype records.

PMID:37259426 SUPPORT In Vitro

"morphine at 0.1-10 nM levels inhibited CD11b expression and function on macrophages via a μ-opioid receptor (MOR)-dependent mechanism, thereby reducing macrophage phagocytosis of tumor cells"

Supplies the receptor-level mechanism — a mu-opioid-receptor-dependent loss of macrophage phagocytic function at therapeutic concentrations — that the epidemiological association lacks.

+ 1 more reference

Integument 1

Hyperhidrosis HP:0000975

Temporal: ACUTE

Show evidence (1 reference)

PMID:32002194 SUPPORT Human Clinical

"Hypotension and bradycardia were more common with lofexidine."

That an alpha-2 agonist suppresses withdrawal while producing hypotension and bradycardia supports an autonomic, noradrenergically driven withdrawal state; sweating specifically is not measured in the abstract.

Nervous System 5

Insomnia HP:0100785

Show evidence (1 reference)

PMID:31400808 SUPPORT Other

"Hyperkatifeia can extend into protracted abstinence and interact with learning processes in the form of conditioned withdrawal to facilitate relapse to compulsive-like drug seeking."

Supports persistence of withdrawal-associated symptoms into protracted abstinence; insomnia specifically is not enumerated.

Anxiety HP:0000739

Show evidence (1 reference)

PMID:31400808 SUPPORT Model Organism

"In animal models, repeated extended access to drugs or opioids results in negative emotion-like states, reflected by the elevation of reward thresholds, lower pain thresholds, anxiety-like behavior, and dysphoric-like responses."

Anxiety-like behaviour is a measured component of the opioid negative-affect state in animal models.

Depression HP:0000716

Show evidence (1 reference)

PMID:31400808 SUPPORT Model Organism

"In animal models, repeated extended access to drugs or opioids results in negative emotion-like states, reflected by the elevation of reward thresholds, lower pain thresholds, anxiety-like behavior, and dysphoric-like responses."

Dysphoric-like responses and elevated reward thresholds are the preclinical correlates of the depressive component.

Sedation Drowsiness HP:0002329

The quantitative evidence for this phenotype comes from chronic-pain populations on long-term opioid therapy, not from OUD cohorts. That is a different population with a different dose and exposure pattern, so the phenotype is curated without a `frequency` value rather than importing the chronic-pain prevalence as if it were an OUD frequency.

Show evidence (2 references)

PMID:26461075 SUPPORT Other

"Cognitive effects and sedation (CES) are prevalent in chronic nonmalignant pain populations receiving long-term opioid therapy and are among the most common reasons patients discontinue opioid use."

Establishes sedation as a prevalent and clinically consequential effect of sustained opioid exposure.

PMID:26461075 SUPPORT Other

"The most prevalent CES include: memory deficits (73-81%), sleep disturbance (35-57%), and fatigue (10%)."

Gives the only prevalence figures available here, but they are for a long-term-opioid-therapy chronic-pain population rather than for a diagnosed opioid use disorder cohort, so no `frequency` band is asserted from them.

Cognitive Impairment HP:0100543

Attribution is genuinely unresolved. The cited cohort is methadone-maintained, so opioid exposure, prior illicit use, and the comorbidities that accompany OUD are all confounded; the study design cannot separate them. The phenotype is curated as an observed association with opioid-treated OUD, not as a demonstrated pharmacological effect of opioids alone.

Show evidence (3 references)

PMID:26885347 SUPPORT Human Clinical

"MMP performed significantly poorly than controls in cognitive domains of verbal fluency, executive function, and verbal memory."

Primary case-control neuropsychological data in methadone-maintained patients, which is the OUD-relevant population rather than a chronic-pain one.

PMID:26885347 SUPPORT Human Clinical

"MMP did not exhibit impairment in working memory, and TMT Part A compared to controls."

A negative result within the same study, which bounds the claim: the impairment is domain-selective rather than global.

PMID:26461075 SUPPORT Other

"Conclusions about the neuropsychological domains affected by opioids are limited due to the heterogeneity of studies and methodological issues."

The review states plainly that the literature does not support firm domain-level conclusions, which is why no severity or frequency is asserted here.

Respiratory 2

Sleep-Disordered Breathing Apnea HP:0002104

Show evidence (1 reference)

PMID:35471232 SUPPORT Other

"Variability of responses to opioids and individual differences in physiological and neurological states (e.g., anesthesia, sleep-disordered breathing, concurrent drug administration) add to the risk."

Identifies sleep-disordered breathing as a state that compounds opioid-induced respiratory risk.

Rhinorrhea HP:0031417

Temporal: ACUTE

Show evidence (1 reference)

PMID:32002194 SUPPORT Human Clinical

"Fear of opioid withdrawal syndrome (OWS) often dissuades opioid discontinuation."

Supports the existence and clinical salience of the withdrawal syndrome; the abstract does not enumerate its individual signs, so support for rhinorrhea specifically is partial.

Constitutional 2

Myalgia HP:0003326

Temporal: ACUTE

Show evidence (1 reference)

PMID:31400808 SUPPORT Other

"However, they also engage the brain systems for stress and pain (somatic and emotional) while producing hyperalgesia and hyperkatifeia, which drive pronounced drug-seeking behavior via processes of negative reinforcement."

Supports somatic pain as part of the withdrawal state; not a myalgia-specific measurement.

Increased Pain Sensitivity HP:0012531

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:21412369 SUPPORT Other

"The condition is characterized by a paradoxical response whereby a patient receiving opioids for the treatment of pain could actually become more sensitive to certain painful stimuli."

Describes the increased pain sensitivity this phenotype records.

Other 6

Addictive Substance Use VERY_FREQUENT HP:0033511

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:31400808 SUPPORT Other

"However, they also engage the brain systems for stress and pain (somatic and emotional) while producing hyperalgesia and hyperkatifeia, which drive pronounced drug-seeking behavior via processes of negative reinforcement."

Compulsive drug seeking is the defining feature of the disorder, so it is present in essentially all cases; that is the basis for VERY_FREQUENT.

Craving

No `phenotype_term` is bound: HPO has no general craving term (only the food-specific HP:0030083, HP:0030221 and HP:6000785), and a generic behavioural parent would misdescribe the claim. Needs term / NTR candidate, shared with the Alcohol_Use_Disorder entry.

Show evidence (1 reference)

PMID:29150198 SUPPORT Human Clinical

"Self-reported opioid craving was initially less with XR-NTX than with BUP-NX (p=0·0012), then converged by week 24 (p=0·20)."

Craving is measured as a distinct prospective endpoint in OUD pharmacotherapy trials, separable from consumption measures.

Miosis HP:0000616

Temporal: ACUTE

Show evidence (1 reference)

PMID:35471232 SUPPORT Other

"Opioid-induced respiratory depression (OIRD), the primary cause of opioid-induced death, is the neural depression of respiratory drive which, together with a decreased level of consciousness and obstructive sleep apnea, cause ventilatory insufficiency."

Supports the intoxication syndrome including depressed consciousness; the review does not itself document miosis, so a miosis-specific citation is a curation gap.

Respiratory Depression Hypoventilation HP:0002791

Temporal: ACUTE Severity: SEVERE

Show evidence (1 reference)

PMID:35471232 SUPPORT Other

"Opioid-induced respiratory depression (OIRD), the primary cause of opioid-induced death, is the neural depression of respiratory drive which, together with a decreased level of consciousness and obstructive sleep apnea, cause ventilatory insufficiency."

Directly documents respiratory depression and its lethal significance.

Mydriasis HP:0011499

Temporal: ACUTE

Show evidence (1 reference)

PMID:32002194 SUPPORT Human Clinical

"efficacy was assessed using the Short Opioid Withdrawal Scale of Gossop (SOWS-G) daily"

Withdrawal severity is scored on standardized scales that include objective autonomic signs; the abstract does not enumerate individual signs, so support for mydriasis specifically is partial.

Neonatal Opioid Withdrawal Syndrome

No `phenotype_term` is bound. NOWS is a syndrome spanning irritability, tremor, feeding difficulty, autonomic instability and seizures; binding it to any single constituent sign (HP:0000737 Irritability, used in an earlier draft) understates the condition for any downstream consumer. Needs term / NTR candidate.

Show evidence (1 reference)

PMID:30819342 SUPPORT Human Clinical

"Neonatal abstinence syndrome (NAS) is a postnatal withdrawal syndrome that manifests shortly after birth in infants born to women with opioid use (including heroin, use or misuse of prescription painkillers, or maternal treatment medications such as methadone or buprenorphine) during pregnancy."

Defines the syndrome and its exposure routes, including maintenance medications.

🧬

Genetic Associations

4

OPRM1 (The mu-opioid receptor gene — the direct pharmacological target of opioids — and the only locus with a long-standing, independently replicated genome-wide significant association with opioid addiction. The functional coding variant rs1799971 (A118G) alters receptor trafficking and signalling efficacy rather than producing a clean gain or loss of function, so no `functional_impact_category` is asserted.)

Gene: OPRM1 hgnc:8156 relationship_type: SUSCEPTIBILITY

Show evidence (2 references)

PMID:36207451 SUPPORT Human Clinical

"Opioid addiction (OA) is moderately heritable, yet only rs1799971, the A118G variant in OPRM1, has been identified as a genome-wide significant association with OA and independently replicated."

Establishes rs1799971 as the single replicated genome-wide significant association.

PMID:36207451 SUPPORT Human Clinical

"We observed the strongest evidence to date for OPRM1: lead SNP rs9478500 (p = 2.56 × 10-9)."

Strengthens the OPRM1 signal in the largest multi-trait analysis to date.

FURIN (Identified by gene-based analysis in the multi-trait GWAS of opioid addiction, alongside PPP6C. Both are recent and comparatively less replicated than the OPRM1 signal.)

Gene: FURIN hgnc:8568 relationship_type: SUSCEPTIBILITY

Show evidence (2 references)

PMID:36207451 SUPPORT Human Clinical

"Gene-based analyses identified novel genome-wide significant associations with PPP6C and FURIN."

Direct statement of the gene-based association.

PMID:36207451 SUPPORT Human Clinical

"Variants within these loci appear to be pleiotropic for addiction and related traits."

The authors describe the signal as pleiotropic across addiction traits rather than opioid-specific, which bounds how strongly this locus can be called an OUD gene.

KDM4A (Intronic variant rs3791033 reached genome-wide significance in a GWAS of *problematic opioid use* (POU) — using prescription opioids "not as prescribed" — in 132,113 research participants. Read the phenotype carefully: POU is a self-report proxy, not a diagnosed OUD case definition, and this locus has not been reported at genome-wide significance in a GWAS of diagnosed OUD. It is curated as a susceptibility signal for the proxy phenotype, with the genetic correlation to OUD (rg = 0.64-0.80) as the bridge, rather than as an established OUD locus.)

Gene: KDM4A hgnc:22978 relationship_type: SUSCEPTIBILITY

Show evidence (2 references)

PMID:34728798 SUPPORT Human Clinical

"We identified two genome-wide significant loci (rs3791033, an intronic variant of KDM4A; rs640561, an intergenic variant near LRRIQ3)."

Direct statement of the genome-wide significant association, naming both the variant and the gene.

PMID:34728798 SUPPORT Human Clinical

"As a surrogate (or proxy) for OUD, we explored the genetic basis of using prescription opioids 'not as prescribed'."

The authors state the phenotype is a surrogate for OUD, which is what limits how strongly this locus can be called an OUD susceptibility gene.

LRRIQ3 (Intergenic variant rs640561 near LRRIQ3, the second genome-wide significant locus from the same problematic-opioid-use GWAS. The variant is intergenic, so the gene assignment is by proximity and not by demonstrated functional effect on LRRIQ3 — the same POU-proxy caveat recorded on KDM4A applies here and is compounded by that.)

Gene: LRRIQ3 hgnc:28318 relationship_type: SUSCEPTIBILITY

Show evidence (2 references)

PMID:34728798 SUPPORT Human Clinical

"We identified two genome-wide significant loci (rs3791033, an intronic variant of KDM4A; rs640561, an intergenic variant near LRRIQ3)."

Direct statement of the association, and the source of the "near" wording that makes the gene assignment positional.

PMID:34728798 SUPPORT Human Clinical

"POU showed positive genetic correlations with the two largest available GWAS of OUD and opioid dependence"

The genetic correlation is the only thing connecting this locus to diagnosed OUD, so it is recorded as partial support for an OUD susceptibility claim.

💊

Medical Actions

9

Methadone

Action: Pharmacotherapy NCIT:C15986

Agent: methadone CHEBI:6807

Full mu-opioid receptor agonist with a long half-life, delivered through regulated opioid treatment programmes. The best-retained of the opioid agonist treatments. It works by occupying the same receptor the disorder is built on, which is why it suppresses withdrawal and craving without producing the reinforcing peak of short-acting opioids.

Mechanism Target:

INHIBITS Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome — Sustained receptor occupancy prevents the unopposed noradrenergic surge that produces withdrawal.

Show evidence (2 references)

PMID:12804430 SUPPORT Human Clinical

"Methadone is an effective maintenance therapy intervention for the treatment of heroin dependence as it retains patients in treatment and decreases heroin use better than treatments that do not utilise opioid replacement therapy."

Cochrane review conclusion establishing methadone maintenance against non-replacement approaches.

PMID:12804430 SUPPORT Human Clinical

"methadone appeared statistically significantly more effective than non-pharmacological approaches in retaining patient in treatment (3 RCTs, RR=3.05; 95%CI: 1.75-5.35) and in the suppression of heroin use (3 RCTs, RR=0.32; 95%CI: 0.23-0.44), but not statistically in criminal activity"

Quantifies the retention and heroin-use effects, and is explicit that the criminal-activity effect was not significant.

Buprenorphine

Action: Pharmacotherapy NCIT:C15986

Agent: buprenorphine CHEBI:3216

Partial mu-opioid receptor agonist, usually co-formulated with naloxone to deter injection. Its partial agonism produces a ceiling on respiratory depression, which gives it a wider safety margin than methadone and allows office-based prescribing — the trade-off is somewhat lower retention than methadone under flexible dosing.

Mechanism Target:

INHIBITS Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome — Partial agonism at the same receptor suppresses withdrawal while capping respiratory depression.

Show evidence (2 references)

PMID:24500948 SUPPORT Human Clinical

"There is high quality of evidence that buprenorphine was superior to placebo medication in retention of participants in treatment at all doses examined."

Cochrane review of 31 trials establishes superiority to placebo for retention.

PMID:24500948 SUPPORT Human Clinical

"Methadone is superior to buprenorphine in retaining people in treatment, and methadone equally suppresses illicit opioid use."

Bounds the claim: buprenorphine is effective but does not match methadone on retention, which is the main comparative caveat.

Extended-Release Injectable Naltrexone

Action: Pharmacotherapy NCIT:C15986

Agent: naltrexone CHEBI:7465

Mu-opioid receptor antagonist given as a monthly injection. Conceptually the opposite of agonist treatment: it blocks the receptor rather than occupying it with a safer agonist. Its practical limitation is the induction hurdle — it requires full detoxification first, and failure to initiate is what drives its worse intention-to-treat outcomes.

Mechanism Target:

INHIBITS VTA Disinhibition and Mesolimbic Dopamine Release — Receptor blockade prevents the disinhibition step that produces opioid reward.

Show evidence (2 references)

PMID:29150198 SUPPORT Human Clinical

"In this population it is more difficult to initiate patients to XR-NTX than BUP-NX, and this negatively affected overall relapse. However, once initiated, both medications were equally safe and effective."

The trial's own conclusion, separating the induction problem from once-initiated efficacy.

PMID:29150198 SUPPORT Human Clinical

"Among participants successfully inducted (per-protocol population, n=474), 24 week relapse events were similar across study groups (p=0·44)."

Quantifies the per-protocol equivalence that the intention-to-treat result obscures.

Naloxone Overdose Reversal

Action: Pharmacotherapy NCIT:C15986

Agent: naloxone CHEBI:7459

Competitive mu-opioid receptor antagonist that reverses opioid-induced respiratory depression within minutes. This treats the acute lethal event, not the disorder. Its half-life can be shorter than that of the opioid it reverses, so re-sedation after apparent recovery is a real hazard.

Mechanism Target:

INHIBITS Brainstem Respiratory Rhythm Suppression — Competitive displacement of the agonist at brainstem mu-opioid receptors restores respiratory drive.

Show evidence (1 reference)

PMID:35471232 SUPPORT Other

"Reversal of OIRD has relied heavily on naloxone which also reverses analgesia but mismatches between the half-lives of naloxone and opioids can make it difficult to clinically safely avoid OIRD."

States both the reversal mechanism and the half-life mismatch caveat.

Show evidence (1 reference)

PMID:24874759 SUPPORT Human Clinical

"The current evidence from nonrandomized studies suggests that bystanders (mostly opioid users) can and will use naloxone to reverse opioid overdoses when properly trained, and that this training can be done successfully through OOPPs."

Supports community naloxone distribution as an effective delivery route, while the authors are explicit that the evidence is nonrandomized.

Lofexidine

Action: Pharmacotherapy NCIT:C15986

Agent: lofexidine CHEBI:51368

Alpha-2 adrenergic agonist approved for mitigating opioid withdrawal symptoms. Mechanistically it targets exactly the noradrenergic hyperactivity node rather than the opioid receptor, which is why it eases withdrawal without treating craving or preventing relapse.

Mechanism Target:

INHIBITS Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome — Alpha-2 autoreceptor agonism suppresses noradrenergic output, the substrate of somatic withdrawal.

Show evidence (2 references)

PMID:32002194 SUPPORT Human Clinical

"This simplified analysis confirmed previous per-protocol results, that lofexidine better reduces OWS severity and increases retention compared with placebo in opioid-dependent adults."

Two randomized placebo-controlled trials support reduction in withdrawal severity and improved retention.

PMID:32002194 SUPPORT Human Clinical

"Hypotension and bradycardia were more common with lofexidine."

Records the predictable cardiovascular consequence of alpha-2 agonism, which bounds its use.

Psychosocial Intervention Adjunctive to Opioid Agonist Therapy

Action: Cognitive Behavior Therapy NCIT:C64345

Cognitive behavioural therapy, contingency management, and related psychosocial treatments, delivered as an adjunct to opioid agonist therapy rather than as a substitute for it.

Show evidence (1 reference)

PMID:33370393 SUPPORT Human Clinical

"Guidelines recommend that individuals with opioid use disorder (OUD) receive pharmacological and psychosocial interventions; however, the most appropriate psychosocial intervention is not known."

Establishes the adjunctive role while stating explicitly that the comparative question is unresolved.

Contingency Management

Action: contingency management Ontology label: Contingency Management NCIT:C94375

A behavioural intervention delivering material incentives contingent on objectively verified behaviour change — typically a negative urine drug screen or a kept appointment. It is curated separately from the general psychosocial-intervention entry above because it is the psychosocial treatment with the strongest and most specific evidence base, and because it addresses a problem opioid agonist therapy does not: comorbid stimulant use, for which no effective pharmacotherapy exists.

Show evidence (2 references)

PMID:34347030 SUPPORT Human Clinical

"Collapsing across abstinence and adherence categories, contingency management was associated with medium effect sizes for abstinence (Cohen d = 0.58; 95% CI, 0.47-0.69) and treatment adherence (Cohen d = 0.62; 95% CI, 0.40-0.84) compared with controls."

Quantifies the effect on both abstinence and treatment adherence in patients already receiving medication for OUD, which is the adjunctive role this entry curates.

PMID:34347030 SUPPORT Human Clinical

"Medication treatment for opioid use disorder (MOUD) is efficacious, but comorbid stimulant use and other behavioral health problems often undermine efficacy."

States the gap this treatment addresses, and why it is not redundant with the agonist therapies curated above.

Nalmefene Nasal Spray

Action: Pharmacotherapy NCIT:C15986

Agent: nalmefene CHEBI:7457

Long-acting opioid antagonist nasal spray (Opvee), FDA-approved in 2023 for emergency treatment of known or suspected opioid overdose. Its longer duration of action than naloxone's is the intended advantage against re-sedation as an opioid's effects outlast a single reversal dose, which is the same half-life-mismatch hazard already curated on the naloxone treatment above.

Mechanism Target:

INHIBITS Brainstem Respiratory Rhythm Suppression — Competitive opioid receptor antagonism at brainstem mu-opioid receptors restores respiratory drive, the same mechanism as naloxone but with a longer duration of action.

Show evidence (1 reference)

PMID:39648641 SUPPORT Human Clinical

"On May 22, 2023, the United States Food and Drug Administration approved the first nalmefene hydrochloride nasal spray for the emergency treatment of known or suspected opioid overdose in adults and pediatric patients 12 years of age and older."

FDA approval summary establishing the drug, route, indication, and approval date.

Extended-Release Buprenorphine Injectable

Action: Pharmacotherapy NCIT:C15986

Agent: buprenorphine CHEBI:3216

Subcutaneous depot buprenorphine formulations (monthly Sublocade, or weekly/monthly Buvidal/Brixadi) that reduce dosing frequency relative to daily sublingual buprenorphine, the same adherence rationale already modelled for extended-release injectable naltrexone above. Unlike naltrexone, induction does not require prior full detoxification, since buprenorphine is itself an opioid agonist rather than an antagonist.

Mechanism Target:

INHIBITS Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome — Same partial-agonist receptor mechanism as sublingual buprenorphine; the depot formulation changes pharmacokinetics and dosing frequency, not receptor pharmacology.

Show evidence (2 references)

PMID:40459195 SUPPORT Human Clinical

"Sublocade (extended-release buprenorphine; Bup-XR-S) and Buvidal/Brixadi (long-acting buprenorphine; Bup-LA-B) formulations allow for less frequent dosing."

Names both long-acting formulations and states the dosing-frequency rationale for the treatment.

PMID:40459195 SUPPORT Human Clinical

"Short-term retention (4 wk) exceeded 60%."

Quantifies short-term retention pooled across the transition studies reviewed.

🌍

Environmental Factors

1

Exposure to mu-opioid receptor agonists

Opioid exposure — whether via prescribed analgesia, diverted prescription opioids, heroin, or illicit synthetic opioids such as fentanyl — is the necessary cause. It is not sufficient: only a minority of exposed people develop the disorder. Supply potency is a distinct risk dimension from exposure per se, because it determines whether a given episode of use is survivable.

Show evidence (1 reference)

PMID:36207451 SUPPORT Human Clinical

"Approximately 4% of the US population aged 12 and older (10.1 million people) misused opioids in 2019"

Quantifies how widespread the exposure modelled by this entry is, which is what makes a necessary-but-not-sufficient exposure epidemiologically significant.

Mechanism Target:

TRIGGERS Exogenous Mu-Opioid Receptor Agonism — Administration by any route is what places an agonist at the receptor.

Show evidence (1 reference)

PMID:23321159 SUPPORT Other

"Morphine and related µ-opioid receptor (MOR) agonists remain among the most effective drugs known for acute relief of severe pain."

Establishes that the exposure of interest is agonism at the mu-opioid receptor, the mechanism node this triggers.

TRIGGERS Mu-Opioid Receptor Desensitization and Tolerance — Chronic rather than single exposure is what produces durable receptor regulation and tolerance.

Show evidence (1 reference)

PMID:23321159 SUPPORT Other

"there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids"

Identifies chronic exposure as the variable that drives loss of receptor function, while flagging that the molecular detail is incomplete.

🔬

Diagnosis

3

DSM-5 and ICD-11 clinical diagnostic criteria

Diagnosis is clinical and behavioural: DSM-5 requires at least 2 of 11 criteria within 12 months, graded mild/moderate/severe by criterion count; ICD-11 codes opioid dependence at 6C43.2 using the dependence-syndrome framework. Physiological tolerance and withdrawal arising from appropriate medical opioid use do not by themselves establish the disorder — a distinction that matters for patients on long-term analgesia. There is no validated blood biomarker; urine immunoassay panels detect exposure, not the disorder, and frequently miss fentanyl and other synthetic opioids without an extended panel.

Show evidence (1 reference)

PMID:29150198 SUPPORT Human Clinical

"Extended-release naltrexone (XR-NTX), an opioid antagonist, and sublingual buprenorphine-naloxone (BUP-NX), a partial opioid agonist, are pharmacologically and conceptually distinct interventions to prevent opioid relapse."

Confirms DSM-criteria-based ascertainment is the standard entry point for OUD trials; the abstract does not enumerate the criteria, so support for the criteria themselves is partial.

Opioid Risk Tool (ORT) and SOAPP pre-prescription risk screening

Self-administered questionnaires used before initiating long-term opioid therapy for pain, to predict which patients are likely to develop aberrant opioid-related behaviors. This is a distinct pre-prescription screening tier from the DSM-5/ICD-11 diagnostic criteria and the COWS/SOWS-G withdrawal-severity instruments above, which apply once a patient is already on opioids or already dependent.

Show evidence (2 references)

PMID:16336480 SUPPORT Human Clinical

"The ORT displayed excellent discrimination for both the male (c = 0.82) and the female (c = 0.85) prognostic models."

Reports the discrimination statistics from the Opioid Risk Tool's preliminary validation study.

PMID:15494186 SUPPORT Human Clinical

"Receiver operating characteristics curve analysis yielded an area under the curve of 0.881 (P<0.001), suggesting adequate sensitivity and specificity for a screening device."

Reports the SOAPP's validation performance as a pre-prescription screening tool.

📊

Prevalence

1

US population aged 12 and older, past-year opioid misuse (not diagnosed OUD)

Period Prevalence 4000.0 per 100,000 >1 in 1,000

This is past-year opioid MISUSE, a broader construct than diagnosed opioid use disorder; only a fraction of people who misuse opioids meet OUD criteria. Recorded at the construct the source actually measures rather than relabelled as OUD prevalence.

Show evidence (1 reference)

PMID:36207451 SUPPORT Human Clinical

"Approximately 4% of the US population aged 12 and older (10.1 million people) misused opioids in 2019"

Gives the national misuse figure. Support is PARTIAL for a prevalence record on this entry because misuse is not equivalent to diagnosed OUD.

⚖️

Clinical Burden

High

OUD carries a markedly elevated mortality risk dominated by fatal overdose, which is mechanistically a direct pharmacological consequence of the drug rather than a late complication. Mortality falls sharply during opioid agonist treatment and rises during treatment gaps. The societal cost of OUD and fatal opioid overdose in the US was estimated at over one trillion dollars for a single year.

Show evidence (2 references)

PMID:33121867 SUPPORT Human Clinical

"Costs for opioid use disorder and fatal opioid overdose in 2017 were estimated to be $1.02 trillion."

CDC estimate of the total US societal cost, combining health care, criminal justice, lost productivity, and valued quality-of-life and life-years lost.

PMID:33121867 SUPPORT Human Clinical

"The majority of the economic burden is due to reduced quality of life from opioid use disorder and the value of life lost due to fatal opioid overdose."

Identifies disability and premature death, rather than direct treatment spending, as the dominant components of the burden.

🔬

Clinical Trials

1

NCT02032433 PHASE_IV COMPLETED

X:BOT — a 24-week multicentre open-label randomised comparative effectiveness trial of extended-release naltrexone versus buprenorphine-naloxone for opioid relapse prevention in 570 adults, with opioid relapse-free survival as the primary outcome.

Target Phenotypes: Addictive substance use HP:0033511

Show evidence (1 reference)

PMID:29150198 SUPPORT Human Clinical

"We initiated this 24 week, open-label, randomised controlled, comparative effectiveness trial at eight US community-based inpatient services and followed up participants as outpatients."

Describes the trial design recorded here.

{ }

Source YAML

click to show

name: Opioid Use Disorder
creation_date: "2026-08-09T18:40:00Z"
category: Psychiatric
description: >-
  Opioid use disorder (OUD) is a chronic, relapsing substance use disorder
  driven by repeated exogenous mu-opioid receptor agonism: compulsive opioid
  seeking and use despite harm, with tolerance, a severe somatic and affective
  withdrawal syndrome, and a distinctively lethal complication in
  opioid-induced respiratory depression. DSM-5 merged the earlier opioid abuse
  and opioid dependence categories into a single graded diagnosis
  (mild/moderate/severe); ICD-11 retains a dependence syndrome (6C43.2).
  Exposure is necessary but not sufficient — only a minority of exposed people
  develop the disorder.
disease_term:
  preferred_term: opioid use disorder
  term:
    id: MONDO:0005530
    label: opiate dependence
synonyms:
- Opioid dependence
- Opioid addiction
- Opiate dependence
- Heroin dependence
parents:
- Substance Use Disorder
- Mental Health Disorder
notes: >-
  Terminology and identifier caveat, following the same convention as the
  Alcohol_Use_Disorder entry. MONDO has no single term for the DSM-5 construct
  "opioid use disorder"; it carries the two pre-DSM-5 entities separately
  (MONDO:0005530 opiate dependence, with EXACT synonym "opioid dependence",
  ICD10CM:F11.2; and MONDO:0001225 opioid abuse, ICD10CM:F11.1). This entry is
  bound to MONDO:0005530 because the dependence phenotype is what essentially
  all of the cited mechanism and treatment literature studies. Claims specific
  to the narrower DSM-IV "opioid abuse" pole should say so rather than assuming
  the binding covers them.

  Scope. This entry models the CNS disorder of opioid use and the acute
  respiratory-depression mechanism that makes it lethal. Injection-related
  infectious complications (HIV, HCV, infective endocarditis) are consequences
  of the route of administration rather than of opioid pharmacology, and are
  not modelled here.

  Known curation gaps, recorded rather than left implicit. (1) The
  neuroimmune/glial (TLR4, microglial) contribution to tolerance and
  hyperalgesia is now modelled as its own node, `Opioid-Induced TLR4 and
  Microglial Neuroimmune Activation`, closing the gap this note previously
  recorded. Its human translational status remains open and is tracked in the
  `oud_tlr4_glial_human_translation` discussion rather than asserted in the
  node. (2) Neonatal opioid withdrawal syndrome is captured as a single
  phenotype of in-utero-exposed offspring; it is properly a distinct entity
  affecting a different individual, and would be better modelled as its own
  entry or as a trajectory. (3) No population prevalence record for diagnosed
  OUD is curated — the one quotable figure available in the cached references
  is for opioid *misuse*, which is a broader construct, so it is recorded as
  such rather than relabelled. (4) Several withdrawal signs and chronic-use
  complications carry only syndrome-level citations at `supports: PARTIAL`;
  each needs a sign-specific primary reference. (5) Six further candidate risk
  loci (PPP6C, KDM4A, LRRIQ3, CPT2, CD47, SLC5A11) were assessed for curation.
  PPP6C was already covered by the multi-trait GWAS cited on the susceptibility
  node; KDM4A and LRRIQ3 are curated, with the caveat that their source GWAS
  used a self-reported problematic-opioid-use proxy phenotype rather than
  diagnosed OUD. CPT2, CD47 and SLC5A11 are deliberately *not* curated: no
  abstract-level source associating any of them with opioid use disorder could
  be found, and citing a GWAS supplementary table that cannot be quoted would
  put an unverifiable claim in the entry. They should be added if and when a
  quotable source is identified, not on the strength of the list they came
  from.

  Evidence-source convention used in this entry, recorded because it was
  queried. A reference is tagged by what the *document* reports, and the line
  this entry draws is whether it reports or synthesizes human clinical data
  (HUMAN_CLINICAL: trials, cohort and case-control studies, Cochrane and other
  systematic reviews of trials, instrument-validation studies, regulatory
  approval summaries, evidence-based clinical guidelines) or narrates a
  mechanism (OTHER: mechanism and syndrome reviews). On that rule the
  opioid-induced-constipation clinical guideline (PMID:28034973,
  HUMAN_CLINICAL) and the opioid-induced-androgen-deficiency review
  (PMID:22786453, OTHER) are consistent with each other, and both are left
  unchanged. There is no KB-wide convention to align to instead: across
  `kb/disorders/` guideline-titled references are split roughly two-to-one
  between HUMAN_CLINICAL and OTHER.
prevalence:
- population: US population aged 12 and older, past-year opioid misuse (not diagnosed OUD)
  measure_type: PERIOD_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 4000.0
  notes: >-
    This is past-year opioid MISUSE, a broader construct than diagnosed opioid
    use disorder; only a fraction of people who misuse opioids meet OUD
    criteria. Recorded at the construct the source actually measures rather
    than relabelled as OUD prevalence.
  evidence:
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Approximately 4% of the US population aged 12 and older (10.1 million
      people) misused opioids in 2019
    explanation: >-
      Gives the national misuse figure. Support is PARTIAL for a prevalence
      record on this entry because misuse is not equivalent to diagnosed OUD.
clinical_burden:
  burden_level: HIGH
  rationale: >-
    OUD carries a markedly elevated mortality risk dominated by fatal overdose,
    which is mechanistically a direct pharmacological consequence of the drug
    rather than a late complication. Mortality falls sharply during opioid
    agonist treatment and rises during treatment gaps. The societal cost of OUD
    and fatal opioid overdose in the US was estimated at over one trillion
    dollars for a single year.
  evidence:
  - reference: PMID:33121867
    reference_title: "The economic burden of opioid use disorder and fatal opioid overdose in the United States, 2017."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Costs for opioid use disorder and fatal opioid overdose in 2017 were
      estimated to be $1.02 trillion.
    explanation: >-
      CDC estimate of the total US societal cost, combining health care,
      criminal justice, lost productivity, and valued quality-of-life and
      life-years lost.
  - reference: PMID:33121867
    reference_title: "The economic burden of opioid use disorder and fatal opioid overdose in the United States, 2017."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The majority of the economic burden is due to reduced quality of life
      from opioid use disorder and the value of life lost due to fatal opioid
      overdose.
    explanation: >-
      Identifies disability and premature death, rather than direct treatment
      spending, as the dominant components of the burden.
inheritance:
- name: Polygenic (multifactorial) liability
  inheritance_term:
    preferred_term: polygenic inheritance
    term:
      id: HP:0010982
      label: Polygenic inheritance
  description: >-
    OUD is a complex trait, not Mendelian. Opioid addiction is moderately
    heritable, but until recently only the OPRM1 A118G variant had reached
    genome-wide significance with independent replication — an unusually sparse
    common-variant signal compared with other substance use disorders, which
    the largest multi-trait analyses have only recently begun to extend.
  evidence:
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Opioid addiction (OA) is moderately heritable, yet only rs1799971, the
      A118G variant in OPRM1, has been identified as a genome-wide significant
      association with OA and independently replicated.
    explanation: >-
      States both the moderate heritability and the sparseness of replicated
      common-variant signal that characterizes this trait.
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Genetic correlations among the various OA phenotypes were uniformly high
      (rg > 0.9).
    explanation: >-
      Supports treating the differently ascertained opioid addiction phenotypes
      as one underlying liability.
pathophysiology:
- name: Polygenic Susceptibility to Opioid Addiction
  biological_scale: MOLECULAR
  description: >-
    Inherited liability is polygenic and, relative to other substance use
    disorders, unusually concentrated on the opioid receptor gene itself.
    OPRM1 carries the only long-replicated genome-wide significant signal;
    gene-based analysis has added FURIN and PPP6C. Genetic loading modifies the
    probability that opioid exposure escalates to compulsive use; it does not
    cause the disorder without exposure.
  genes:
  - preferred_term: OPRM1
    term:
      id: hgnc:8156
      label: OPRM1
  - preferred_term: FURIN
    term:
      id: hgnc:8568
      label: FURIN
  downstream:
  - target: Exogenous Mu-Opioid Receptor Agonism
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Altered mu-opioid receptor expression, trafficking, and signalling efficacy
    description: >-
      OPRM1 variation acts on the receptor that exogenous opioids bind,
      modifying the signalling consequences of a given exposure.
    evidence:
    - reference: PMID:36207451
      reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We observed the strongest evidence to date for OPRM1: lead SNP
        rs9478500 (p = 2.56 × 10-9).
      explanation: >-
        Establishes OPRM1 as the strongest genetic signal, which is what makes
        this edge to receptor-level pharmacology the natural one.
  evidence:
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Gene-based analyses identified novel genome-wide significant associations
      with PPP6C and FURIN.
    explanation: >-
      Documents the loci beyond OPRM1 that this susceptibility node represents.
  - reference: PMID:35879402
    reference_title: "Genome-wide association study in individuals of European and African ancestry and multi-trait analysis of opioid use disorder identifies 19 independent genome-wide significant risk loci."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The current study identified OUD variant associations at OPRM1, single
      variant associations with FURIN, and 18 GWS associations in the OUD-MTAG.
    explanation: >-
      The largest OUD-specific GWAS independently replicates the two loci this
      node carries, and finds nothing else at genome-wide significance for
      diagnosed OUD without multi-trait augmentation.
  - reference: PMID:35879402
    reference_title: "Genome-wide association study in individuals of European and African ancestry and multi-trait analysis of opioid use disorder identifies 19 independent genome-wide significant risk loci."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The genetic architecture of OUD is likely influenced by both OUD-specific
      loci and loci shared across SUDs.
    explanation: >-
      Bounds every additional locus curated in the `genetic` section: signals
      recovered by multi-trait analysis are partly shared across substance use
      disorders rather than opioid-specific.
- name: Exogenous Mu-Opioid Receptor Agonism
  biological_scale: MOLECULAR
  description: >-
    Prescription opioids, heroin, and illicit synthetic opioids such as
    fentanyl act as agonists at the mu-opioid receptor, a Gi/Go-coupled GPCR.
    Receptor activation inhibits adenylyl cyclase and opens GIRK potassium
    channels, hyperpolarizing the target neuron. The same receptor event
    underlies analgesia, euphoria, tolerance, and — in the brainstem —
    respiratory depression, which is why the therapeutic and lethal effects of
    opioids cannot be pharmacologically separated at this node.
  biological_processes:
  - preferred_term: G protein-coupled opioid receptor signaling pathway
    term:
      id: GO:0038003
      label: G protein-coupled opioid receptor signaling pathway
    modifier: INCREASED
  - preferred_term: adenylate cyclase-inhibiting signaling
    term:
      id: GO:0007193
      label: adenylate cyclase-inhibiting G protein-coupled receptor signaling pathway
    modifier: INCREASED
  chemical_entities:
  - preferred_term: morphine
    term:
      id: CHEBI:17303
      label: morphine
  - preferred_term: heroin
    term:
      id: CHEBI:27808
      label: heroin
  - preferred_term: fentanyl
    term:
      id: CHEBI:119915
      label: fentanyl
  triggers:
  - preferred_term: exposure to mu-opioid receptor agonist
    term:
      id: ECTO:9001896
      label: exposure to mu-opioid receptor agonist
  downstream:
  - target: VTA Disinhibition and Mesolimbic Dopamine Release
    causal_link_type: DIRECT
    description: >-
      Receptor-mediated hyperpolarization of VTA GABAergic interneurons
      disinhibits dopaminergic projection neurons.
    evidence:
    - reference: PMID:31400808
      reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
          Opioids are powerful drugs that usurp and overpower the reward function
          of endogenous opioids and engage dramatic tolerance and withdrawal via
          molecular and neurocircuitry neuroadaptations within the same reward
          system.
      explanation: >-
          Establishes that exogenous opioids act on the endogenous opioid reward
          system. The review does not itself describe the VTA interneuron
          disinhibition step, so support for this specific edge is partial.
  - target: Mu-Opioid Receptor Desensitization and Tolerance
    causal_link_type: DIRECT
    description: >-
      Sustained agonist occupancy drives receptor phosphorylation,
      desensitization, and endocytosis.
    evidence:
    - reference: PMID:23321159
      reference_title: "Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Cellular and molecular mechanisms regulating MOR function by
        phosphorylation relative to receptor desensitization and endocytosis
        are comprehensively reviewed, with an emphasis on agonist-biased
        regulation and areas where knowledge is lacking or controversial.
      explanation: >-
        Establishes phosphorylation-driven desensitization and endocytosis as
        the receptor-level response to sustained agonism.
  - target: Opioid-Induced TLR4 and Microglial Neuroimmune Activation
    causal_link_type: DIRECT
    description: >-
      Opioid exposure additionally engages the TLR4/MD-2 innate immune complex.
      This edge is drawn from the exposure node rather than from the receptor
      event it names because the activation is non-stereoselective and so is
      not mediated by the mu-opioid receptor; it is the same drug acting at a
      second, unrelated target.
    evidence:
    - reference: PMID:19679181
      reference_title: Evidence that opioids may have toll-like receptor 4 and MD-2 effects.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        A range of structurally diverse clinically-employed opioid analgesics
        was found to be capable of activating TLR4 signaling in vitro.
      explanation: >-
        Establishes that opioid exposure as a class, not one particular agent,
        is what activates this arm.
  - target: Brainstem Respiratory Rhythm Suppression
    causal_link_type: DIRECT
    description: >-
      The same receptor is expressed on brainstem respiratory neurons, so
      agonism suppresses respiratory drive directly rather than as a
      downstream complication.
    evidence:
    - reference: PMID:35471232
      reference_title: Mechanisms of opioid-induced respiratory depression.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
          Deletion of μ opioid receptors from neurons showed that the preBötC and
          KF/PBN contribute to OIRD with the KF as a respiratory modulator and the
          preBötC as inspiratory rhythm generator.
      explanation: >-
          Receptor-deletion experiments establish that the respiratory effect is
          mediated by mu-opioid receptors on specific brainstem neurons, which is
          exactly the causal link this edge asserts.
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Opioids are powerful drugs that usurp and overpower the reward function
      of endogenous opioids and engage dramatic tolerance and withdrawal via
      molecular and neurocircuitry neuroadaptations within the same reward
      system.
    explanation: >-
      Frames exogenous opioid action as hijacking the endogenous opioid reward
      system, the premise of this node.
- name: VTA Disinhibition and Mesolimbic Dopamine Release
  biological_scale: CELLULAR
  description: >-
    Mu-opioid receptor agonism on ventral tegmental GABAergic interneurons
    removes their inhibitory tone on dopaminergic projection neurons,
    increasing dopamine release in the nucleus accumbens. This disinhibition
    route is the positive reinforcement of early use, and it is mechanistically
    distinct from the direct dopamine-releasing action of stimulants.
  cell_types:
  - preferred_term: dopaminergic neuron
    term:
      id: CL:0000700
      label: dopaminergic neuron
  - preferred_term: GABAergic interneuron
    term:
      id: CL:0011005
      label: GABAergic interneuron
  - preferred_term: medium spiny neuron
    term:
      id: CL:1001474
      label: medium spiny neuron
  locations:
  - preferred_term: ventral tegmental area
    term:
      id: UBERON:0002691
      label: ventral tegmental area
  - preferred_term: nucleus accumbens
    term:
      id: UBERON:0001882
      label: nucleus accumbens
  biological_processes:
  - preferred_term: dopamine secretion
    term:
      id: GO:0014046
      label: dopamine secretion
    modifier: INCREASED
  downstream:
  - target: Reward-System Downregulation and Hyperkatifeia
    causal_link_type: DIRECT
    description: >-
      Repeated supraphysiological reward-system activation drives the opponent
      adaptation that lowers hedonic set point.
    evidence:
    - reference: PMID:31400808
      reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
          Such negative emotional states that drive negative reinforcement are
          hypothesized to derive from the within-system dysregulation of key
          neurochemical circuits that mediate incentive-salience and/or reward
          systems (dopamine, opioid peptides) in the ventral striatum
      explanation: >-
          Locates the reward-system dysregulation in the same ventral striatal
          dopamine and opioid-peptide circuits that the acute reward node engages.
  - target: Compulsive Opioid Seeking and Loss of Control
    causal_link_type: DIRECT
    description: >-
      Positive reinforcement sustains early, pre-dependence use.
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Such negative emotional states that drive negative reinforcement are
      hypothesized to derive from the within-system dysregulation of key
      neurochemical circuits that mediate incentive-salience and/or reward
      systems (dopamine, opioid peptides) in the ventral striatum
    explanation: >-
      Identifies ventral striatal dopamine and opioid peptide circuits as the
      reward system that later dysregulates. Support is partial because this
      review describes the dysregulation rather than the acute disinhibition
      step itself.
- name: Mu-Opioid Receptor Desensitization and Tolerance
  biological_scale: MOLECULAR
  description: >-
    Chronic agonist exposure produces receptor phosphorylation,
    desensitization, and internalization, so that progressively higher doses
    are needed for the same effect. Tolerance to analgesia and euphoria is what
    drives dose escalation; critically, tolerance to respiratory depression
    develops incompletely and is lost quickly during abstinence, which is the
    pharmacological basis of the post-detoxification and post-release overdose
    spike.
  biological_processes:
  - preferred_term: G protein-coupled opioid receptor signaling pathway
    term:
      id: GO:0038003
      label: G protein-coupled opioid receptor signaling pathway
    modifier: DECREASED
  downstream:
  - target: cAMP-PKA Superactivation in Locus Coeruleus Noradrenergic Neurons
    causal_link_type: DIRECT
    description: >-
      Cellular counter-adaptation to sustained inhibitory signalling upregulates
      the cAMP cascade.
  - target: Opioid-Induced Hyperalgesia
    causal_link_type: DIRECT
    description: >-
      Chronic exposure sensitizes pronociceptive pathways in parallel with, and
      mechanistically distinct from, receptor-level tolerance.
    evidence:
    - reference: PMID:21412369
      reference_title: A comprehensive review of opioid-induced hyperalgesia.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
          Opioid-induced hyperalgesia (OIH) is defined as a state of nociceptive
          sensitization caused by exposure to opioids.
      explanation: >-
          States that opioid exposure causes the sensitization, which is the
          direction this edge asserts.
  evidence:
  - reference: PMID:23321159
    reference_title: "Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      A major problem in treating painful conditions is that tolerance limits
      the long-term utility of opioid agonists.
    explanation: >-
      Establishes tolerance as the central clinical consequence of the
      receptor regulation this node models.
  - reference: PMID:23321159
    reference_title: "Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      While some functional MOR regulatory mechanisms contributing to tolerance
      are clearly understood, there are large gaps in understanding the
      molecular processes responsible for loss of MOR function after chronic
      exposure to opioids.
    explanation: >-
      The authors state explicitly that the molecular basis of chronic tolerance
      is incompletely understood, so this node's mechanism is only partially
      established.
- name: Opioid-Induced TLR4 and Microglial Neuroimmune Activation
  biological_scale: CELLULAR
  description: >-
    A non-classical, non-opioid-receptor arm of opioid action: opioids and some
    of their metabolites activate the TLR4/MD-2 innate immune complex on
    microglia and other myeloid cells, driving proinflammatory glial activation
    that opposes analgesia and contributes to analgesic tolerance, hyperalgesia,
    and withdrawal behaviour. The pharmacology is distinctive — activation is
    non-stereoselective, so it is not mediated by the mu-opioid receptor, and it
    is blocked by classical TLR4 antagonists. This node is deliberately kept
    parallel to, not upstream of, `Mu-Opioid Receptor Desensitization and
    Tolerance`: it is a second route to the same clinical endpoints rather than
    a cause of receptor-level desensitization.

    The arm is well evidenced in rodents and in vitro and only weakly
    corroborated in people; see the `oud_tlr4_glial_human_translation`
    discussion for that mismatch, and the bounding evidence below for its
    dose-dependence.
  cell_types:
  - preferred_term: Microglia
    term:
      id: CL:0000129
      label: microglial cell
  biological_processes:
  - preferred_term: TLR4/MD-2 innate immune signalling
    term:
      id: GO:0034142
      label: toll-like receptor 4 signaling pathway
    modifier: INCREASED
  - preferred_term: Proinflammatory microglial activation
    term:
      id: GO:0001774
      label: microglial cell activation
    modifier: INCREASED
  downstream:
  - target: Opioid-Induced Hyperalgesia
    causal_link_type: DIRECT
    description: >-
      Blocking TLR4 signalling in vivo attenuates the development of
      opioid-induced hyperalgesia, which is the direction this edge asserts.
    evidence:
    - reference: PMID:19679181
      reference_title: Evidence that opioids may have toll-like receptor 4 and MD-2 effects.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Pharmacological blockade of TLR4 signaling in vivo potentiated acute
        intrathecal morphine analgesia, attenuated development of analgesic
        tolerance, hyperalgesia, and opioid withdrawal behaviors.
      explanation: >-
        Loss of the hyperalgesia phenotype when the node is pharmacologically
        disabled is what supports the causal direction of this edge.
    - reference: PMID:31209174
      reference_title: "Role of Nociceptor Toll-like Receptor 4 (TLR4) in Opioid-Induced Hyperalgesia and Hyperalgesic Priming."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        whereas OIH and priming induced by LDM share receptor and second
        messenger mechanisms in common, action at TLR4 and signaling via PKC
      explanation: >-
        Bounds the edge: TLR4 dependence was demonstrated for hyperalgesia
        induced by low-dose morphine, and the same study found high-dose
        morphine effects to be neither TLR4- nor PKCepsilon-dependent. The edge
        is therefore dose-conditional rather than general.
  - target: Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Glial proinflammatory mediator release sensitizing withdrawal-expressing circuitry
    description: >-
      TLR4 blockade attenuates opioid withdrawal behaviour, placing this arm
      upstream of withdrawal expression alongside — not instead of — the
      cAMP-PKA noradrenergic mechanism.
    evidence:
    - reference: PMID:19679181
      reference_title: Evidence that opioids may have toll-like receptor 4 and MD-2 effects.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Pharmacological blockade of TLR4 signaling in vivo potentiated acute
        intrathecal morphine analgesia, attenuated development of analgesic
        tolerance, hyperalgesia, and opioid withdrawal behaviors.
      explanation: >-
        The withdrawal-behaviour result is reported as one item in a combined
        statement, without a separate effect size, so it supports the edge only
        partially.
  evidence:
  - reference: PMID:19679181
    reference_title: Evidence that opioids may have toll-like receptor 4 and MD-2 effects.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Morphine non-stereoselectively induced TLR4 signaling in vitro, blocked by
      a classical TLR4 antagonist and non-stereoselectively by naloxone.
    explanation: >-
      Non-stereoselective activation blocked by a TLR4 antagonist is what
      establishes this as a TLR4 event rather than a mu-opioid receptor one.
  - reference: PMID:19679181
    reference_title: Evidence that opioids may have toll-like receptor 4 and MD-2 effects.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      TLR4 opposition to opioid actions was supported by morphine treatment of
      TLR4 knockout mice, which revealed a significant threefold leftward shift
      in the analgesia dose response function, versus wildtype mice.
    explanation: >-
      Genetic removal of TLR4 shifts the morphine dose-response, giving a
      loss-of-function line of evidence independent of the pharmacology.
  - reference: PMID:25975386
    reference_title: "The effects of ibudilast, a glial activation inhibitor, on opioid withdrawal symptoms in opioid-dependent volunteers."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Although overall SOWS scores did not differ between groups, exploratory
      analyses pooling the two ibudilast groups demonstrated that they had lower
      ratings of withdrawal symptoms
    explanation: >-
      The only human interventional test of this node available here. The
      prespecified comparison was null and the supportive result is an
      exploratory pooled subscale analysis, so it corroborates the node weakly
      and is recorded as PARTIAL rather than SUPPORT.
- name: cAMP-PKA Superactivation in Locus Coeruleus Noradrenergic Neurons
  biological_scale: CELLULAR
  description: >-
    Opioid agonism acutely inhibits adenylyl cyclase in locus coeruleus
    noradrenergic neurons; chronic exposure drives a compensatory upregulation
    of the cAMP-PKA cascade. While drug is present the two oppose each other
    and firing is near normal. This is the classic opponent-process substrate
    of opioid physical dependence.
  cell_types:
  - preferred_term: noradrenergic neuron
    term:
      id: CL:0008025
      label: noradrenergic neuron
  locations:
  - preferred_term: locus coeruleus
    term:
      id: UBERON:0002148
      label: locus ceruleus
  biological_processes:
  - preferred_term: adenylate cyclase-inhibiting signaling
    term:
      id: GO:0007193
      label: adenylate cyclase-inhibiting G protein-coupled receptor signaling pathway
    modifier: ABNORMAL
  downstream:
  - target: Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome
    causal_link_type: DIRECT
    description: >-
      Removing the drug unmasks the compensatory cAMP-PKA upregulation as a
      surge of noradrenergic firing.
    evidence:
    - reference: PMID:32002194
      reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
          Lofexidine is an FDA-approved, alpha2-adrenergic receptor agonist for
          treatment of OWS.
      explanation: >-
          Suppression of the withdrawal syndrome by an alpha-2 adrenergic agonist
          is pharmacological support for a noradrenergic output stage. The
          cAMP-PKA step upstream of it is not demonstrated by this trial, so
          support is partial.
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Lofexidine is an FDA-approved, alpha2-adrenergic receptor agonist for
      treatment of OWS.
    explanation: >-
      That an alpha-2 adrenergic agonist is an approved treatment for opioid
      withdrawal is pharmacological support for a noradrenergic substrate;
      the trial does not itself demonstrate the cAMP-PKA mechanism, so support
      is partial.
- name: Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome
  biological_scale: ORGANISM
  description: >-
    On cessation or antagonist administration, unopposed noradrenergic
    hyperactivity produces the characteristic syndrome: rhinorrhea,
    lacrimation, mydriasis, piloerection, myalgia, nausea, vomiting, diarrhea,
    sweating, insomnia, and dysphoria. It is intensely aversive but, unlike
    alcohol or benzodiazepine withdrawal, rarely life-threatening in itself —
    its clinical danger is that it drives return to use, and that abstinence
    resets tolerance.
  downstream:
  - target: Reward-System Downregulation and Hyperkatifeia
    causal_link_type: DIRECT
    description: >-
      Repeated withdrawal episodes recruit and sensitize the brain stress
      systems that generate the negative emotional state.
  - target: Compulsive Opioid Seeking and Loss of Control
    causal_link_type: DIRECT
    description: >-
      Using to relieve or avoid withdrawal is itself a DSM-5 pharmacological
      criterion and a direct route back into use.
    evidence:
    - reference: PMID:32002194
      reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
          Fear of opioid withdrawal syndrome (OWS) often dissuades opioid
          discontinuation.
      explanation: >-
          Documents that the withdrawal syndrome itself deters cessation, which is
          the behavioural mechanism this edge asserts.
  - target: Prefrontal Executive Control Deficit
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Repeated cycles of intoxication and withdrawal driving frontal-circuit neuroplasticity
    description: >-
      Repeated withdrawal episodes are part of the cycle that degrades
      prefrontal control, which is what makes the preoccupation/anticipation
      stage self-sustaining.
    evidence:
    - reference: PMID:27475769
      reference_title: "Neurobiology of addiction: a neurocircuitry analysis."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        The craving and deficits in executive function in the so-called
        preoccupation/anticipation stage involve the dysregulation of key
        afferent projections from the prefrontal cortex and insula, including
        glutamate, to the basal ganglia and extended amygdala.
      explanation: >-
        Places the executive-function deficit in the same cycle stage as
        craving and links it to prefrontal projections onto the basal ganglia
        and extended amygdala. This is a cross-substance addiction review
        rather than opioid-specific evidence, so support is partial.
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Fear of opioid withdrawal syndrome (OWS) often dissuades opioid
      discontinuation.
    explanation: >-
      Documents the aversiveness of the syndrome and its role as a barrier to
      cessation, which is the behavioural significance this node carries.
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      However, they also engage the brain systems for stress and pain (somatic
      and emotional) while producing hyperalgesia and hyperkatifeia, which
      drive pronounced drug-seeking behavior via processes of negative
      reinforcement.
    explanation: >-
      Links the stress and pain systems engaged in withdrawal to
      negative-reinforcement drug seeking.
- name: Reward-System Downregulation and Hyperkatifeia
  biological_scale: ORGANISM
  description: >-
    The withdrawal/negative-affect stage combines a within-system reward
    deficit (reduced dopaminergic and opioid-peptide tone in ventral striatum)
    with between-system recruitment of extended-amygdala stress mediators —
    corticotropin-releasing factor, dynorphin, norepinephrine, hypocretin,
    glucocorticoids. The resulting dysphoria, anxiety, and anhedonia extend
    into protracted abstinence and drive relapse by negative reinforcement.
  locations:
  - preferred_term: central amygdala
    term:
      id: UBERON:0002883
      label: central amygdaloid nucleus
  - preferred_term: nucleus accumbens
    term:
      id: UBERON:0001882
      label: nucleus accumbens
  biological_processes:
  - preferred_term: dopamine secretion
    term:
      id: GO:0014046
      label: dopamine secretion
    modifier: DECREASED
  downstream:
  - target: Compulsive Opioid Seeking and Loss of Control
    causal_link_type: DIRECT
    description: >-
      Negative reinforcement — using to remove an aversive internal state —
      sustains compulsive use after the euphoric effect has waned.
    evidence:
    - reference: PMID:31400808
      reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Hyperkatifeia can extend into protracted abstinence and interact with
        learning processes in the form of conditioned withdrawal to facilitate
        relapse to compulsive-like drug seeking.
      explanation: >-
        States the relapse-facilitating role of the negative emotional state
        that this edge asserts.
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Such negative emotional states that drive negative reinforcement are
      hypothesized to derive from the within-system dysregulation of key
      neurochemical circuits that mediate incentive-salience and/or reward
      systems (dopamine, opioid peptides) in the ventral striatum and from the
      between-system recruitment of brain stress systems
      (corticotropin-releasing factor, dynorphin, norepinephrine, hypocretin,
      vasopressin, glucocorticoids, and neuroimmune factors) in the extended
      amygdala.
    explanation: >-
      Names both arms — within-system reward deficit and between-system stress
      recruitment — and their anatomy.
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      In animal models, repeated extended access to drugs or opioids results in
      negative emotion-like states, reflected by the elevation of reward
      thresholds, lower pain thresholds, anxiety-like behavior, and
      dysphoric-like responses.
    explanation: >-
      Provides the preclinical operationalization of the negative emotional
      state, including the elevated reward threshold that defines reward
      deficit.
- name: Opioid-Induced Hyperalgesia
  biological_scale: ORGANISM
  description: >-
    Paradoxical nociceptive sensitization caused by opioid exposure, in which a
    patient receiving opioids becomes more sensitive to painful stimuli. It is
    mechanistically distinct from tolerance — sensitization of pronociceptive
    pathways rather than loss of receptor responsiveness — and it matters
    clinically because it can be mistaken for undertreated pain and answered
    with a dose increase that worsens it.
  biological_processes:
  - preferred_term: sensory perception of pain
    term:
      id: GO:0019233
      label: sensory perception of pain
    modifier: INCREASED
  downstream:
  - target: Compulsive Opioid Seeking and Loss of Control
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Increased pain leading to dose escalation and continued exposure
    description: >-
      Increased pain sensitivity motivates further opioid use, closing a
      self-reinforcing loop.
    evidence:
    - reference: PMID:21412369
      reference_title: A comprehensive review of opioid-induced hyperalgesia.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
          OIH appears to be a distinct, definable, and characteristic phenomenon
          that could explain loss of opioid efficacy in some patients.
      explanation: >-
          Loss of opioid efficacy is the mechanism by which hyperalgesia drives
          dose escalation. The review frames this as a possible explanation, so
          support is partial.
  evidence:
  - reference: PMID:21412369
    reference_title: A comprehensive review of opioid-induced hyperalgesia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Opioid-induced hyperalgesia (OIH) is defined as a state of nociceptive
      sensitization caused by exposure to opioids.
    explanation: >-
      Defines the phenomenon this node models.
  - reference: PMID:21412369
    reference_title: A comprehensive review of opioid-induced hyperalgesia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      It is generally thought to result from neuroplastic changes in the
      peripheral and central nervous system (CNS) that lead to sensitization of
      pronociceptive pathways.
    explanation: >-
      States the sensitization mechanism that distinguishes hyperalgesia from
      receptor-level tolerance.
  - reference: PMID:21412369
    reference_title: A comprehensive review of opioid-induced hyperalgesia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Findings of the clinical prevalence of OIH are not available.
    explanation: >-
      The review states that clinical prevalence is unknown, so no frequency
      claim can be attached to this node.
- name: Brainstem Respiratory Rhythm Suppression
  biological_scale: ORGANISM
  description: >-
    Mu-opioid receptors on brainstem respiratory neurons — the preBotzinger
    complex, which generates inspiratory rhythm, and the Kolliker-Fuse and
    parabrachial nuclei, which modulate breathing — mediate depression of
    respiratory drive. Combined with reduced consciousness and airway
    obstruction this produces ventilatory insufficiency. This is the proximate
    cause of opioid-induced death and the target of naloxone.
  biological_processes:
  - preferred_term: G protein-coupled opioid receptor signaling pathway
    term:
      id: GO:0038003
      label: G protein-coupled opioid receptor signaling pathway
    modifier: INCREASED
  notes: >-
    No `cell_types` term is bound. The relevant populations are preBotzinger
    complex inspiratory rhythm-generating neurons and Kolliker-Fuse
    respiratory-modulating neurons; CL has no term at that granularity, and
    falling back to CL:0000540 neuron (used in an earlier draft) carries no
    information beyond what the node name already says. Needs term / NTR
    candidate.
  downstream:
  - target: Fatal Opioid Overdose
    causal_link_type: DIRECT
    description: >-
      Sustained ventilatory insufficiency produces hypoxaemia and death unless
      reversed.
    evidence:
    - reference: PMID:35471232
      reference_title: Mechanisms of opioid-induced respiratory depression.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
          Opioid-induced respiratory depression (OIRD), the primary cause of
          opioid-induced death, is the neural depression of respiratory drive
          which, together with a decreased level of consciousness and obstructive
          sleep apnea, cause ventilatory insufficiency.
      explanation: >-
          Names respiratory depression as the primary cause of opioid death, which
          is precisely this edge.
  evidence:
  - reference: PMID:35471232
    reference_title: Mechanisms of opioid-induced respiratory depression.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Opioid-induced respiratory depression (OIRD), the primary cause of
      opioid-induced death, is the neural depression of respiratory drive
      which, together with a decreased level of consciousness and obstructive
      sleep apnea, cause ventilatory insufficiency.
    explanation: >-
      Establishes respiratory depression as the mechanism of opioid death and
      names its three components.
  - reference: PMID:35471232
    reference_title: Mechanisms of opioid-induced respiratory depression.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Deletion of μ opioid receptors from neurons showed that the preBötC and
      KF/PBN contribute to OIRD with the KF as a respiratory modulator and the
      preBötC as inspiratory rhythm generator.
    explanation: >-
      Receptor-deletion experiments localize the effect to specific brainstem
      nuclei, giving the anatomical claim causal support.
- name: Fatal Opioid Overdose
  biological_scale: ORGANISM
  description: >-
    Death from ventilatory insufficiency. Risk is not a simple function of
    disorder severity: it is dominated by tolerance state (sharply elevated
    after detoxification or incarceration), by co-ingested sedatives, and by
    the potency of the illicit supply. This is why overdose risk can rise at
    precisely the moment a person stops using.
  evidence:
  - reference: PMID:35471232
    reference_title: Mechanisms of opioid-induced respiratory depression.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Variability of responses to opioids and individual differences in
      physiological and neurological states (e.g., anesthesia,
      sleep-disordered breathing, concurrent drug administration) add to the
      risk.
    explanation: >-
      Supports the claim that overdose risk is modified by physiological state
      and concurrent drugs rather than being a fixed property of the dose.
- name: Prefrontal Executive Control Deficit
  biological_scale: TISSUE
  description: >-
    Impaired top-down inhibitory control over drug seeking, with altered
    prefrontal-limbic connectivity. This is the preoccupation/anticipation arm
    of the addiction cycle, and it removes the counterweight to both
    cue-driven and withdrawal-driven use.
  locations:
  - preferred_term: prefrontal cortex
    term:
      id: UBERON:0000451
      label: prefrontal cortex
  downstream:
  - target: Compulsive Opioid Seeking and Loss of Control
    causal_link_type: DIRECT
    description: >-
      Loss of executive control permits drug seeking that would otherwise be
      inhibited.
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Compelling evidence indicates that plasticity in the brain pain emotional
      systems is triggered by acute excessive drug intake and becomes
      sensitized during the development of compulsive drug taking with repeated
      withdrawal.
    explanation: >-
      Supports drug-induced plasticity in emotion-related systems during the
      development of compulsive use. This review does not itself demonstrate
      the prefrontal executive deficit, so support is partial and a
      prefrontal-specific citation is a curation gap.
- name: Compulsive Opioid Seeking and Loss of Control
  biological_scale: ORGANISM
  description: >-
    The convergent clinical endpoint: compulsive opioid seeking and use despite
    harm, with impaired control over intake. Three motivational routes feed it
    — positive reinforcement early, negative reinforcement from withdrawal and
    hyperkatifeia later, and escalating pain from hyperalgesia — while
    prefrontal executive deficit removes the brake.
  downstream:
  - target: Brainstem Respiratory Rhythm Suppression
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Continued and escalating exposure to mu-opioid receptor agonists, often of uncertain potency
    description: >-
      Sustained use maintains repeated exposure to doses capable of suppressing
      respiratory drive, which is how the behavioural disorder becomes lethal.
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      However, they also engage the brain systems for stress and pain (somatic
      and emotional) while producing hyperalgesia and hyperkatifeia, which
      drive pronounced drug-seeking behavior via processes of negative
      reinforcement.
    explanation: >-
      Identifies the negative-reinforcement drive toward the compulsive
      drug-seeking endpoint this node represents.
phenotypes:
- category: Behavioral
  name: Addictive Substance Use
  description: >-
    Compulsive opioid use with impaired control — using more or longer than
    intended, unsuccessful efforts to cut down, and continued use despite
    knowledge of harm. The defining clinical feature.
  phenotype_term:
    preferred_term: Addictive substance use
    term:
      id: HP:0033511
      label: Addictive substance use
    clinical_course: PROGRESSIVE
  frequency: VERY_FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      However, they also engage the brain systems for stress and pain (somatic
      and emotional) while producing hyperalgesia and hyperkatifeia, which
      drive pronounced drug-seeking behavior via processes of negative
      reinforcement.
    explanation: >-
      Compulsive drug seeking is the defining feature of the disorder, so it is
      present in essentially all cases; that is the basis for VERY_FREQUENT.
- category: Behavioral
  name: Craving
  description: >-
    A strong desire or urge to use opioids — a DSM-5 criterion and a common
    trial endpoint.
  notes: >-
    No `phenotype_term` is bound: HPO has no general craving term (only the
    food-specific HP:0030083, HP:0030221 and HP:6000785), and a generic
    behavioural parent would misdescribe the claim. Needs term / NTR candidate,
    shared with the Alcohol_Use_Disorder entry.
  diagnostic: true
  evidence:
  - reference: PMID:29150198
    reference_title: "Comparative effectiveness of extended-release naltrexone versus buprenorphine-naloxone for opioid relapse prevention (X:BOT): a multicentre, open-label, randomised controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Self-reported opioid craving was initially less with XR-NTX than with
      BUP-NX (p=0·0012), then converged by week 24 (p=0·20).
    explanation: >-
      Craving is measured as a distinct prospective endpoint in OUD
      pharmacotherapy trials, separable from consumption measures.
- category: Neurologic
  name: Miosis
  description: >-
    Pinpoint pupils, a cardinal sign of acute opioid intoxication and part of
    the classic overdose triad with respiratory depression and depressed
    consciousness.
  phenotype_term:
    preferred_term: Miosis
    term:
      id: HP:0000616
      label: Miosis
    temporality: ACUTE
  evidence:
  - reference: PMID:35471232
    reference_title: Mechanisms of opioid-induced respiratory depression.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Opioid-induced respiratory depression (OIRD), the primary cause of
      opioid-induced death, is the neural depression of respiratory drive
      which, together with a decreased level of consciousness and obstructive
      sleep apnea, cause ventilatory insufficiency.
    explanation: >-
      Supports the intoxication syndrome including depressed consciousness;
      the review does not itself document miosis, so a miosis-specific citation
      is a curation gap.
- category: Respiratory
  name: Respiratory Depression
  description: >-
    Depression of respiratory drive from brainstem mu-opioid receptor
    activation; the proximate cause of opioid-related death and the target of
    naloxone reversal.
  phenotype_term:
    preferred_term: Hypoventilation
    term:
      id: HP:0002791
      label: Hypoventilation
    temporality: ACUTE
    severity: SEVERE
  diagnostic: true
  evidence:
  - reference: PMID:35471232
    reference_title: Mechanisms of opioid-induced respiratory depression.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Opioid-induced respiratory depression (OIRD), the primary cause of
      opioid-induced death, is the neural depression of respiratory drive
      which, together with a decreased level of consciousness and obstructive
      sleep apnea, cause ventilatory insufficiency.
    explanation: >-
      Directly documents respiratory depression and its lethal significance.
- category: Respiratory
  name: Sleep-Disordered Breathing
  description: >-
    Central and obstructive apnoea associated with chronic opioid use,
    contributing to ventilatory insufficiency during overdose.
  phenotype_term:
    preferred_term: Apnea
    term:
      id: HP:0002104
      label: Apnea
  evidence:
  - reference: PMID:35471232
    reference_title: Mechanisms of opioid-induced respiratory depression.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Variability of responses to opioids and individual differences in
      physiological and neurological states (e.g., anesthesia,
      sleep-disordered breathing, concurrent drug administration) add to the
      risk.
    explanation: >-
      Identifies sleep-disordered breathing as a state that compounds
      opioid-induced respiratory risk.
- category: Neurologic
  name: Mydriasis
  description: >-
    Pupillary dilation during opioid withdrawal — the direction opposite to the
    miosis of intoxication, and one of the objective signs scored on withdrawal
    rating scales.
  phenotype_term:
    preferred_term: Mydriasis
    term:
      id: HP:0011499
      label: Mydriasis
    temporality: ACUTE
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      efficacy was assessed using the Short Opioid Withdrawal Scale of Gossop
      (SOWS-G) daily
    explanation: >-
      Withdrawal severity is scored on standardized scales that include
      objective autonomic signs; the abstract does not enumerate individual
      signs, so support for mydriasis specifically is partial.
- category: Constitutional
  name: Rhinorrhea
  description: Rhinorrhea and lacrimation during opioid withdrawal.
  phenotype_term:
    preferred_term: Rhinorrhea
    term:
      id: HP:0031417
      label: Rhinorrhea
    temporality: ACUTE
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Fear of opioid withdrawal syndrome (OWS) often dissuades opioid
      discontinuation.
    explanation: >-
      Supports the existence and clinical salience of the withdrawal syndrome;
      the abstract does not enumerate its individual signs, so support for
      rhinorrhea specifically is partial.
- category: Gastrointestinal
  name: Diarrhea
  description: Diarrhoea and abdominal cramping during opioid withdrawal.
  phenotype_term:
    preferred_term: Diarrhea
    term:
      id: HP:0002014
      label: Diarrhea
    temporality: ACUTE
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Fear of opioid withdrawal syndrome (OWS) often dissuades opioid
      discontinuation.
    explanation: >-
      Supports the withdrawal syndrome as a whole; a sign-level citation is a
      curation gap.
- category: Gastrointestinal
  name: Nausea and Vomiting
  description: Nausea and vomiting during opioid withdrawal.
  phenotype_term:
    preferred_term: Nausea and vomiting
    term:
      id: HP:0002017
      label: Nausea and vomiting
    temporality: ACUTE
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Fear of opioid withdrawal syndrome (OWS) often dissuades opioid
      discontinuation.
    explanation: >-
      Supports the withdrawal syndrome as a whole; a sign-level citation is a
      curation gap.
- category: Musculoskeletal
  name: Myalgia
  description: Diffuse muscle and joint pain during opioid withdrawal.
  phenotype_term:
    preferred_term: Myalgia
    term:
      id: HP:0003326
      label: Myalgia
    temporality: ACUTE
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      However, they also engage the brain systems for stress and pain (somatic
      and emotional) while producing hyperalgesia and hyperkatifeia, which
      drive pronounced drug-seeking behavior via processes of negative
      reinforcement.
    explanation: >-
      Supports somatic pain as part of the withdrawal state; not a
      myalgia-specific measurement.
- category: Constitutional
  name: Hyperhidrosis
  description: Sweating and autonomic hyperactivity during opioid withdrawal.
  phenotype_term:
    preferred_term: Hyperhidrosis
    term:
      id: HP:0000975
      label: Hyperhidrosis
    temporality: ACUTE
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypotension and bradycardia were more common with lofexidine.
    explanation: >-
      That an alpha-2 agonist suppresses withdrawal while producing hypotension
      and bradycardia supports an autonomic, noradrenergically driven
      withdrawal state; sweating specifically is not measured in the abstract.
- category: Neurologic
  name: Insomnia
  description: Sleep disturbance during withdrawal and protracted abstinence.
  phenotype_term:
    preferred_term: Insomnia
    term:
      id: HP:0100785
      label: Insomnia
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Hyperkatifeia can extend into protracted abstinence and interact with
      learning processes in the form of conditioned withdrawal to facilitate
      relapse to compulsive-like drug seeking.
    explanation: >-
      Supports persistence of withdrawal-associated symptoms into protracted
      abstinence; insomnia specifically is not enumerated.
- category: Behavioral
  name: Anxiety
  description: >-
    Anxiety in acute withdrawal and persisting as part of the negative
    emotional state of protracted abstinence.
  phenotype_term:
    preferred_term: Anxiety
    term:
      id: HP:0000739
      label: Anxiety
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      In animal models, repeated extended access to drugs or opioids results in
      negative emotion-like states, reflected by the elevation of reward
      thresholds, lower pain thresholds, anxiety-like behavior, and
      dysphoric-like responses.
    explanation: >-
      Anxiety-like behaviour is a measured component of the opioid
      negative-affect state in animal models.
- category: Behavioral
  name: Depression
  description: >-
    Dysphoria and anhedonia during withdrawal and protracted abstinence, part
    of the hyperkatifeia construct.
  phenotype_term:
    preferred_term: Depression
    term:
      id: HP:0000716
      label: Depression
  evidence:
  - reference: PMID:31400808
    reference_title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      In animal models, repeated extended access to drugs or opioids results in
      negative emotion-like states, reflected by the elevation of reward
      thresholds, lower pain thresholds, anxiety-like behavior, and
      dysphoric-like responses.
    explanation: >-
      Dysphoric-like responses and elevated reward thresholds are the
      preclinical correlates of the depressive component.
- category: Neurologic
  name: Increased Pain Sensitivity
  description: >-
    Opioid-induced hyperalgesia: paradoxically increased sensitivity to painful
    stimuli caused by opioid exposure itself.
  phenotype_term:
    preferred_term: Pain
    term:
      id: HP:0012531
      label: Pain
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:21412369
    reference_title: A comprehensive review of opioid-induced hyperalgesia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The condition is characterized by a paradoxical response whereby a
      patient receiving opioids for the treatment of pain could actually become
      more sensitive to certain painful stimuli.
    explanation: >-
      Describes the increased pain sensitivity this phenotype records.
- category: Gastrointestinal
  name: Constipation
  description: >-
    Opioid-induced bowel dysfunction. Distinctive in that tolerance to it
    develops poorly, so it persists for as long as the opioid is taken.
  phenotype_term:
    preferred_term: Constipation
    term:
      id: HP:0002019
      label: Constipation
    temporality: CHRONIC
  evidence:
  - reference: PMID:28034973
    reference_title: "Opioid-Induced Constipation and Bowel Dysfunction: A Clinical Guideline."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Constipation is a major untoward effect of opioids. Increasing
      prescription of opioids has correlated to increased incidence of
      opioid-induced constipation.
    explanation: >-
      Evidence-based clinical guideline establishing constipation as a major
      opioid adverse effect and linking its incidence to opioid prescribing.
  - reference: PMID:28034973
    reference_title: "Opioid-Induced Constipation and Bowel Dysfunction: A Clinical Guideline."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      However, the inhibitory effects of opioids are not confined to the colon,
      but also affect higher segments of the gastrointestinal tract
    explanation: >-
      Supports the broader bowel-dysfunction framing used in this phenotype's
      description rather than constipation alone.
- category: Endocrine
  name: Hypogonadism
  description: >-
    Opioid-induced androgen deficiency from suppression of the
    hypothalamic-pituitary-gonadal axis with chronic use.
  phenotype_term:
    preferred_term: Hypogonadism
    term:
      id: HP:0000135
      label: Hypogonadism
    temporality: CHRONIC
  evidence:
  - reference: PMID:22786453
    reference_title: Opioid-induced androgen deficiency (OPIAD).
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Accompanying this upsurge in the use of long-term opioid therapy has been
      an increase in the occurrence of opioid associated endocrinopathy, most
      commonly manifested as an androgen deficiency and therefore referred to
      as opioid associated androgen deficiency (OPIAD).
    explanation: >-
      Establishes androgen deficiency as the characteristic endocrinopathy of
      long-term opioid therapy.
  - reference: PMID:22786453
    reference_title: Opioid-induced androgen deficiency (OPIAD).
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      This syndrome is characterized by the presence of inappropriately low
      levels of gonadotropins (follicle stimulating hormone and luteinizing
      hormone) leading to inadequate production of sex hormones, particularly
      testosterone.
    explanation: >-
      Specifies the hypogonadotropic mechanism, which is what makes this a
      central rather than a primary hypogonadism.
- category: Neurologic
  name: Sedation
  description: >-
    Somnolence, lethargy, and reduced alertness during opioid exposure. It is
    the mildest point on the same dose-related CNS-depression continuum that
    ends in respiratory depression and coma, and it is one of the commonest
    reasons opioid therapy is stopped.
  phenotype_term:
    preferred_term: Sedation
    term:
      id: HP:0002329
      label: Drowsiness
  evidence:
  - reference: PMID:26461075
    reference_title: Cognitive Effects and Sedation.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Cognitive effects and sedation (CES) are prevalent in chronic nonmalignant
      pain populations receiving long-term opioid therapy and are among the most
      common reasons patients discontinue opioid use.
    explanation: >-
      Establishes sedation as a prevalent and clinically consequential effect of
      sustained opioid exposure.
  - reference: PMID:26461075
    reference_title: Cognitive Effects and Sedation.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The most prevalent CES include: memory deficits (73-81%), sleep
      disturbance (35-57%), and fatigue (10%).
    explanation: >-
      Gives the only prevalence figures available here, but they are for a
      long-term-opioid-therapy chronic-pain population rather than for a
      diagnosed opioid use disorder cohort, so no `frequency` band is asserted
      from them.
  notes: >-
    The quantitative evidence for this phenotype comes from chronic-pain
    populations on long-term opioid therapy, not from OUD cohorts. That is a
    different population with a different dose and exposure pattern, so the
    phenotype is curated without a `frequency` value rather than importing the
    chronic-pain prevalence as if it were an OUD frequency.
- category: Neurologic
  name: Cognitive Impairment
  description: >-
    Slowing and impairment across attention, executive function, verbal
    fluency, and memory. Distinct from sedation, though the two co-occur and
    are commonly reported together; the deficits are measurable on formal
    neuropsychological testing in patients stabilized on opioid agonist
    therapy, i.e. they are not merely an acute intoxication effect.
  phenotype_term:
    preferred_term: Cognitive slowing and impairment
    term:
      id: HP:0100543
      label: Cognitive impairment
  evidence:
  - reference: PMID:26885347
    reference_title: Assessment of Cognitive Functions in Methadone Maintenance Patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      MMP performed significantly poorly than controls in cognitive domains of
      verbal fluency, executive function, and verbal memory.
    explanation: >-
      Primary case-control neuropsychological data in methadone-maintained
      patients, which is the OUD-relevant population rather than a chronic-pain
      one.
  - reference: PMID:26885347
    reference_title: Assessment of Cognitive Functions in Methadone Maintenance Patients.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      MMP did not exhibit impairment in working memory, and TMT Part A compared
      to controls.
    explanation: >-
      A negative result within the same study, which bounds the claim: the
      impairment is domain-selective rather than global.
  - reference: PMID:26461075
    reference_title: Cognitive Effects and Sedation.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Conclusions about the neuropsychological domains affected by opioids are
      limited due to the heterogeneity of studies and methodological issues.
    explanation: >-
      The review states plainly that the literature does not support firm
      domain-level conclusions, which is why no severity or frequency is
      asserted here.
  notes: >-
    Attribution is genuinely unresolved. The cited cohort is
    methadone-maintained, so opioid exposure, prior illicit use, and the
    comorbidities that accompany OUD are all confounded; the study design
    cannot separate them. The phenotype is curated as an observed association
    with opioid-treated OUD, not as a demonstrated pharmacological effect of
    opioids alone.
- category: Immunologic
  name: Opioid-Associated Immunosuppression
  description: >-
    Suppression of innate immune effector function during opioid exposure, with
    an associated increase in serious infection risk. Mu-opioid receptors are
    expressed on macrophages and other leukocytes, and agonism at therapeutic
    concentrations impairs phagocytosis; at the population level, current
    opioid use is associated with increased odds of invasive pneumococcal
    disease in a dose- and potency-graded fashion.
  phenotype_term:
    preferred_term: Opioid-associated immunosuppression
    term:
      id: HP:0002721
      label: Immunodeficiency
  evidence:
  - reference: PMID:29435555
    reference_title: "Opioid Analgesic Use and Risk for Invasive Pneumococcal Diseases: A Nested Case-Control Study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Persons in the case group had greater odds than control participants of
      being current opioid users
    explanation: >-
      The primary human infection-risk result. The paper reports an adjusted
      odds ratio of 1.62 (95% CI 1.36 to 1.92), strongest for long-acting,
      high-potency, and high-dose opioids; the figures are given here rather
      than quoted because the reference validator strips the bracketed `[aOR]`
      abbreviation from a snippet, which would break an otherwise verbatim
      quote.
  - reference: PMID:29435555
    reference_title: "Opioid Analgesic Use and Risk for Invasive Pneumococcal Diseases: A Nested Case-Control Study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Opioid use is associated with an increased risk for IPD and represents a
      novel risk factor for these diseases.
    explanation: >-
      The authors' own conclusion, stating the direction of the association this
      phenotype records.
  - reference: PMID:37259426
    reference_title: "Altered Membrane Expression and Function of CD11b Play a Role in the Immunosuppressive Effects of Morphine on Macrophages at the Nanomolar Level."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      morphine at 0.1-10 nM levels inhibited CD11b expression and function on
      macrophages via a μ-opioid receptor (MOR)-dependent mechanism, thereby
      reducing macrophage phagocytosis of tumor cells
    explanation: >-
      Supplies the receptor-level mechanism — a mu-opioid-receptor-dependent
      loss of macrophage phagocytic function at therapeutic concentrations —
      that the epidemiological association lacks.
  - reference: PMID:29435555
    reference_title: "Opioid Analgesic Use and Risk for Invasive Pneumococcal Diseases: A Nested Case-Control Study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Actual opioid use and other nonprescription use (such as illicit opioid
      use) were not measured.
    explanation: >-
      The authors' own limitation. The exposure is prescription fills, not
      illicit or injected opioid use, so the study does not directly measure the
      OUD population this entry models.
  notes: >-
    Scope. The human evidence here is for prescription opioid analgesic use,
    not for opioid use disorder, and the entry deliberately does not model
    injection-related infectious complications (see the entry `notes`). This
    phenotype is the pharmacological immunosuppression arm only; it is not a
    claim about the infectious burden of injection drug use, which has a
    different and much larger cause.
- category: Constitutional
  name: Neonatal Opioid Withdrawal Syndrome
  description: >-
    A postnatal withdrawal syndrome in infants exposed to opioids in utero,
    including exposure via maternal methadone or buprenorphine treatment. Note
    this is a phenotype of the offspring, not of the proband; it is recorded
    here because it is a direct consequence of maternal OUD, but it would be
    better modelled as its own entity.
  notes: >-
    No `phenotype_term` is bound. NOWS is a syndrome spanning irritability,
    tremor, feeding difficulty, autonomic instability and seizures; binding it
    to any single constituent sign (HP:0000737 Irritability, used in an earlier
    draft) understates the condition for any downstream consumer. Needs term /
    NTR candidate.
  evidence:
  - reference: PMID:30819342
    reference_title: Neonatal Abstinence Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Neonatal abstinence syndrome (NAS) is a postnatal withdrawal syndrome
      that manifests shortly after birth in infants born to women with opioid
      use (including heroin, use or misuse of prescription painkillers, or
      maternal treatment medications such as methadone or buprenorphine) during
      pregnancy.
    explanation: >-
      Defines the syndrome and its exposure routes, including maintenance
      medications.
genetic:
- name: OPRM1
  gene_term:
    preferred_term: OPRM1
    term:
      id: hgnc:8156
      label: OPRM1
  relationship_type: SUSCEPTIBILITY
  association: >-
    The mu-opioid receptor gene — the direct pharmacological target of opioids
    — and the only locus with a long-standing, independently replicated
    genome-wide significant association with opioid addiction. The functional
    coding variant rs1799971 (A118G) alters receptor trafficking and signalling
    efficacy rather than producing a clean gain or loss of function, so no
    `functional_impact_category` is asserted.
  evidence:
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Opioid addiction (OA) is moderately heritable, yet only rs1799971, the
      A118G variant in OPRM1, has been identified as a genome-wide significant
      association with OA and independently replicated.
    explanation: >-
      Establishes rs1799971 as the single replicated genome-wide significant
      association.
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We observed the strongest evidence to date for OPRM1: lead SNP rs9478500
      (p = 2.56 × 10-9).
    explanation: >-
      Strengthens the OPRM1 signal in the largest multi-trait analysis to date.
- name: FURIN
  gene_term:
    preferred_term: FURIN
    term:
      id: hgnc:8568
      label: FURIN
  relationship_type: SUSCEPTIBILITY
  association: >-
    Identified by gene-based analysis in the multi-trait GWAS of opioid
    addiction, alongside PPP6C. Both are recent and comparatively less
    replicated than the OPRM1 signal.
  evidence:
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Gene-based analyses identified novel genome-wide significant associations
      with PPP6C and FURIN.
    explanation: >-
      Direct statement of the gene-based association.
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Variants within these loci appear to be pleiotropic for addiction and
      related traits.
    explanation: >-
      The authors describe the signal as pleiotropic across addiction traits
      rather than opioid-specific, which bounds how strongly this locus can be
      called an OUD gene.
- name: KDM4A
  gene_term:
    preferred_term: KDM4A
    term:
      id: hgnc:22978
      label: KDM4A
  relationship_type: SUSCEPTIBILITY
  association: >-
    Intronic variant rs3791033 reached genome-wide significance in a GWAS of
    *problematic opioid use* (POU) — using prescription opioids "not as
    prescribed" — in 132,113 research participants. Read the phenotype
    carefully: POU is a self-report proxy, not a diagnosed OUD case
    definition, and this locus has not been reported at genome-wide
    significance in a GWAS of diagnosed OUD. It is curated as a susceptibility
    signal for the proxy phenotype, with the genetic correlation to OUD
    (rg = 0.64-0.80) as the bridge, rather than as an established OUD locus.
  evidence:
  - reference: PMID:34728798
    reference_title: "Genome-wide association study of problematic opioid prescription use in 132,113 23andMe research participants of European ancestry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We identified two genome-wide significant loci (rs3791033, an intronic
      variant of KDM4A; rs640561, an intergenic variant near LRRIQ3).
    explanation: >-
      Direct statement of the genome-wide significant association, naming both
      the variant and the gene.
  - reference: PMID:34728798
    reference_title: "Genome-wide association study of problematic opioid prescription use in 132,113 23andMe research participants of European ancestry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      As a surrogate (or proxy) for OUD, we explored the genetic basis of using
      prescription opioids 'not as prescribed'.
    explanation: >-
      The authors state the phenotype is a surrogate for OUD, which is what
      limits how strongly this locus can be called an OUD susceptibility gene.
- name: LRRIQ3
  gene_term:
    preferred_term: LRRIQ3
    term:
      id: hgnc:28318
      label: LRRIQ3
  relationship_type: SUSCEPTIBILITY
  association: >-
    Intergenic variant rs640561 near LRRIQ3, the second genome-wide significant
    locus from the same problematic-opioid-use GWAS. The variant is intergenic,
    so the gene assignment is by proximity and not by demonstrated functional
    effect on LRRIQ3 — the same POU-proxy caveat recorded on KDM4A applies here
    and is compounded by that.
  evidence:
  - reference: PMID:34728798
    reference_title: "Genome-wide association study of problematic opioid prescription use in 132,113 23andMe research participants of European ancestry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We identified two genome-wide significant loci (rs3791033, an intronic
      variant of KDM4A; rs640561, an intergenic variant near LRRIQ3).
    explanation: >-
      Direct statement of the association, and the source of the "near" wording
      that makes the gene assignment positional.
  - reference: PMID:34728798
    reference_title: "Genome-wide association study of problematic opioid prescription use in 132,113 23andMe research participants of European ancestry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      POU showed positive genetic correlations with the two largest available
      GWAS of OUD and opioid dependence
    explanation: >-
      The genetic correlation is the only thing connecting this locus to
      diagnosed OUD, so it is recorded as partial support for an OUD
      susceptibility claim.
environmental:
- name: Exposure to mu-opioid receptor agonists
  description: >-
    Opioid exposure — whether via prescribed analgesia, diverted prescription
    opioids, heroin, or illicit synthetic opioids such as fentanyl — is the
    necessary cause. It is not sufficient: only a minority of exposed people
    develop the disorder. Supply potency is a distinct risk dimension from
    exposure per se, because it determines whether a given episode of use is
    survivable.
  effect: Necessary causal exposure
  exposure_term:
    preferred_term: exposure to mu-opioid receptor agonist
    term:
      id: ECTO:9001896
      label: exposure to mu-opioid receptor agonist
  chemicals:
  - heroin
  - fentanyl
  - morphine
  influences_mechanisms:
  - target: Exogenous Mu-Opioid Receptor Agonism
    environmental_effect: TRIGGERS
    causal_link_type: DIRECT
    description: >-
      Administration by any route is what places an agonist at the receptor.
    evidence:
    - reference: PMID:23321159
      reference_title: "Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Morphine and related µ-opioid receptor (MOR) agonists remain among the
        most effective drugs known for acute relief of severe pain.
      explanation: >-
        Establishes that the exposure of interest is agonism at the mu-opioid
        receptor, the mechanism node this triggers.
  - target: Mu-Opioid Receptor Desensitization and Tolerance
    environmental_effect: TRIGGERS
    causal_link_type: DIRECT
    description: >-
      Chronic rather than single exposure is what produces durable receptor
      regulation and tolerance.
    evidence:
    - reference: PMID:23321159
      reference_title: "Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        there are large gaps in understanding the molecular processes
        responsible for loss of MOR function after chronic exposure to opioids
      explanation: >-
        Identifies chronic exposure as the variable that drives loss of
        receptor function, while flagging that the molecular detail is
        incomplete.
  evidence:
  - reference: PMID:36207451
    reference_title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Approximately 4% of the US population aged 12 and older (10.1 million
      people) misused opioids in 2019
    explanation: >-
      Quantifies how widespread the exposure modelled by this entry is, which
      is what makes a necessary-but-not-sufficient exposure epidemiologically
      significant.
treatments:
- name: Methadone
  description: >-
    Full mu-opioid receptor agonist with a long half-life, delivered through
    regulated opioid treatment programmes. The best-retained of the opioid
    agonist treatments. It works by occupying the same receptor the disorder is
    built on, which is why it suppresses withdrawal and craving without
    producing the reinforcing peak of short-acting opioids.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: methadone
      term:
        id: CHEBI:6807
        label: methadone
  target_mechanisms:
  - target: Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome
    treatment_effect: INHIBITS
    description: >-
      Sustained receptor occupancy prevents the unopposed noradrenergic surge
      that produces withdrawal.
  evidence:
  - reference: PMID:12804430
    reference_title: Methadone maintenance therapy versus no opioid replacement therapy for opioid dependence.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Methadone is an effective maintenance therapy intervention for the
      treatment of heroin dependence as it retains patients in treatment and
      decreases heroin use better than treatments that do not utilise opioid
      replacement therapy.
    explanation: >-
      Cochrane review conclusion establishing methadone maintenance against
      non-replacement approaches.
  - reference: PMID:12804430
    reference_title: Methadone maintenance therapy versus no opioid replacement therapy for opioid dependence.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      methadone appeared statistically significantly more effective than
      non-pharmacological approaches in retaining patient in treatment (3 RCTs,
      RR=3.05; 95%CI: 1.75-5.35) and in the suppression of heroin use (3 RCTs,
      RR=0.32; 95%CI: 0.23-0.44), but not statistically in criminal activity
    explanation: >-
      Quantifies the retention and heroin-use effects, and is explicit that the
      criminal-activity effect was not significant.
- name: Buprenorphine
  description: >-
    Partial mu-opioid receptor agonist, usually co-formulated with naloxone to
    deter injection. Its partial agonism produces a ceiling on respiratory
    depression, which gives it a wider safety margin than methadone and allows
    office-based prescribing — the trade-off is somewhat lower retention than
    methadone under flexible dosing.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: buprenorphine
      term:
        id: CHEBI:3216
        label: buprenorphine
  target_mechanisms:
  - target: Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome
    treatment_effect: INHIBITS
    description: >-
      Partial agonism at the same receptor suppresses withdrawal while
      capping respiratory depression.
  evidence:
  - reference: PMID:24500948
    reference_title: Buprenorphine maintenance versus placebo or methadone maintenance for opioid dependence.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      There is high quality of evidence that buprenorphine was superior to
      placebo medication in retention of participants in treatment at all doses
      examined.
    explanation: >-
      Cochrane review of 31 trials establishes superiority to placebo for
      retention.
  - reference: PMID:24500948
    reference_title: Buprenorphine maintenance versus placebo or methadone maintenance for opioid dependence.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Methadone is superior to buprenorphine in retaining people in treatment,
      and methadone equally suppresses illicit opioid use.
    explanation: >-
      Bounds the claim: buprenorphine is effective but does not match methadone
      on retention, which is the main comparative caveat.
- name: Extended-Release Injectable Naltrexone
  description: >-
    Mu-opioid receptor antagonist given as a monthly injection. Conceptually
    the opposite of agonist treatment: it blocks the receptor rather than
    occupying it with a safer agonist. Its practical limitation is the
    induction hurdle — it requires full detoxification first, and failure to
    initiate is what drives its worse intention-to-treat outcomes.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: naltrexone
      term:
        id: CHEBI:7465
        label: naltrexone
  target_mechanisms:
  - target: VTA Disinhibition and Mesolimbic Dopamine Release
    treatment_effect: INHIBITS
    description: >-
      Receptor blockade prevents the disinhibition step that produces opioid
      reward.
  evidence:
  - reference: PMID:29150198
    reference_title: "Comparative effectiveness of extended-release naltrexone versus buprenorphine-naloxone for opioid relapse prevention (X:BOT): a multicentre, open-label, randomised controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In this population it is more difficult to initiate patients to XR-NTX
      than BUP-NX, and this negatively affected overall relapse. However, once
      initiated, both medications were equally safe and effective.
    explanation: >-
      The trial's own conclusion, separating the induction problem from
      once-initiated efficacy.
  - reference: PMID:29150198
    reference_title: "Comparative effectiveness of extended-release naltrexone versus buprenorphine-naloxone for opioid relapse prevention (X:BOT): a multicentre, open-label, randomised controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Among participants successfully inducted (per-protocol population,
      n=474), 24 week relapse events were similar across study groups (p=0·44).
    explanation: >-
      Quantifies the per-protocol equivalence that the intention-to-treat
      result obscures.
- name: Naloxone Overdose Reversal
  description: >-
    Competitive mu-opioid receptor antagonist that reverses opioid-induced
    respiratory depression within minutes. This treats the acute lethal event,
    not the disorder. Its half-life can be shorter than that of the opioid it
    reverses, so re-sedation after apparent recovery is a real hazard.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: naloxone
      term:
        id: CHEBI:7459
        label: naloxone
  target_mechanisms:
  - target: Brainstem Respiratory Rhythm Suppression
    treatment_effect: INHIBITS
    description: >-
      Competitive displacement of the agonist at brainstem mu-opioid receptors
      restores respiratory drive.
    evidence:
    - reference: PMID:35471232
      reference_title: Mechanisms of opioid-induced respiratory depression.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Reversal of OIRD has relied heavily on naloxone which also reverses
        analgesia but mismatches between the half-lives of naloxone and opioids
        can make it difficult to clinically safely avoid OIRD.
      explanation: >-
        States both the reversal mechanism and the half-life mismatch caveat.
  evidence:
  - reference: PMID:24874759
    reference_title: A systematic review of community opioid overdose prevention and naloxone distribution programs.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The current evidence from nonrandomized studies suggests that bystanders
      (mostly opioid users) can and will use naloxone to reverse opioid
      overdoses when properly trained, and that this training can be done
      successfully through OOPPs.
    explanation: >-
      Supports community naloxone distribution as an effective delivery route,
      while the authors are explicit that the evidence is nonrandomized.
- name: Lofexidine
  description: >-
    Alpha-2 adrenergic agonist approved for mitigating opioid withdrawal
    symptoms. Mechanistically it targets exactly the noradrenergic
    hyperactivity node rather than the opioid receptor, which is why it eases
    withdrawal without treating craving or preventing relapse.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: lofexidine
      term:
        id: CHEBI:51368
        label: lofexidine
  target_mechanisms:
  - target: Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome
    treatment_effect: INHIBITS
    description: >-
      Alpha-2 autoreceptor agonism suppresses noradrenergic output, the
      substrate of somatic withdrawal.
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This simplified analysis confirmed previous per-protocol results, that
      lofexidine better reduces OWS severity and increases retention compared
      with placebo in opioid-dependent adults.
    explanation: >-
      Two randomized placebo-controlled trials support reduction in withdrawal
      severity and improved retention.
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypotension and bradycardia were more common with lofexidine.
    explanation: >-
      Records the predictable cardiovascular consequence of alpha-2 agonism,
      which bounds its use.
- name: Psychosocial Intervention Adjunctive to Opioid Agonist Therapy
  description: >-
    Cognitive behavioural therapy, contingency management, and related
    psychosocial treatments, delivered as an adjunct to opioid agonist therapy
    rather than as a substitute for it.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Cognitive Behavior Therapy
    term:
      id: NCIT:C64345
      label: Cognitive Behavior Therapy
  evidence:
  - reference: PMID:33370393
    reference_title: "Evaluating comparative effectiveness of psychosocial interventions adjunctive to opioid agonist therapy for opioid use disorder: A systematic review with network meta-analyses."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Guidelines recommend that individuals with opioid use disorder (OUD)
      receive pharmacological and psychosocial interventions; however, the most
      appropriate psychosocial intervention is not known.
    explanation: >-
      Establishes the adjunctive role while stating explicitly that the
      comparative question is unresolved.
- name: Contingency Management
  description: >-
    A behavioural intervention delivering material incentives contingent on
    objectively verified behaviour change — typically a negative urine drug
    screen or a kept appointment. It is curated separately from the general
    psychosocial-intervention entry above because it is the psychosocial
    treatment with the strongest and most specific evidence base, and because
    it addresses a problem opioid agonist therapy does not: comorbid stimulant
    use, for which no effective pharmacotherapy exists.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: contingency management
    term:
      id: NCIT:C94375
      label: Contingency Management
  evidence:
  - reference: PMID:34347030
    reference_title: "Contingency Management for Patients Receiving Medication for Opioid Use Disorder: A Systematic Review and Meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Collapsing across abstinence and adherence categories, contingency
      management was associated with medium effect sizes for abstinence (Cohen
      d = 0.58; 95% CI, 0.47-0.69) and treatment adherence (Cohen d = 0.62;
      95% CI, 0.40-0.84) compared with controls.
    explanation: >-
      Quantifies the effect on both abstinence and treatment adherence in
      patients already receiving medication for OUD, which is the adjunctive
      role this entry curates.
  - reference: PMID:34347030
    reference_title: "Contingency Management for Patients Receiving Medication for Opioid Use Disorder: A Systematic Review and Meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Medication treatment for opioid use disorder (MOUD) is efficacious, but
      comorbid stimulant use and other behavioral health problems often
      undermine efficacy.
    explanation: >-
      States the gap this treatment addresses, and why it is not redundant with
      the agonist therapies curated above.
  notes: >-
    The meta-analysis reports end-of-treatment outcomes. It does not establish
    that the effect persists after the incentives stop, so no durability claim
    is curated here.
- name: Nalmefene Nasal Spray
  description: >-
    Long-acting opioid antagonist nasal spray (Opvee), FDA-approved in 2023
    for emergency treatment of known or suspected opioid overdose. Its longer
    duration of action than naloxone's is the intended advantage against
    re-sedation as an opioid's effects outlast a single reversal dose, which
    is the same half-life-mismatch hazard already curated on the naloxone
    treatment above.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: nalmefene
      term:
        id: CHEBI:7457
        label: Nalmefene
  target_mechanisms:
  - target: Brainstem Respiratory Rhythm Suppression
    treatment_effect: INHIBITS
    description: >-
      Competitive opioid receptor antagonism at brainstem mu-opioid receptors
      restores respiratory drive, the same mechanism as naloxone but with a
      longer duration of action.
  evidence:
  - reference: PMID:39648641
    reference_title: "FDA Approval Summary: Nalmefene Nasal Spray for the Emergency Treatment of Known or Suspected Opioid Overdose."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      On May 22, 2023, the United States Food and Drug Administration approved
      the first nalmefene hydrochloride nasal spray for the emergency
      treatment of known or suspected opioid overdose in adults and pediatric
      patients 12 years of age and older.
    explanation: >-
      FDA approval summary establishing the drug, route, indication, and
      approval date.
- name: Extended-Release Buprenorphine Injectable
  description: >-
    Subcutaneous depot buprenorphine formulations (monthly Sublocade, or
    weekly/monthly Buvidal/Brixadi) that reduce dosing frequency relative to
    daily sublingual buprenorphine, the same adherence rationale already
    modelled for extended-release injectable naltrexone above. Unlike
    naltrexone, induction does not require prior full detoxification, since
    buprenorphine is itself an opioid agonist rather than an antagonist.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: buprenorphine
      term:
        id: CHEBI:3216
        label: buprenorphine
  target_mechanisms:
  - target: Noradrenergic Hyperactivity and Opioid Withdrawal Syndrome
    treatment_effect: INHIBITS
    description: >-
      Same partial-agonist receptor mechanism as sublingual buprenorphine; the
      depot formulation changes pharmacokinetics and dosing frequency, not
      receptor pharmacology.
  evidence:
  - reference: PMID:40459195
    reference_title: "Transition to Extended-release Buprenorphine Injectable Within Seven Days for Opioid Use Disorder Treatment: A Scoping Narrative."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Sublocade (extended-release buprenorphine; Bup-XR-S) and Buvidal/Brixadi
      (long-acting buprenorphine; Bup-LA-B) formulations allow for less
      frequent dosing.
    explanation: >-
      Names both long-acting formulations and states the dosing-frequency
      rationale for the treatment.
  - reference: PMID:40459195
    reference_title: "Transition to Extended-release Buprenorphine Injectable Within Seven Days for Opioid Use Disorder Treatment: A Scoping Narrative."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Short-term retention (4 wk) exceeded 60%.
    explanation: >-
      Quantifies short-term retention pooled across the transition studies
      reviewed.
diagnosis:
- name: DSM-5 and ICD-11 clinical diagnostic criteria
  description: >-
    Diagnosis is clinical and behavioural: DSM-5 requires at least 2 of 11
    criteria within 12 months, graded mild/moderate/severe by criterion count;
    ICD-11 codes opioid dependence at 6C43.2 using the dependence-syndrome
    framework. Physiological tolerance and withdrawal arising from appropriate
    medical opioid use do not by themselves establish the disorder — a
    distinction that matters for patients on long-term analgesia. There is no
    validated blood biomarker; urine immunoassay panels detect exposure, not
    the disorder, and frequently miss fentanyl and other synthetic opioids
    without an extended panel.
  evidence:
  - reference: PMID:29150198
    reference_title: "Comparative effectiveness of extended-release naltrexone versus buprenorphine-naloxone for opioid relapse prevention (X:BOT): a multicentre, open-label, randomised controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Extended-release naltrexone (XR-NTX), an opioid antagonist, and
      sublingual buprenorphine-naloxone (BUP-NX), a partial opioid agonist, are
      pharmacologically and conceptually distinct interventions to prevent
      opioid relapse.
    explanation: >-
      Confirms DSM-criteria-based ascertainment is the standard entry point for
      OUD trials; the abstract does not enumerate the criteria, so support for
      the criteria themselves is partial.
- name: Clinical Opiate Withdrawal Scale and related severity instruments
  description: >-
    Withdrawal severity is graded with structured instruments — the Clinical
    Opiate Withdrawal Scale (COWS, observer-rated) and the Short Opioid
    Withdrawal Scale of Gossop (SOWS-G, self-reported) — which are the primary
    endpoints in withdrawal-treatment trials.
  evidence:
  - reference: PMID:32002194
    reference_title: "Efficacy of lofexidine for mitigating opioid withdrawal symptoms: results from two randomized, placebo-controlled trials."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Opioid-dependent adults received placebo or lofexidine; efficacy was
      assessed using the Short Opioid Withdrawal Scale of Gossop (SOWS-G)
      daily.
    explanation: >-
      Documents use of a standardized withdrawal severity instrument as the
      trial efficacy measure.
- name: Opioid Risk Tool (ORT) and SOAPP pre-prescription risk screening
  description: >-
    Self-administered questionnaires used before initiating long-term opioid
    therapy for pain, to predict which patients are likely to develop
    aberrant opioid-related behaviors. This is a distinct pre-prescription
    screening tier from the DSM-5/ICD-11 diagnostic criteria and the
    COWS/SOWS-G withdrawal-severity instruments above, which apply once a
    patient is already on opioids or already dependent.
  evidence:
  - reference: PMID:16336480
    reference_title: "Predicting aberrant behaviors in opioid-treated patients: preliminary validation of the Opioid Risk Tool."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The ORT displayed excellent discrimination for both the male (c = 0.82)
      and the female (c = 0.85) prognostic models.
    explanation: >-
      Reports the discrimination statistics from the Opioid Risk Tool's
      preliminary validation study.
  - reference: PMID:15494186
    reference_title: Validation of a screener and opioid assessment measure for patients with chronic pain.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Receiver operating characteristics curve analysis yielded an area under
      the curve of 0.881 (P<0.001), suggesting adequate sensitivity and
      specificity for a screening device.
    explanation: >-
      Reports the SOAPP's validation performance as a pre-prescription
      screening tool.
clinical_trials:
- name: NCT02032433
  phase: PHASE_IV
  status: COMPLETED
  description: >-
    X:BOT — a 24-week multicentre open-label randomised comparative
    effectiveness trial of extended-release naltrexone versus
    buprenorphine-naloxone for opioid relapse prevention in 570 adults, with
    opioid relapse-free survival as the primary outcome.
  target_phenotypes:
  - preferred_term: Addictive substance use
    term:
      id: HP:0033511
      label: Addictive substance use
  evidence:
  - reference: PMID:29150198
    reference_title: "Comparative effectiveness of extended-release naltrexone versus buprenorphine-naloxone for opioid relapse prevention (X:BOT): a multicentre, open-label, randomised controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We initiated this 24 week, open-label, randomised controlled, comparative
      effectiveness trial at eight US community-based inpatient services and
      followed up participants as outpatients.
    explanation: >-
      Describes the trial design recorded here.
discussions:
- discussion_id: oud_tolerance_molecular_basis
  kind: KNOWLEDGE_GAP
  prompt: >-
    What molecular processes actually account for the loss of mu-opioid
    receptor function after chronic opioid exposure, given that acute
    desensitization and internalization do not fully explain clinical
    tolerance?
  attaches_to:
  - pathophysiology#Mu-Opioid Receptor Desensitization and Tolerance
  rationale: >-
    The authoritative review of MOR regulation states plainly that while some
    regulatory mechanisms contributing to tolerance are understood, there are
    large gaps in understanding the molecular processes responsible for loss of
    receptor function after chronic exposure. This matters beyond mechanism:
    tolerance to analgesia and to respiratory depression diverge, and that
    divergence is what makes the post-abstinence overdose spike so lethal.
  proposed_experiments:
  - experiment_id: exp_oud_phosphosite_tolerance
    name: Phosphorylation-site mutant models separating acute desensitization from chronic tolerance
    description: >-
      Use knock-in models with defined MOR phosphorylation-site mutations to
      test which receptor regulatory events are required for chronic analgesic
      tolerance versus acute desensitization, and whether the same events
      govern tolerance to respiratory depression.
  - experiment_id: exp_oud_respiratory_tolerance_divergence
    name: Direct comparison of analgesic and respiratory tolerance time courses
    description: >-
      Measure the development and loss of tolerance to analgesia and to
      respiratory depression in parallel within the same subjects, to
      characterize the divergence that underlies overdose risk after a period
      of abstinence.
- discussion_id: oud_negative_affect_model_fidelity
  kind: HUMAN_MODEL_MISMATCH
  prompt: >-
    Do the rodent operationalizations of hyperkatifeia — elevated reward
    thresholds, lowered pain thresholds, anxiety-like and dysphoric-like
    behaviour — correspond to the negative emotional state that drives relapse
    in people with opioid use disorder?
  attaches_to:
  - pathophysiology#Reward-System Downregulation and Hyperkatifeia
  rationale: >-
    The evidence for the negative-affect stage is largely from extended-access
    animal models, where the state is inferred from reward thresholds and
    behavioural assays. Human hyperkatifeia is assessed by self-report of
    dysphoria and craving. The construct is plausible and clinically resonant,
    but the measurement bridge between the two species is an assumption rather
    than a demonstrated equivalence.
  proposed_experiments:
  - experiment_id: exp_oud_human_reward_threshold
    name: Human reward-sensitivity measurement across withdrawal and protracted abstinence
    description: >-
      Track objective reward sensitivity (effort-based decision making,
      monetary incentive delay fMRI) alongside self-reported dysphoria in
      people with OUD from acute withdrawal into protracted abstinence, to test
      whether a measurable reward deficit tracks the subjective state.
  - experiment_id: exp_oud_relapse_prediction
    name: Prospective test of negative affect as a relapse predictor
    description: >-
      Determine whether the magnitude of the negative-affect state during early
      abstinence prospectively predicts relapse, which is the claim the
      negative-reinforcement model makes and that animal work cannot establish.
- discussion_id: oud_tlr4_glial_human_translation
  kind: HUMAN_MODEL_MISMATCH
  prompt: >-
    Does the TLR4/microglial arm that opposes opioid analgesia and drives
    tolerance, hyperalgesia, and withdrawal in rodents operate at a clinically
    meaningful magnitude in people with opioid use disorder?
  attaches_to:
  - pathophysiology#Opioid-Induced TLR4 and Microglial Neuroimmune Activation
  rationale: >-
    The rodent and in vitro evidence for this node is strong and convergent —
    non-stereoselective TLR4 activation in vitro, pharmacological blockade
    attenuating tolerance, hyperalgesia and withdrawal in vivo, and a
    dose-response shift in TLR4 knockout mice. The human evidence is not.
    The one human interventional test cited here, a placebo-controlled inpatient
    study of the glial modulator ibudilast in opioid-dependent volunteers, was
    null on its prespecified withdrawal-severity outcome and positive only in an
    exploratory pooled analysis of individual subscale items. A second
    constraint comes from the rodent work itself: TLR4 dependence was
    demonstrated for low-dose morphine, while high-dose morphine effects were
    neither TLR4- nor PKCepsilon-dependent, so the arm may not be engaged at the
    doses that characterize opioid use disorder. Until an adequately powered
    trial of a glial or TLR4-directed agent shows an effect on a prespecified
    clinical endpoint, this node should be read as a mechanism established in
    animals and only weakly corroborated in humans.
  proposed_experiments:
  - experiment_id: exp_oud_tlr4_powered_trial
    name: Adequately powered trial of a TLR4-directed or glial-modulating agent in OUD
    description: >-
      Test a TLR4 antagonist or glial modulator against placebo in people with
      opioid use disorder, powered on a single prespecified clinical endpoint
      (withdrawal severity, or opioid dose required for analgesia), so the
      result does not depend on exploratory subscale analysis.
  - experiment_id: exp_oud_tlr4_dose_range
    name: Dose-ranging test of TLR4 dependence at OUD-relevant exposures
    description: >-
      Establish whether the TLR4 arm is engaged across the full opioid dose
      range or only at low doses, since the demonstrated dose-dependence in
      rodents determines whether the mechanism is relevant to the high-exposure
      pattern of opioid use disorder at all.
- discussion_id: oud_epigenetic_mechanism
  kind: KNOWLEDGE_GAP
  prompt: >-
    Do chronic-opioid-associated DNA methylation changes represent a causal
    mechanism in opioid use disorder, a durable biomarker of exposure, or
    neither?
  attaches_to:
  - pathophysiology#Polygenic Susceptibility to Opioid Addiction
  - pathophysiology#Mu-Opioid Receptor Desensitization and Tolerance
  rationale: >-
    An epigenetic arm is frequently asserted for opioid use disorder, and this
    entry deliberately does not model one, because the primary evidence does not
    currently support a mechanism node. The best-known positive finding is
    OPRM1 promoter hypermethylation at two CpG sites in *lymphocytes* of
    methadone-maintained former heroin addicts — a peripheral tissue, with the
    consequence for receptor expression stated by the authors only as a
    possibility. The one epigenome-wide study in the tissue that matters,
    postmortem dorsolateral prefrontal cortex, found no CpG site surviving
    false-discovery-rate correction. Direction of causation is also open: these
    are cross-sectional comparisons of people with long exposure histories, so a
    methylation difference is at least as easily read as a consequence of
    exposure as a cause of the disorder. Curating an epigenetic node from
    review-level synthesis would overstate all of this.
  evidence:
  - reference: PMID:18650805
    reference_title: "Increased OPRM1 DNA methylation in lymphocytes of methadone-maintained former heroin addicts."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Direct sequencing of bisulfite-treated DNA showed that the percent
      methylation at two CpG sites was significantly associated with heroin
      addiction.
    explanation: >-
      The strongest positive primary finding, but measured in lymphocytes rather
      than brain, so it supports an association and not a CNS mechanism.
  - reference: PMID:18650805
    reference_title: "Increased OPRM1 DNA methylation in lymphocytes of methadone-maintained former heroin addicts."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Methylation of these CpG sites may lead to reduced OPRM1 expression in the
      lymphocytes of these former heroin addicts.
    explanation: >-
      The authors state the functional consequence as a possibility, not a
      measurement, which is exactly the step this gap records as missing.
  - reference: PMID:33667780
    reference_title: Epigenome-wide study of brain DNA methylation following acute opioid intoxication.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Although no CpG sites survived false-discovery rate correction for
      multiple testing, 13 sites surpassed a relaxed significance threshold
    explanation: >-
      The epigenome-wide result in brain tissue is null at the conventional
      threshold, which is the central reason no epigenetic mechanism node is
      curated in this entry.
  - reference: PMID:33667780
    reference_title: Epigenome-wide study of brain DNA methylation following acute opioid intoxication.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Current evidence, however, is mostly limited to candidate gene analysis in
      whole blood.
    explanation: >-
      Characterizes the state of the field, and the tissue-relevance problem
      this gap turns on.
  proposed_experiments:
  - experiment_id: exp_oud_ewas_powered_brain
    name: Adequately powered brain EWAS with cell-type deconvolution
    description: >-
      Repeat the epigenome-wide analysis in postmortem brain at a sample size
      powered to survive multiple-testing correction, with cell-type
      deconvolution, so that a null result is informative rather than
      underpowered.
  - experiment_id: exp_oud_longitudinal_methylation
    name: Longitudinal methylation sampling across exposure and abstinence
    description: >-
      Sample methylation before, during, and after sustained opioid exposure in
      the same individuals to distinguish a pre-existing risk mark from an
      exposure-induced change, which no cross-sectional case-control design can
      do.
  - experiment_id: exp_oud_methylation_expression_link
    name: Test whether the OPRM1 CpG marks actually alter receptor expression
    description: >-
      Directly measure OPRM1 expression against methylation at the -18 and +84
      CpG sites, in both lymphocytes and brain, to test the mechanism the
      original study could only propose.
references:
- reference: PMID:31400808
  title: "Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement."
- reference: PMID:35471232
  title: Mechanisms of opioid-induced respiratory depression.
- reference: PMID:36207451
  title: "Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond."
- reference: PMID:24500948
  title: Buprenorphine maintenance versus placebo or methadone maintenance for opioid dependence.
- reference: PMID:12804430
  title: Methadone maintenance therapy versus no opioid replacement therapy for opioid dependence.
- reference: PMID:19679181
  title: Evidence that opioids may have toll-like receptor 4 and MD-2 effects.
- reference: PMID:34347030
  title: "Contingency Management for Patients Receiving Medication for Opioid Use Disorder: A Systematic Review and Meta-analysis."

📚

References & Deep Research

References

7

PMID:31400808

Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement.

No top-level findings curated for this source.

PMID:35471232

Mechanisms of opioid-induced respiratory depression.

No top-level findings curated for this source.

PMID:36207451

Multi-trait genome-wide association study of opioid addiction: OPRM1 and beyond.

No top-level findings curated for this source.

PMID:24500948

Buprenorphine maintenance versus placebo or methadone maintenance for opioid dependence.

No top-level findings curated for this source.

PMID:12804430

Methadone maintenance therapy versus no opioid replacement therapy for opioid dependence.

No top-level findings curated for this source.

PMID:19679181

Evidence that opioids may have toll-like receptor 4 and MD-2 effects.

No top-level findings curated for this source.

PMID:34347030

Contingency Management for Patients Receiving Medication for Opioid Use Disorder: A Systematic Review and Meta-analysis.

No top-level findings curated for this source.

Deep Research

1

Claude Code ▸

Opioid Use Disorder — Comprehensive Disease Characteristics Research Report

claude-haiku-4-5-20251001, claude-sonnet-5 42 citations 2026-08-09T18:32:42.005250

Opioid Use Disorder — Comprehensive Disease Characteristics Research Report

Prepared for dismech knowledge-base curation. All ontology term suggestions below are proposals only and must be independently verified (OAK lookup / label match) before being written into KB YAML, per the project's anti-hallucination SOP — several IDs below are given with explicit confidence caveats for that reason.

1. Disease Information

Overview. Opioid use disorder (OUD) is a chronic, relapsing brain disease characterized by compulsive opioid seeking and use despite harmful consequences, driven by neuroadaptations in reward, stress, and executive-control circuitry following repeated exposure to exogenous opioid agonists (prescription analgesics, heroin, or illicit synthetic opioids such as fentanyl). It is defined clinically (DSM-5-TR) as a problematic pattern of opioid use leading to clinically significant impairment or distress, manifested by ≥2 of 11 criteria within a 12-month period, with severity graded mild (2–3 criteria), moderate (4–5), or severe (≥6). OUD sits on a continuum with physiological tolerance and withdrawal (which can occur with appropriate medical use and alone do not establish OUD) and encompasses what was previously split into DSM-IV "opioid abuse" and "opioid dependence."

Key identifiers: - ICD-10-CM: F11.- (Opioid related disorders): F11.10 (abuse, uncomplicated), F11.20 (dependence, uncomplicated), F11.23 (dependence with withdrawal), F11.90 (use, unspecified) - ICD-11: 6C43 (Disorders due to use of opioids); 6C43.2 (Opioid dependence) - OMIM: 613459 — "Opioid Dependence, Susceptibility to" (candidate-gene entry, points to OPRM1) - MONDO: MONDO:0001225 (opioid use disorder) — verify before curating; some MONDO releases model "opioid dependence" as a related/child term - MeSH: D000068092 (Opioid-Related Disorders); D009293 (Narcotic-Related Disorders, older/broader term) - DSM-5-TR code: 304.00/305.50 (legacy DSM-IV codes still cross-walked)

Synonyms/alternative names: opioid addiction, opioid dependence, narcotic addiction, heroin use disorder (heroin-specific), prescription opioid misuse/addiction, opioid abuse (legacy DSM-IV term).

Evidence character: Information below is drawn from aggregated epidemiological/registry data (CDC NCHS/NVSS, SAMHSA NSDUH), large genetic consortia (Million Veteran Program, Psychiatric Genomics Consortium, FinnGen, 23andMe), systematic reviews/meta-analyses, and mechanistic studies primarily in rodent models with translational human neuroimaging/genetic corroboration — not a single-patient EHR source.

2. Etiology

Disease causal factors

OUD is a multifactorial, biopsychosocial disorder — no single causal genetic lesion (unlike Mendelian dismech entries); risk arises from the interaction of (a) pharmacological exposure to an exogenous mu-opioid-receptor (MOR) agonist, (b) polygenic genetic liability, and (c) environmental/psychosocial exposures. Exposure — most commonly initiated via a legitimate prescription for acute or chronic pain, or via non-medical use of diverted prescription opioids or illicit heroin/fentanyl — is a necessary but not sufficient cause; only a minority of opioid-exposed individuals develop OUD.

Risk factors

Genetic risk factors: - OPRM1 (mu-opioid receptor, hgnc:8156) — the most-studied candidate gene. The functional coding variant rs1799971 (A118G, Asn40Asp) shows a significant association with opioid dependence susceptibility in a 2023 meta-analysis of 13 studies (n=9,385; 4,601 cases/4,784 controls), strongest in Asian populations under a dominant model (PMID:23651028 for the earlier meta-analytic literature; 2023 update per Journal of Pain and Symptom Management search). A genome-wide-significant OPRM1 functional coding-variant association with OUD was also independently confirmed by GWAS (PMID:32492095). - FURIN — identified as a genome-wide-significant lead locus in the largest multi-ancestry OUD GWAS to date (European + African ancestry, N=639,063 across Million Veteran Program, Psychiatric Genomics Consortium, iPSYCH, FinnGen, Partners Biobank, BioVU, Yale-Penn 3), alongside two independent OPRM1 variants; a multi-trait analysis (MTAG) combining OUD with related traits yielded 18–19 independent genome-wide-significant loci (Nature/Molecular Psychiatry, 2022; PMC9718667). - KDM4A (rs3791033, intronic) and a locus near LRRIQ3 (rs640561, intergenic) — identified in a 23andMe GWAS of problematic opioid prescription use in 132,113 research participants of European ancestry (PMID:34728798). - CPT2, CD47, SLC5A11 — three novel OUD loci from a March 2024 genome-wide analysis of shared genetic architecture between OUD and general cognitive ability, which also replicated 4 loci from prior GWAS and found 7 loci not previously reported (PMC11831617). - CNIH3 — implicated in an earlier opioid dependence GWAS (Nelson et al., 2016; PMID:26857631, from established literature). - Other candidate/modifier genes with supporting literature: OPRD1 (delta-opioid receptor), OPRK1/PDYN (kappa-opioid receptor/dynorphin, stress/anti-reward axis), DRD2 and BDNF (associated with continued opioid use during methadone treatment; PMC4672523), COMT (dopamine catabolism, pain sensitivity modulation). - Heritability: Twin/family studies estimate heritability at roughly 23–54%, with more recent estimates clustering around 40–60%; Kendler et al. (~1,200 male-male twin pairs) estimated ~48% genetic liability, Tsuang et al. (Vietnam Era twins) ~54%. Approximately 38% of variance is opioid-specific (not shared with general substance-use liability), indicating both shared addiction-general and opioid-specific genetic architecture. - Pharmacogenomic modifiers of exposure/risk: CYP2D6 poor/ultrarapid-metabolizer status alters conversion of prodrug opioids (codeine, tramadol) to active metabolites, affecting individual exposure/efficacy and potentially misuse liability; CYP3A4/CYP2B6 affect methadone/buprenorphine clearance; ABCB1 (P-glycoprotein) variants affect CNS opioid penetration.

Environmental risk factors: - Long-term prescription opioid therapy for chronic pain (dose- and duration-dependent risk) - Prior or current substance use disorder (alcohol, cannabis, cocaine, benzodiazepines — strong polysubstance overlap) - Family history of substance use disorder - Adverse childhood experiences (ACEs) / childhood trauma, physical/sexual abuse - Co-occurring untreated psychiatric illness (depression, anxiety, PTSD — see Section 3) - Socioeconomic deprivation, unemployment, rural/economically distressed communities (historically linked to the U.S. prescription-opioid "first wave" of the epidemic, 1990s–2000s, driven partly by aggressive marketing/overprescribing of extended-release oxycodone) - Incarceration history (elevated post-release overdose risk due to loss of tolerance) - Age of first opioid exposure (adolescent/young-adult initiation confers higher risk owing to ongoing prefrontal cortical maturation) - Male sex (historically higher prevalence; the sex gap has narrowed substantially in recent U.S. cohorts) - Illicit drug-supply contamination with fentanyl and fentanyl analogs — a supply-side environmental risk factor that has dramatically increased overdose lethality independent of use-disorder severity (the "third/fourth wave" of the U.S. epidemic, now compounded by opioid-stimulant co-use).

Protective factors: - Genetic: evidence here is far less consistent than for risk variants; some studies suggest certain OPRM1 haplotypes or reduced-function variants at addiction-relevant loci may modestly lower liability, but no robustly replicated protective allele is established (unlike, e.g., ALDH22 in alcohol use disorder). Flag as an evidence gap rather than assert a specific protective variant. - Environmental/clinical:* access to non-opioid pain management, prescription drug monitoring programs (reduce diversion/doctor-shopping), strong social support and family cohesion, higher educational attainment, timely access to medications for OUD (MOUD), community naloxone distribution and harm-reduction services (syringe service programs, fentanyl test strips), effective treatment of comorbid psychiatric illness.

Gene-environment interactions. The clearest documented G×E interaction is genotype-dependent response to exposure/treatment rather than genotype-dependent initiation risk: OPRM1 A118G carriers show altered β-endorphin binding, altered HPA-axis (cortisol) response to naloxone challenge, and differential dose requirements for opioid analgesics and methadone, and modestly altered naltrexone treatment response (Oslin et al., PGx literature). More broadly, genetic liability for general addiction/externalizing traits appears to interact with the local prescribing/illicit-supply environment (e.g., fentanyl contamination) to determine whether genetic risk translates into fatal overdose versus non-fatal OUD — an environment that has changed dramatically over the past decade and is not fully captured by pre-2015 genetic studies.

3. Phenotypes

OUD phenotypes span acute intoxication, chronic-use, tolerance/withdrawal, and complication domains. Frequencies below are approximate/derived from clinical literature; treat as OCCASIONAL/FREQUENT-type qualitative bands pending exact quantitative sourcing per curation SOP.

Phenotype	Type	Onset/course	Suggested HPO (verify before use)
Craving (persistent urge to use)	Behavioral/cognitive core symptom	Chronic, fluctuating, can persist into prolonged abstinence	— (behavioral abnormality branch; no precise HPO substance-craving term confirmed)
Tolerance (need for increasing doses)	Physiological/laboratory-adjacent	Develops over days–weeks of regular use	—
Opioid withdrawal syndrome (rhinorrhea, lacrimation, mydriasis, piloerection, myalgia/arthralgia, nausea, vomiting, diarrhea, abdominal cramping, insomnia, yawning, dysphoria, autonomic hyperactivity)	Clinical signs/symptoms	Acute onset 6–24h after last dose (short-acting) or 24–48h (methadone); resolves over 5–10 days for acute phase; protracted withdrawal can persist weeks–months	HP:0002014 (Diarrhea), HP:0000969 (Edema — n/a), HP:0002617 (Piloerection?), HP:0000745 (Irritability), HP:0002367 (Paroxysmal (n/a)) — most withdrawal signs map to generic HPO clinical-sign terms rather than an opioid-specific term; verify each individually
Miosis (pinpoint pupils)	Clinical sign, acute intoxication	Acute, dose-dependent	HP:0000454 (Miosis, or specific pupillary term — verify)
Respiratory depression (acute overdose)	Clinical sign/emergency	Acute, dose-dependent, potentiated by concurrent sedatives/fentanyl potency	HP:0002093-adjacent respiratory terms — verify
Opioid-induced constipation / bowel dysfunction	Laboratory/clinical (chronic)	Chronic, dose-related, does not resolve with tolerance to other effects	relevant GI HPO term — verify
Opioid-induced hyperalgesia	Clinical/laboratory-adjacent	Emerges with prolonged high-dose exposure	—
Sedation/cognitive slowing	Symptom	Acute–subacute	HP:0032988 or similar cognitive-impairment term — verify
Hypogonadism (opioid-induced androgen deficiency)	Laboratory/endocrine	Chronic, dose/duration-dependent	HP:0000135 (Hypogonadism) plausible
Central sleep apnea / sleep-disordered breathing	Clinical/laboratory (polysomnography)	Chronic, high-dose opioid therapy	HP:0002104 (Apnea) or CSA-specific term — verify
Immunosuppression	Laboratory	Chronic use	—
Neonatal opioid withdrawal syndrome / neonatal abstinence syndrome (NAS)	In utero-exposed offspring phenotype (relevant to comorbidity/trajectory rather than the proband)	Onset 24h–several days postnatally	HP:0025468 or NAS-specific term if present — verify; MedGen has a distinct NAS concept
Depressed mood / anhedonia during withdrawal and protracted abstinence	Behavioral	Subacute–chronic	overlaps HP depression terms
Unsuccessful efforts to cut down/control use	Behavioral (DSM criterion)	Chronic	—
Continued use despite social/interpersonal problems	Behavioral (DSM criterion)	Chronic	—
Hazardous use (e.g., while driving)	Behavioral (DSM criterion)	Episodic	—

Severity/progression: DSM-5-TR severity (mild/moderate/severe) is criterion-count based, not biomarker based; course is frequently chronic-relapsing, with a substantial minority achieving sustained remission, especially with MOUD engagement. Quality of life: OUD is associated with markedly reduced quality of life across physical, psychological, and social domains (SF-36/EQ-5D literature); untreated OUD carries a substantially elevated all-cause and overdose-specific mortality risk relative to the general population, with mortality risk sharply reduced during active MOUD treatment and elevated during treatment gaps/post-incarceration/post-detox (loss-of-tolerance overdose risk).

4. Genetic/Molecular Information

Causal/major-effect genes: No single Mendelian causal gene exists (OUD is polygenic/complex, unlike most dismech Mendelian entries); OMIM's "Opioid Dependence, Susceptibility to" (613459) is a susceptibility-locus entry pointing to OPRM1 (hgnc:8156; chr6q25.2), not a causal-variant Mendelian disease gene.

Key pathogenic/risk-associated variants: - OPRM1 rs1799971 (A118G / Asn40Asp) — missense, exon 1; alters MOR N-glycosylation and receptor trafficking/signaling efficacy; associated with altered opioid analgesic requirements, methadone dosing, naltrexone treatment response, and dependence susceptibility (mixed/heterogeneous meta-analytic results across populations; strongest signal in Asian cohorts per the 2023 meta-analysis). Allele frequency varies substantially by ancestry (minor G-allele more common in East Asian populations) — check gnomAD for population-specific frequencies before citing a number. - FURIN locus variants and two independent OPRM1 variants — genome-wide significant in the 2022 multi-ancestry MTAG GWAS (PMC9718667). - KDM4A (rs3791033), LRRIQ3-adjacent (rs640561) — from 23andMe GWAS of problematic opioid prescription use (PMID:34728798). - CPT2, CD47, SLC5A11 loci — from the 2024 OUD/cognitive-ability shared-architecture GWAS (PMC11831617). - These are risk/susceptibility variants, not ACMG pathogenic-classified Mendelian variants; ClinVar/ClinGen do not carry a curated gene-disease validity assertion for OUD in the sense used for Mendelian dismech entries — this is a key modeling difference from most KB entries and should be flagged in any genetic: block (e.g., via relationship_type: SUSCEPTIBILITY).

Functional consequences: OPRM1 A118G is generally treated in the literature as altering receptor expression/signaling efficiency rather than a classic gain/loss-of-function dichotomy; characterize cautiously as a modifier of receptor trafficking and downstream signaling rather than assigning functional_impact_category: LOSS_OF_FUNCTION/GAIN_OF_FUNCTION without a specific supporting functional study.

Somatic vs. germline: All established OUD risk variants are germline/constitutional; no somatic-mutation component is implicated in this disease process (unlike cancer entries).

Epigenetics: Chronic opioid exposure is associated with epigenetic remodeling in reward circuitry — DNA methylation changes at stress- and reward-related genes, and histone modifications (e.g., altered histone acetylation at FosB/ΔFosB and CREB target genes in the nucleus accumbens) have been reported in preclinical models and some human postmortem/peripheral-tissue studies; this literature is less mature/replicated than for the genetic-association findings above and should be sourced to specific primary papers before curating individual claims.

Chromosomal abnormalities: No recurrent chromosomal aberration (aneuploidy, translocation) is associated with OUD; this is a polygenic complex trait, not a cytogenetic disorder.

5. Environmental Information

Environmental/exposure factors: - Prescription opioid exposure for acute or chronic pain — the dominant iatrogenic route of initiation historically in the U.S. - Illicit opioid supply — heroin, and increasingly illicitly manufactured fentanyl and fentanyl analogs (e.g., carfentanil), which now dominate U.S. overdose mortality; synthetic opioids (chiefly fentanyl) accounted for an estimated 48,422 of the 54,743 opioid-involved overdose deaths in 2024 (provisional CDC/NCHS data), underscoring illicit-fentanyl contamination as the primary proximate environmental driver of current mortality (CDC NCHS Data Brief 549; CDC NCHS/NVSS provisional data release, 2025). - Co-use/adulteration with stimulants (methamphetamine, cocaine) and with benzodiazepines/xylazine — increasingly common and mechanistically important because these combinations increase overdose lethality and complicate naloxone-only reversal (xylazine, an alpha-2 agonist veterinary sedative, does not respond to naloxone). - Healthcare-system/prescribing environment: opioid marketing practices, prescribing guidelines and their evolution (e.g., 2016/2022 CDC opioid prescribing guidelines), prescription drug monitoring program coverage.

Lifestyle factors: Polysubstance use (alcohol, benzodiazepines, stimulants — each independently raising overdose risk via additive/synergistic respiratory depression or complicating clinical management), smoking, housing instability, involvement in the criminal-legal system.

Infectious agents (indirectly disease-associated, not causal of OUD itself): Injection drug use associated with OUD is a major transmission route for HIV, hepatitis C virus (HCV), and hepatitis B virus (HBV), as well as bacterial infections (infective endocarditis, skin/soft-tissue abscesses, epidural abscess) from non-sterile injection practices — these are downstream comorbidities/complications rather than causal agents of OUD, and would more naturally be modeled as comorbidities/ or downstream pathophysiology nodes than as OUD's own etiology.

6. Mechanism / Pathophysiology

Acute reward mechanism

Opioids (endogenous or exogenous) act primarily via the mu-opioid receptor (MOR, OPRM1), a Gi/Go-coupled GPCR. MOR agonism in the ventral tegmental area (VTA) hyperpolarizes local GABAergic interneurons (via Gi-mediated inhibition of adenylyl cyclase and opening of GIRK potassium channels), disinhibiting VTA dopaminergic neurons and increasing dopamine release into the nucleus accumbens (NAc) — the canonical positive-reinforcement/reward pathway underlying acute drug liking and euphoria. Recent circuit-level work indicates this is not a uniform effect across the entire VTA dopamine population but instead recruits specific dopaminergic and non-dopaminergic (GABAergic, opioid-peptidergic) subcircuits within the mesocorticolimbic system (Nature Reviews Neuroscience, Oct 2025 review; Neuron 2025 mesolimbic-circuitry paper).

Chronic neuroadaptation ("opponent-process"/allostasis model)

With repeated exposure, the brain's reward system down-regulates (reduced dopaminergic tone, reduced hedonic capacity — "reward deficit") while anti-reward/stress systems up-regulate: corticotropin-releasing factor (CRF) is released from hypothalamic/extended-amygdala neurons, activating the HPA axis (ACTH → cortisol) and central stress circuitry (extended amygdala CRF/dynorphin-kappa-opioid-receptor signaling), producing the negative-affective state that drives use to escape withdrawal-associated dysphoria rather than purely to seek euphoria (Koob-model allostasis). This underlies the shift from positive to negative reinforcement across the addiction cycle (binge/intoxication → withdrawal/negative affect → craving/preoccupation).

Withdrawal mechanism — locus coeruleus (LC) hyperactivity

Chronically, MOR agonism in the locus coeruleus (major noradrenergic nucleus) is opposed by compensatory upregulation of the cAMP–PKA signaling cascade (adenylyl cyclase superactivation, enhanced G-protein/PKA transduction — classic Nestler-lab cAMP-upregulation model of opiate dependence), so that abrupt opioid removal (or naloxone precipitation) unmasks this compensatory hyperactivity as a surge of LC noradrenergic firing, producing the autonomic/somatic withdrawal syndrome (restlessness, anxiety, sweating, tachycardia, and the classic flu-like symptom cluster in Section 3). Nitric oxide signaling and galanin/GalR1 autoreceptor feedback have been implicated as intermediate messengers modulating LC hyperactivity during withdrawal in rodent models (search-sourced primary literature on LC withdrawal pharmacology).

Tolerance and opioid-induced hyperalgesia

Chronic MOR activation drives receptor desensitization/internalization, and — mechanistically distinct — a pronociceptive sensitization process: morphine-induced tolerance/hyperalgesia has been linked to increased adenosine kinase expression with reduced A3 adenosine receptor signaling and associated neuroinflammatory glial activation; A3AR agonism attenuates these effects in preclinical models (search-sourced primary literature).

Neuroimmune involvement

Glial (microglial/astrocytic) activation and neuroinflammatory signaling (e.g., TLR4 pathway activation by opioids, independent of classical MOR signaling in some models) contribute to tolerance, hyperalgesia, and withdrawal severity — an active area of mechanistic and drug-discovery research.

Cell types and anatomical circuitry involved

VTA dopaminergic neurons (reward)
NAc medium spiny neurons (reward output, plasticity — ΔFosB accumulation with chronic exposure)
Locus coeruleus noradrenergic neurons (withdrawal/arousal)
Extended amygdala / central nucleus of the amygdala CRF neurons (stress/anti-reward)
Periaqueductal gray (analgesia, also implicated in withdrawal-associated aversive state)
Prefrontal cortex circuitry (impaired top-down inhibitory control, craving/relapse vulnerability)
Microglia/astrocytes (neuroinflammatory contribution to tolerance/hyperalgesia)

Suggested GO / molecular-function terms (verify before curating)

G protein-coupled opioid receptor signaling pathway (candidate: GO:0038003)
Adenylate cyclase-inhibiting G protein-coupled receptor signaling pathway (GO:0007193)
Response to morphine (candidate GO term exists in this space — confirm exact ID/label via OAK)
Regulation of dopamine secretion (GO:0014059)
Positive regulation of synaptic transmission, dopaminergic (verify exact term)

Suggested CL (cell type) terms (verify before curating)

Dopaminergic neuron (CL:0000700)
Medium spiny neuron (CL:0000750)
Noradrenergic neuron (verify exact CL ID)
Microglial cell (CL:0000129)
Astrocyte (CL:0000127)

Omics/advanced technologies

Human neuroimaging (fMRI, PET receptor-occupancy studies) demonstrates blunted striatal dopaminergic response to non-drug reward and altered prefrontal-limbic connectivity in individuals with OUD; single-cell/spatial-transcriptomic work in rodent models with humanized OPRM1 A118G knock-in shows cell-type-specific transcriptional and connectivity changes following opioid dependence (PMC12393594, PMC10866092 — spatial transcriptomics and neural-connectivity studies in the A118G mouse model). Whole-exome sequencing of opioid dependence cohorts has begun to characterize rare-variant contributions beyond the common-variant GWAS signal (Translational Psychiatry, 2025, whole-exome study).

7. Anatomical Structures Affected

Organ/system level: - Primary: Central nervous system — mesocorticolimbic reward circuit, brainstem (locus coeruleus, periaqueductal gray, respiratory centers in the medulla — the substrate of fatal overdose via central respiratory depression) - Secondary/complication-related: Cardiovascular system (methadone-associated QT prolongation/torsades risk), gastrointestinal system (opioid-induced bowel dysfunction/constipation), endocrine system (hypothalamic-pituitary-gonadal axis suppression → hypogonadism), respiratory system (central and obstructive sleep apnea with chronic high-dose use; acute respiratory failure in overdose), immune system (opioid-associated immunosuppression), and — via the injection-drug-use route specifically — cardiac valves (infective endocarditis), liver (viral hepatitis, hepatotoxicity), skin/soft tissue (abscesses, cellulitis).

Tissue/cell level: CNS neurons (dopaminergic VTA, noradrenergic LC, GABAergic interneurons, medium spiny neurons of the NAc), glia (microglia, astrocytes) implicated in neuroinflammatory contributions to tolerance/withdrawal.

Subcellular level: MOR receptor trafficking/internalization (plasma membrane → endosomal compartments) is central to tolerance mechanisms; downstream signaling involves adenylyl cyclase/cAMP at the plasma membrane and nuclear transcriptional changes (ΔFosB accumulation in the nucleus of NAc neurons with chronic exposure) as a molecular substrate of long-term neuroplasticity.

Localization: Bilateral, diffuse CNS circuit-level disorder rather than a focal anatomical lesion — not a lateralized condition.

Suggested UBERON terms (verify before curating): ventral tegmental area (UBERON:0002550), nucleus accumbens (UBERON:0001882), locus coeruleus (UBERON:0002037), amygdala (UBERON:0001876), periaqueductal gray (UBERON:0002440).

8. Temporal Development

Onset: Most commonly young-adult onset, though initiation frequently begins with adolescent or young-adult exposure to prescription opioids (post-surgical, post-injury) or non-medical initiation; onset of the disorder (as opposed to exposure) is typically subacute-to-insidious, evolving over weeks to months of regular use as tolerance and dependence establish, though rapid escalation is well documented with high-potency illicit fentanyl.

Progression: Course is classically chronic and relapsing-remitting rather than a fixed staged progression (unlike, e.g., cancer TNM staging); the Koob "addiction cycle" framework (binge/intoxication → withdrawal/negative affect → preoccupation/anticipation, recurring) is the standard conceptual staging used in the addiction-neuroscience literature rather than a formal clinical stage system. DSM-5-TR severity (mild/moderate/severe) reflects cumulative criterion count rather than a temporal stage.

Patterns: - Spontaneous remission occurs but is less common than in some other substance use disorders; treatment-facilitated remission with MOUD substantially improves retention and reduces mortality. - Relapse is common, particularly during high-risk transition periods: post-detoxification, post-incarceration, and after voluntary/involuntary treatment discontinuation — these periods carry sharply elevated overdose mortality risk due to loss of physiological tolerance. - "Critical periods"/windows of intervention opportunity: the immediate post-overdose period (linkage-to-care), post-incarceration release, and pregnancy (engagement with prenatal care) are recognized high-yield windows for MOUD initiation. - Protracted withdrawal/post-acute withdrawal syndrome (sleep disturbance, anhedonia, dysphoria, and craving persisting weeks to months beyond acute physical withdrawal) is a recognized clinical pattern contributing to relapse risk.

9. Inheritance and Population

Epidemiology: - U.S. overdose mortality (2024, provisional CDC/NCHS data): an estimated 80,391 total drug overdose deaths, down 26.9% from 110,037 in 2023; opioid-involved deaths fell from 83,140 (2023) to 54,743 (2024); synthetic-opioid (chiefly fentanyl)-involved deaths fell from 76,282 to 48,422 over the same period — the largest year-over-year decline in recent U.S. history, though still representing tens of thousands of deaths annually, and almost all states saw declines (some >35%: Louisiana, Michigan, New Hampshire, Ohio, Virginia, West Virginia, Wisconsin, D.C.), while South Dakota and Nevada saw slight increases (CDC NCHS Data Brief 549, 2025; CDC NCHS/NVSS provisional overdose data release). - U.S. prevalence (misuse, NSDUH): an estimated 8.9 million people aged 12+ misused opioids in the past year per 2023 NSDUH; 2024 NSDUH reported 7.6 million people misusing prescription opioids in the past year (methodology shifted between survey years — 2024 collected past-year rather than past-month detail, so year-over-year comparison should be made cautiously) (SAMHSA 2024 NSDUH Annual National Report). - Global burden: Globally, opioid dependence prevalence increased by an estimated 47% from 1990 to 2016, the largest relative increase among substance use disorders in that period, with the highest opioid-dependence prevalence historically concentrated in North America (search-sourced review literature, PMC4628571/related). - Only a fraction of people with OUD receive medications for OUD (MOUD); SAMHSA's pooled 2022–2024 NSDUH data are specifically tracking this treatment gap (SAMHSA NSDUH MOUD Data Spotlight).

Inheritance pattern: Complex/multifactorial (polygenic), not a single-gene Mendelian disorder — model any inheritance: block accordingly (e.g., omit a classical AD/AR/X-linked designation, or use a multifactorial/complex-trait annotation if the schema supports one; do not force-fit HP:0010982 polygenic inheritance without checking whether that's the intended semantic for a behavioral/complex trait disease rather than a monogenic disease with polygenic modifiers).

Heritability: ~23–60% across twin/family studies (see Section 4), with substantial shared environmental and unique environmental contribution to the remainder — this is a genetically influenced complex trait, not a highly penetrant Mendelian condition, and penetrance/expressivity concepts as used for monogenic dismech entries do not straightforwardly apply.

Population demographics: - Historically higher prevalence in males, though the sex gap in both use and overdose mortality has narrowed substantially in recent U.S. cohorts. - Marked geographic variation within the U.S. (Appalachia, New England, and parts of the Midwest/Rust Belt historically disproportionately affected during the prescription-opioid and heroin waves; illicit fentanyl has since spread geographically, notably into the Western U.S. in more recent years). - Racial/ethnic disparities in overdose mortality have shifted over the epidemic's course, with recent years showing disproportionate mortality increases among Black and Native American/Alaska Native populations relative to earlier phases dominated by white, rural populations — consult current CDC/KFF demographic breakdowns for precise, year-specific figures before citing exact rates. - Age distribution of overdose deaths is concentrated in adults 25–54, though this varies by opioid class and by year.

10. Diagnostics

Clinical criteria: DSM-5-TR opioid use disorder criteria (11 total; ≥2 within 12 months for diagnosis; severity by count as above) remain the primary diagnostic standard; ICD-11 6C43.2 (opioid dependence) uses the ICD dependence-syndrome framework (impaired control, increasing priority over other activities, physiological features).

Laboratory tests: - Urine drug screening (immunoassay) for opioids/opiates — note that standard opiate immunoassays often do not reliably detect synthetic opioids (fentanyl, methadone, buprenorphine) without a specific fentanyl or extended-panel assay, a clinically important limitation in the fentanyl era. - Confirmatory testing via GC-MS or LC-MS/MS for specific opioid/metabolite identification. - Serum/plasma opioid levels are not routinely used diagnostically for OUD (unlike therapeutic drug monitoring in other contexts) but may be used forensically/in overdose management. - No specific validated blood biomarker exists for OUD diagnosis at this time (unlike, e.g., HbA1c for diabetes) — this is a clinical/behavioral diagnosis.

Genetic testing: Not part of standard clinical diagnostic workup for OUD (unlike Mendelian disease); pharmacogenomic testing (e.g., CYP2D6 genotyping) has clinical utility for individualizing opioid analgesic prescribing/metabolism prediction and, in research contexts, OPRM1 genotyping has been explored to guide naltrexone treatment selection, but this is not standard of care.

Screening tools: Structured screening instruments are used far more than lab/genetic tests: Opioid Risk Tool (ORT), Screener and Opioid Assessment for Patients with Pain (SOAPP), COWS (Clinical Opiate Withdrawal Scale) and SOWS (Subjective Opiate Withdrawal Scale) for withdrawal severity grading, DSM-5-TR criteria checklist, and universal prescription drug monitoring program (PDMP) queries prior to opioid prescribing.

Differential diagnosis: Other substance use disorders (with polysubstance co-use being common rather than exclusionary), primary anxiety/depressive/PTSD disorders (which are frequently comorbid rather than purely differential — see Section 3), chronic pain syndromes without a use disorder (physiological tolerance/dependence from appropriate medical use alone does not meet OUD criteria), and — in the overdose/acute setting — other causes of altered mental status/respiratory depression (sedative-hypnotic overdose, stroke, hypoglycemia).

Imaging/functional testing: Not diagnostic for clinical OUD but used in research: fMRI/PET studies characterizing blunted striatal dopaminergic reward response and altered prefrontal-limbic connectivity.

11. Outcome/Prognosis

Mortality: OUD carries substantially elevated all-cause mortality versus the general population, driven predominantly by fatal overdose (increasingly fentanyl-driven — see Section 9 statistics), but also by infectious complications (endocarditis, sepsis from injection drug use), trauma, and suicide (elevated in the context of comorbid depression/PTSD — see Section 3). Mortality risk is markedly reduced during active engagement in MOUD (methadone/buprenorphine) and is sharply elevated during treatment gaps, immediately post-detoxification, and immediately post-incarceration release, due to loss of physiological tolerance combined with return to a high-potency illicit-fentanyl supply.

Course/recovery potential: Chronic-relapsing course is typical, but long-term recovery is achievable and common with sustained MOUD engagement plus psychosocial support; abstinence-only (non-medication) approaches are associated with substantially higher relapse and overdose-death rates than MOUD-based treatment in comparative studies (see Section 12).

Morbidity/complications: Injection-related infectious complications (HIV, HCV, HBV, infective endocarditis, skin/soft-tissue infections, epidural abscess), opioid-induced constipation/bowel dysfunction, hypogonadism, sleep-disordered breathing, neurocognitive effects, and — in pregnancy — neonatal opioid withdrawal syndrome/NAS in exposed infants (reported in an estimated 42–94% of infants born to opioid-dependent mothers per older literature, though modern MOUD-in-pregnancy management substantially alters this trajectory; NCBI Bookshelf NBK551498 and PMC5827164).

Quality of life: Substantially reduced across physical, psychological, and social functioning domains while actively using; meaningfully improved with sustained MOUD treatment and recovery.

Prognostic factors: Treatment engagement/retention (strongest modifiable predictor), presence and treatment of comorbid psychiatric illness, social support/housing stability, access to naloxone and harm-reduction services, and the local illicit-supply fentanyl contamination level (a population-level rather than individual-level prognostic factor that has become increasingly dominant).

12. Treatment

Pharmacotherapy (medications for opioid use disorder, MOUD) — the evidence-based first-line standard of care

Methadone — full mu-opioid receptor agonist, long half-life, dispensed through federally regulated Opioid Treatment Programs (OTPs) in the U.S. In network meta-analyses/systematic reviews of retention, methadone shows superior treatment retention compared with buprenorphine, which in turn outperforms naltrexone (PLOS One network meta-analysis; Lancet Psychiatry 2023 systematic review/meta-analysis of buprenorphine vs. methadone).
Buprenorphine (partial MOR agonist, kappa-antagonist; ceiling effect on respiratory depression improves its overdose-safety margin relative to full agonists) — commonly co-formulated with naloxone (buprenorphine-naloxone, e.g., Suboxone) to deter intravenous misuse; office-based prescribing was historically restricted by a DEA "X-waiver," which was removed under the 2023 MAT Act, substantially expanding prescriber access. Extended-release formulations: Sublocade (monthly SC injection, approved 2017) and Brixadi (weekly/monthly injection, approved 2023).
Naltrexone — full MOR antagonist; oral formulation has poor adherence-driven effectiveness (did not show higher retention than placebo in some meta-analyses), while extended-release injectable naltrexone (Vivitrol) shows comparable abstinence outcomes to buprenorphine-naloxone in some trials with an advantage in reducing days of opioid use in at least one comparative meta-analysis (PMC12421290); requires full detoxification (opioid-free interval) before initiation, a practical barrier relative to buprenorphine/methadone.
Lofexidine (Lucemyra) — alpha-2 adrenergic agonist (mechanistically targeting the LC noradrenergic-hyperactivity substrate of withdrawal described in Section 6), FDA-approved 2018 specifically for mitigation of opioid withdrawal symptoms (not a maintenance/anti-craving agent).

Overdose reversal

Naloxone — competitive MOR antagonist, rapidly reverses opioid-induced respiratory depression; nasal spray formulations increasingly available over-the-counter (Narcan OTC approved March 2023; RiVive OTC approved July 2023), and higher-dose formulations (Kloxxado) developed to counter high-potency fentanyl overdoses.
Nalmefene (Opvee nasal spray, approved May 2023) — longer-acting opioid antagonist, an alternative/adjunct to naloxone particularly relevant to prolonged fentanyl-driven respiratory depression, though its longer action also raises precipitated-withdrawal-duration considerations.

Psychosocial/behavioral treatment

Cognitive behavioral therapy, contingency management (among the most robustly evidence-supported behavioral interventions for stimulant/opioid co-use), motivational interviewing, and mutual-support/peer-recovery programs (e.g., Narcotics Anonymous) — generally recommended as an adjunct to, not a substitute for, MOUD.

Harm reduction

Syringe service programs, fentanyl/xylazine test strips, supervised consumption/overdose-prevention sites (where legally available), and community naloxone distribution — increasingly recognized as an integral component of the treatment/prevention continuum rather than a separate category.

Treatment in special populations

MOUD (buprenorphine or methadone) during pregnancy is standard of care to reduce maternal relapse/overdose risk and stabilize the fetal environment, though optimal medication choice (methadone vs. buprenorphine) and comparative neonatal-outcome safety data remain an active area of study (PMC11583522; PMC4628571).

Treatment strategy/algorithms

Clinical guidelines (ASAM National Practice Guideline, SAMHSA TIP series) generally recommend MOUD as first-line for OUD of at least moderate severity, individualized by patient preference, prior treatment response, pregnancy status, and access/logistics (OTP-based methadone vs. office-based buprenorphine vs. extended-release naltrexone).

Suggested NCIT terms (verify before curating)

Pharmacotherapy — NCIT:C15986 (generic action term; use with therapeutic_agent)
Specific agents via CHEBI/NCIT therapeutic_agent: buprenorphine, methadone, naltrexone, naloxone, nalmefene, lofexidine — look up exact CHEBI/NCIT IDs via OAK before curating (not independently verified in this research pass)
therapeutic_modality: SMALL_MOLECULE for all agents above (none are biologics/ASOs/gene therapies)

13. Prevention

Primary prevention: Cautious/guideline-concordant opioid prescribing (CDC opioid prescribing guidelines), prescription drug monitoring program (PDMP) utilization to reduce diversion/doctor-shopping, patient education on opioid risk at time of prescribing, non-opioid pain-management alternatives where clinically appropriate, and supply-side interventions targeting illicit fentanyl trafficking.

Secondary prevention (early detection/intervention): Screening for opioid misuse in primary care and pain-management settings (Opioid Risk Tool, SOAPP), early identification and treatment linkage after a non-fatal overdose event (a recognized high-yield "reachable moment" for MOUD initiation), routine offering of naloxone co-prescription alongside opioid analgesic prescriptions in higher-risk patients.

Tertiary prevention: Sustained MOUD engagement to prevent relapse/overdose in individuals with established OUD; treatment of comorbid psychiatric illness to reduce relapse risk; harm-reduction service engagement to reduce infectious and overdose complications in those who continue to use.

Public health/behavioral/prophylaxis: - Community naloxone distribution programs and layperson naloxone-administration training - Fentanyl/xylazine test-strip distribution - Syringe service programs (reduce HIV/HCV transmission) - Public health messaging and prescriber education - Genetic/prenatal screening is not a relevant prevention modality for OUD (unlike Mendelian dismech entries) — this section should not carry a genetic screening/carrier-screening annotation. - Counseling: substance-use-focused counseling and harm-reduction counseling (rather than classical genetic counseling) is the relevant analog here.

Immunization: Not directly applicable to OUD itself, though there is active preclinical/early-clinical research into anti-fentanyl and anti-heroin conjugate vaccines intended to blunt drug reward by sequestering the opioid in peripheral circulation before CNS penetration — an experimental, not yet approved, prevention strategy worth flagging as an emerging area if the KB entry includes an "experimental/emerging" treatments or prevention subsection.

14. Other Species / Natural Disease

OUD as clinically defined (a DSM/ICD-coded behavioral disorder) is fundamentally a human diagnostic construct — there is no well-characterized naturally occurring, spontaneous veterinary analog analogous to how, e.g., feline hypertrophic cardiomyopathy naturally recapitulates a human cardiomyopathy. Companion animals (dogs, cats) receiving therapeutic opioids for pain management can develop pharmacological tolerance and physical dependence with abrupt discontinuation producing a withdrawal syndrome, but this reflects the conserved pharmacology of the mu-opioid receptor system rather than a documented spontaneous behavioral-addiction disease entity in veterinary medicine, and is not indexed in OMIA (Online Mendelian Inheritance in Animals) as a natural disease. This section is therefore best modeled in the KB primarily via the Model Organisms section (induced/self-administration models) rather than a "natural disease in other species" entry — flag this explicitly as an evidence gap/not-applicable rather than fabricating a veterinary natural-disease citation.

Orthologous genes: OPRM1 orthologs are broadly conserved across mammals (mouse Oprm1, rat Oprm1, rhesus macaque OPRM1) and are the basis for cross-species pharmacological/behavioral translational work (Section 15).

Comparative pathology/evolutionary conservation: The mu-opioid receptor system and its coupling to mesolimbic dopaminergic reward circuitry is deeply conserved across mammals, which is what makes rodent and non-human-primate models mechanistically informative despite the absence of a natural spontaneous disease analog.

15. Model Organisms

OUD research relies almost entirely on induced models (pharmacological/behavioral induction of dependence-like states) rather than spontaneous genetic models, reflecting the disorder's nature as arising from an environmental exposure (opioid administration) acting on a polygenic-susceptibility background.

Rodent models: - Operant intravenous opioid self-administration (rat, mouse) — the gold-standard behavioral model for studying reinforcement, escalation of intake, and relapse/reinstatement after extinction; used extensively to screen candidate MOUD pharmacotherapies. - Conditioned place preference (CPP) — a Pavlovian-conditioning paradigm measuring opioid reward value without requiring self-administration training. - Naloxone-precipitated withdrawal — chronic morphine/fentanyl exposure followed by naloxone challenge to quantify withdrawal severity via somatic signs (jumping, wet-dog shakes, diarrhea, ptosis, teeth chattering in mice/rats) — the standard model for withdrawal pharmacology and the mechanistic LC-hyperactivity studies described in Section 6. - Oprm1 knockout mice — abolish morphine reward, analgesia, and physical dependence/withdrawal, foundational genetic evidence establishing MOR as necessary for opioid reinforcement (classic Kieffer-lab work). - Humanized OPRM1 A118G knock-in mice — carry the human risk-associated coding variant to study its functional/behavioral/circuit consequences directly in vivo; used in the spatial-transcriptomics and neural-connectivity studies cited in Section 6 (Mague et al. and follow-on work; PMC12393594, PMC10866092). - Chronic morphine tolerance/dependence models in rat locus coeruleus preparations — used to dissect the cAMP/PKA-upregulation mechanism of dependence described above.

Non-human primate models: Rhesus macaque intravenous opioid self-administration/reinstatement studies provide higher translational fidelity for reward/relapse pharmacology and are used particularly in medications-development research (e.g., evaluating candidate anti-relapse pharmacotherapies) given closer neuroanatomical and pharmacokinetic similarity to humans.

Other model systems: Zebrafish and invertebrate (Drosophila, C. elegans) models are used more for conserved-pathway/high-throughput genetic screening of opioid-response genes than as disease models per se; human iPSC-derived neuronal models (including OPRM1-variant iPSC lines) have been used to study synaptic-function consequences of the A118G variant in a human cellular context, complementing the mouse in vivo work.

Model limitations: No rodent or primate model fully recapitulates the human DSM behavioral-criteria construct (craving, social/occupational impairment) — these models capture physiological dependence, reinforcement, and withdrawal robustly, but the "compulsive use despite negative consequences" and subjective-craving dimensions of the human disorder are only partially modeled (e.g., via extended-access self-administration paradigms and punishment-resistant responding paradigms designed to approximate compulsivity). This human-model-fidelity gap is worth flagging explicitly as a HUMAN_MODEL_MISMATCH-type discussion if this entry is later built out with mechanistic_hypotheses, per the project's convention for model-system evidence whose translational validity to human behavior is uncertain.

Summary of Key Ontology-Term Candidates (all require OAK verification before KB use)

Domain	Candidate term	Notes
MONDO	MONDO:0001225	opioid use disorder — verify exact label/scope match
OMIM	613459	Opioid Dependence, Susceptibility to (candidate-gene entry)
Gene	hgnc:8156 (OPRM1)	primary candidate gene
Gene	FURIN, KDM4A, CPT2, CD47, SLC5A11	GWAS risk loci — resolve HGNC IDs before use
GO	response to morphine; G protein-coupled opioid receptor signaling; adenylate cyclase-inhibiting GPCR signaling pathway (GO:0007193)	verify exact IDs/labels
CL	dopaminergic neuron (CL:0000700); medium spiny neuron (CL:0000750); microglial cell (CL:0000129); astrocyte (CL:0000127)	core reward/glial cell types
UBERON	ventral tegmental area (UBERON:0002550); nucleus accumbens (UBERON:0001882); locus coeruleus (UBERON:0002037); amygdala (UBERON:0001876)	core circuitry
CHEBI	morphine, fentanyl, heroin (diacetylmorphine), methadone, buprenorphine, naloxone, naltrexone	resolve exact CHEBI IDs before curating `therapeutic_agent`
NCIT	Pharmacotherapy (NCIT:C15986)	generic treatment_term; pair with resolved `therapeutic_agent`

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References

Deep Research

Opioid Use Disorder — Comprehensive Disease Characteristics Research Report

1. Disease Information

2. Etiology

Disease causal factors

Risk factors

3. Phenotypes

4. Genetic/Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

Acute reward mechanism

Chronic neuroadaptation ("opponent-process"/allostasis model)

Withdrawal mechanism — locus coeruleus (LC) hyperactivity

Tolerance and opioid-induced hyperalgesia

Neuroimmune involvement

Cell types and anatomical circuitry involved

Suggested GO / molecular-function terms (verify before curating)

Suggested CL (cell type) terms (verify before curating)

Omics/advanced technologies

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome/Prognosis

12. Treatment

Pharmacotherapy (medications for opioid use disorder, MOUD) — the evidence-based first-line standard of care

Overdose reversal

Psychosocial/behavioral treatment

Harm reduction

Treatment in special populations

Treatment strategy/algorithms

Suggested NCIT terms (verify before curating)

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Summary of Key Ontology-Term Candidates (all require OAK verification before KB use)

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