Opioid-related disorders are among the most lethal psychiatric conditions in modern practice, driven by a synthetic-opioid overdose crisis dominated by illicitly manufactured fentanyl.^[1-2] Opioid use disorder (OUD) is defined in DSM-5-TR by a problematic pattern of opioid use producing clinically significant impairment, meeting at least 2 of 11 criteria within a 12-month period.^[3] Three medications for opioid use disorder (MOUD) are FDA-approved and reduce all-cause and overdose mortality: methadone, buprenorphine, and extended-release naltrexone.^[1,4] Naloxone, a competitive mu-opioid antagonist, reverses opioid-induced respiratory depression and is the single most important harm-reduction tool for overdose.^[5] The clinical bottom line: OUD is a chronic, relapsing, treatable disorder, and withholding agonist therapy is associated with excess mortality.^[1,4]

Opioid use disorder is common, undertreated, and disproportionately fatal. Synthetic opioids now account for the majority of US overdose deaths.^[1-2]

Prevalence and burden

• Past-year prevalence of OUD among US adults is estimated at approximately 0.6-1% based on national household surveys, with higher rates among adults aged 18-25 and among those with chronic pain; household surveys likely underestimate prevalence in marginally housed and incarcerated populations.^[1]
• US drug overdose deaths exceeded 100,000 per year in recent surveillance, with synthetic opioids (primarily illicitly manufactured fentanyl) implicated in the majority.^[2]
• OUD carries a standardized mortality ratio markedly elevated over the general population, with overdose and suicide as leading causes.^[1]

Demographics

• Onset typically in adolescence or young adulthood; prescription opioid misuse often precedes transition to heroin or fentanyl.^[1]
• Male:female ratio is approximately 1.5-2:1 for heroin use, narrower for prescription-opioid OUD.^[1]

Comorbidity

• High co-occurrence with other substance use disorders (alcohol, benzodiazepines, stimulants, tobacco).^[1]
• Major depressive disorder, PTSD, anxiety disorders, and personality disorders are over-represented among individuals with OUD.^[1]
• Suicide risk is markedly elevated; OUD independently predicts both attempted and completed suicide.^[1]

Risk factors

• Chronic pain, exposure to prescription opioids, adverse childhood experiences, family history of substance use disorder, and untreated psychiatric illness.^[1]
• Recent release from incarceration and recent discharge from inpatient detoxification confer markedly elevated overdose risk due to lost tolerance.^[1,4]

Opioids produce reinforcement and dependence through mu-opioid receptor agonism in mesolimbic reward circuitry, with adaptation in stress and antireward systems driving the chronic-relapsing course.^[8-9]

Neurobiology

• Mu-opioid receptor (MOR) agonism in the ventral tegmental area disinhibits dopaminergic neurons projecting to the nucleus accumbens, producing acute reinforcement.^[8]
• Repeated exposure causes neuroadaptation in the locus coeruleus (noradrenergic), amygdala (extended stress system, including corticotropin-releasing factor), and prefrontal cortex (executive control).^[8-9]
• Withdrawal reflects unopposed noradrenergic activity from the locus coeruleus, manifesting as autonomic hyperactivity.^[8]

Genetics and environment

• Heritability of OUD is estimated at approximately 50%, comparable to other substance use disorders.^[8]
• Functional variants in OPRM1 (e.g., A118G) and genes affecting opioid metabolism (CYP2D6, CYP3A4) influence response and risk, but no variant is clinically actionable for routine screening.^[8]
• Environmental contributors include opioid prescribing patterns, drug availability (especially fentanyl contamination of the illicit supply), trauma exposure, and socioeconomic stressors.^[1-2,6]

Integrative model

• Koob and Volkow's three-stage model frames addiction as cycling through binge/intoxication (basal ganglia), withdrawal/negative affect (extended amygdala), and preoccupation/anticipation (prefrontal cortex), with neuroplastic changes accumulating over time.^[9]

OUD is diagnosed when a problematic pattern of opioid use produces clinically significant impairment or distress, manifested by at least 2 of 11 DSM-5-TR criteria within a 12-month period.^[3]

DSM-5-TR criteria (paraphrased)

• Opioids taken in larger amounts or over a longer period than intended.^[3]
• Persistent desire or unsuccessful efforts to cut down or control use.^[3]
• Substantial time spent obtaining, using, or recovering from opioids.^[3]
• Craving or strong urge to use opioids.^[3]
• Recurrent use resulting in failure to fulfill major role obligations.^[3]
• Continued use despite persistent social or interpersonal problems caused or worsened by opioids.^[3]
• Important activities given up or reduced because of use.^[3]
• Recurrent use in physically hazardous situations.^[3]
• Continued use despite knowledge of a physical or psychological problem likely caused or worsened by opioids.^[3]
• Tolerance, defined as need for increased amounts or diminished effect at the same amount.^[3]
• Withdrawal, manifested by the characteristic syndrome or by use of opioids to relieve or avoid withdrawal.^[3]

Severity specifiers

• Mild: 2-3 criteria; moderate: 4-5 criteria; severe: 6 or more criteria.^[3]

Course specifiers

• In early remission: no criteria (except craving) for at least 3 months but less than 12 months.^[3]
• In sustained remission: no criteria (except craving) for 12 months or longer.^[3]
• On maintenance therapy: receiving a prescribed agonist medication (e.g., methadone, buprenorphine) with no other criteria met (except tolerance/withdrawal from the prescribed agent).^[3]
• In a controlled environment: access to opioids is restricted.^[3]

ICD-11 differences

• ICD-11 separates opioid dependence, harmful pattern of use, and hazardous use, rather than using the DSM-5-TR severity spectrum.^[10]
• ICD-11 also codes opioid intoxication, withdrawal, and induced mental disorders as distinct entities under the same disorders-due-to-substance-use chapter.^[10]

Presentations span acute intoxication, withdrawal, the chronic disorder, and overdose. Recognition of the opioid intoxication and withdrawal syndromes is a board-tested core skill.^[1,11]

Opioid intoxication

• Classic triad: miosis (pinpoint pupils), respiratory depression, and depressed level of consciousness.^[1,11]
• Additional features: bradycardia, hypotension, hypothermia, decreased bowel sounds, slurred speech.^[1,11]
• Meperidine, tramadol, and severe hypoxia may produce mydriasis rather than miosis.^[1,11]

Opioid withdrawal

• Onset 6-12 hours after last short-acting opioid; 24-48 hours for methadone; peaks 36-72 hours; resolves over 5-7 days for short-acting and 10-14 days for methadone.^[1,11]
• Features: dysphoria, anxiety, restlessness, myalgia, arthralgia, abdominal cramps, nausea, vomiting, diarrhea, lacrimation, rhinorrhea, piloerection, yawning, mydriasis, tachycardia, hypertension, low-grade fever.^[1,3,11]
• Subjective severity is high but, in adults without major comorbidity, opioid withdrawal is rarely life-threatening (in contrast to alcohol or sedative-hypnotic withdrawal).^[1,11]

Neonatal opioid withdrawal syndrome can be life-threatening and requires inpatient pharmacologic management.^[12]

Prototypical course

• Initiation often via prescription opioids, recreational use, or self-medication of pain or trauma symptoms.^[1,6]
• Escalation to daily use, tolerance, withdrawal-avoidance use, and transition to injection or to fentanyl-contaminated supplies.^[1-2]
• Repeated cycles of use, attempted abstinence, and return to use; relapse rates after detoxification alone exceed 80% within 12 months.^[1,4]

Red flags

• Track marks, skin abscesses, endocarditis, hepatitis C, HIV seroconversion.^[1]
• Multiple prescribers or pharmacies on the prescription drug monitoring program (PDMP).^[1]
• Recent loss of tolerance (post-incarceration, post-detox, post-hospitalization) markedly increases overdose risk.^[1,4]

The differential turns on distinguishing opioid intoxication and withdrawal from look-alike toxidromes and medical mimics, and on parsing OUD from appropriate opioid analgesia.^[1,11]

Toxidromes and substance-induced states

• Sedative-hypnotic intoxication: CNS depression and respiratory depression without miosis; benzodiazepines alone rarely cause severe respiratory depression but markedly potentiate opioid risk.^[11]
• Alcohol intoxication: nystagmus, ataxia, normal-to-large pupils; smell of alcohol does not exclude opioid co-ingestion.^[11]
• Clonidine and other alpha-2 agonist overdose: CNS depression, bradycardia, hypotension, and miosis — can mimic opioid overdose; partial response to naloxone is reported.^[11]
• Cholinergic toxidrome (organophosphates): miosis, bronchorrhea, bradycardia, secretions — distinguished by SLUDGE/DUMBELS features and treated with atropine and pralidoxime.^[11]
• Pontine hemorrhage: miosis, depressed consciousness; distinguished by focal neurologic findings and imaging.^[11]

Other substance use disorders

• Polysubstance use is the rule, not the exception; document each substance and apply DSM-5-TR criteria per substance.^[1,3]

Medical and psychiatric mimics of withdrawal

• Influenza, gastroenteritis, sepsis, and adrenal insufficiency can mimic features of opioid withdrawal.^[1,11]
• Generalized anxiety disorder and panic disorder share dysphoria, restlessness, and autonomic features but lack the temporal link to opioid cessation.^[1]
• Akathisia and serotonin syndrome should be considered in patients on psychotropics.^[11]

Appropriate analgesic use vs. OUD:

• Physiologic tolerance and withdrawal in patients on chronic prescribed opioids do not, by themselves, establish OUD; the diagnosis requires the additional behavioral, control, and consequence criteria.^[3]

Assessment integrates a substance-focused history, validated rating scales, and targeted medical workup. Avoid ordering unnecessary investigations that delay treatment.^[1,4]

History and interview

• Quantify opioid type, route, daily amount, duration of use, last use, and longest period of abstinence.^[1]
• Screen for polysubstance use, especially benzodiazepines, alcohol, stimulants, and gabapentinoids.^[1]
• Assess prior treatment, prior overdoses, naloxone availability, and treatment goals.^[1,4]
• Screen for suicide risk, psychiatric comorbidity, and trauma exposure.^[1]
• Review the PDMP, prior prescriptions, and any controlled-substance agreements.^[1]

Validated instruments

• Clinical Opiate Withdrawal Scale (COWS): 11-item clinician-rated scale; scores 5-12 mild, 13-24 moderate, 25-36 moderately severe, >36 severe; a score of approximately 8-12 is the conventional threshold for buprenorphine induction with full-agonist short-acting opioids.^[1,4]
• Subjective Opiate Withdrawal Scale and Objective Opiate Withdrawal Scale are alternatives where COWS is unavailable.^[1]
• TAPS, NIDA Quick Screen, and AUDIT-C are useful for primary-care screening.^[1]

Physical examination

• Vital signs (respiratory rate, oxygen saturation, blood pressure, heart rate, temperature).^[1,11]
• Pupils, mental status, bowel sounds, skin (track marks, abscesses), murmurs (endocarditis), and signs of withdrawal.^[1,11]

Laboratory and imaging

• Urine drug screen with confirmatory testing as indicated; standard immunoassays may miss fentanyl, oxycodone, methadone, buprenorphine, and tramadol unless specific assays are ordered.^[1,11]
• HIV, hepatitis B and C, syphilis, and pregnancy testing in those at risk; tuberculosis screening as indicated.^[1]
• Liver and renal function before starting MOUD; ECG in patients starting or on methadone, or with risk factors for QT prolongation.^[4,13]
• Avoid routine head CT, lumbar puncture, or extensive metabolic workup in uncomplicated intoxication or withdrawal.^[1,11]

Medications for opioid use disorder are the cornerstone of treatment and reduce all-cause and overdose mortality; psychosocial interventions complement but do not replace pharmacotherapy.^[1,4]

Pharmacotherapy

• Methadone, buprenorphine, and extended-release naltrexone are the three FDA-approved medications for OUD; strong evidence supports methadone and buprenorphine for retention in treatment and reduction in illicit opioid use.^[1,4,14]
• Strong evidence supports both methadone and buprenorphine for reductions in overdose mortality and all-cause mortality compared to no MOUD.^[4,14]
• methadone 20-30 mg PO QD initial dose with titration by 5-10 mg every several days, target typically 60-120 mg/day for full effect; dispensed only through federally certified opioid treatment programs in the US.^[4,13]
• buprenorphine-naloxone 2-4 mg SL once initial induction dose when COWS is approximately 8-12, with additional doses on day 1 to total 8-16 mg; maintenance typically 16-24 mg/day; available in office-based settings, no special waiver required since the 2023 elimination of the X-waiver.^[4,15]
• Buprenorphine micro-induction (low-dose initiation) is increasingly used when patients cannot tolerate a withdrawal-mediated induction or are using fentanyl, where precipitated withdrawal risk is higher.^[15]
• naltrexone 380 mg IM monthly extended-release after 7-10 days opioid-free (10-14 days for long-acting opioids); requires confirmed abstinence to avoid precipitated withdrawal.^[4,16]
• Comparative evidence: in patients who can be successfully inducted, extended-release naltrexone is non-inferior to buprenorphine-naloxone for opioid relapse, but induction failure is substantially more common with naltrexone in real-world settings.^[16]

Opioid withdrawal management

• Buprenorphine is first-line for managed withdrawal; transition to maintenance pharmacotherapy is the goal.^[4]
• lofexidine 0.54 mg PO QID is FDA-approved for non-opioid management of withdrawal symptoms; clonidine is widely used off-label.^[1,4]
• Symptomatic adjuncts: loperamide for diarrhea, ondansetron for nausea, NSAIDs for myalgia, hydroxyzine for anxiety and insomnia.^[1,4]

Overdose reversal

• naloxone 4 mg IN once intranasal or 0.4-2 mg IM/IV; repeat every 2-3 minutes as needed; the goal is restoration of adequate ventilation, not full arousal.^[5]
• Higher and repeated doses are commonly needed in fentanyl and analogue overdoses due to potency; continuous infusion may be required after large or sustained-release ingestions.^[2,5]
• Post-reversal observation is required because naloxone half-life (30-90 minutes) is shorter than that of most opioids; recurrence of respiratory depression is common.^[5]
• All patients with OUD should be offered take-home naloxone; provision is associated with reduced overdose death rates at the community level.^[5]

Psychotherapy

• Evidence suggests cognitive-behavioral therapy, contingency management, and the matrix model improve outcomes when combined with MOUD; they do not substitute for it.^[1,4]
• Contingency management has the strongest evidence for reducing concurrent stimulant use, which is increasingly prevalent.^[1,4]
• Motivational interviewing supports treatment engagement and is a low-cost addition.^[1,4]

Neuromodulation

• It is uncertain whether transcranial magnetic stimulation, deep-brain stimulation, or transcutaneous auricular vagus-nerve stimulation provide meaningful benefit for OUD; current evidence is limited and they are not standard of care.^[1]

Adjunctive

• Hepatitis C treatment with direct-acting antivirals, HIV pre-exposure prophylaxis, vaccination (hepatitis A, hepatitis B, tetanus), and contraception counseling are commonly recommended.^[1]
• Syringe service programs reduce HIV and hepatitis C transmission and increase entry into treatment.^[1,5]
• Fentanyl test strips support harm reduction by detecting fentanyl contamination of the illicit supply.^[2,5]
• 12-step facilitation and mutual-help groups (Narcotics Anonymous, SMART Recovery) are commonly recommended as adjuncts, though high-quality evidence specific to OUD is limited.^[1]

All three FDA-approved MOUDs carry distinct risk profiles, and the evidence base, while strong for mortality benefit, has important gaps in long-term outcomes and special populations.^[1,4]

Common adverse effects

• Methadone and buprenorphine commonly cause constipation, sweating, sedation, and sexual dysfunction.^[4,7]
• Buprenorphine can produce dose-dependent transaminase elevations; baseline and periodic LFTs are recommended in patients with hepatitis C or other liver disease.^[7]
• Extended-release naltrexone commonly causes injection-site reactions, nausea, and headache.^[11]

Serious or rare adverse effects

• Methadone prolongs the QTc interval in a dose-dependent fashion and has been associated with torsades de pointes, particularly above 100 mg/day or with coadministered QT-prolonging agents.^[9]
• Buprenorphine alone causes minimal respiratory depression because of its partial-agonist ceiling, but combination with benzodiazepines, alcohol, or other CNS depressants can be fatal.^[7]
• Naltrexone discontinuation is followed by a period of reduced tolerance; opioid relapse during this window carries a markedly elevated overdose risk.^[11]

Monitoring burden, withdrawal, and discontinuation

• Methadone in US opioid treatment programs requires near-daily observed dosing for the first months, a logistical barrier to retention.^[4]
• Abrupt discontinuation of methadone or buprenorphine produces a protracted withdrawal syndrome; tapering should be slow and patient-led.^[1,4]
• Discontinuation of MOUD without a clear plan is associated with sharply increased overdose mortality compared with continued treatment.^[14]

Limitations of the evidence base

• Most RCTs predate the fentanyl era and may underestimate the difficulty of buprenorphine induction with high-potency synthetic opioids.^[2,15]
• Long-term (more than 2 years) head-to-head data on methadone versus buprenorphine outcomes remain limited.^[6-7]
• Pregnant patients, adolescents, and patients with serious mental illness are under-represented in pivotal trials.^[1]
• Publication bias favors positive trials of agonist therapy; comparative effectiveness with extended-release naltrexone is constrained by induction failures.^[11]

Treatment principles are consistent across populations, but dosing, formulation choice, and monitoring change in ways that affect mortality.^[1]

Pregnancy and perinatal care

• Methadone and buprenorphine are both recommended in pregnancy; medically supervised withdrawal is associated with high relapse and is not preferred.^[1,16]
• Buprenorphine in pregnancy is associated with shorter neonatal opioid withdrawal syndrome and shorter neonatal hospital stay compared with methadone.^[16]
• Naltrexone is not first-line in pregnancy due to limited safety data.^[1]

Pediatric and adolescent patients

• Buprenorphine is FDA-approved for patients 16 years and older; methadone in patients under 18 requires documented prior failed treatment attempts in the US.^[1,4]
• Adolescents have lower retention on MOUD than adults, but MOUD still outperforms detoxification alone.^[1]

Geriatric patients

• Older adults on chronic opioid therapy are at increased risk of falls, cognitive impairment, and respiratory depression with concurrent benzodiazepines.^[1]
• Dose reductions and slower titrations are appropriate; OUD prevalence in older adults is rising.^[1]

Comorbid medical illness

• Liver disease: buprenorphine doses should be reduced in severe hepatic impairment; monitor LFTs.^[7]
• HIV and hepatitis C: MOUD improves engagement with HIV and HCV care; methadone has more drug-drug interactions via CYP3A4 than buprenorphine.^[1]
• Chronic pain: split dosing of methadone or buprenorphine improves analgesia; full-agonist short-acting opioids for acute pain are not contraindicated and can be used alongside MOUD with dose adjustment.^[1]

Comorbid psychiatric illness

• Co-occurring depression, anxiety, and PTSD are common; integrated treatment with MOUD plus psychiatric pharmacotherapy improves outcomes.^[1,13]
• Stimulant use disorder commonly co-occurs and worsens fentanyl-era overdose mortality; contingency management has the strongest evidence for stimulant use.^[1,13]

OUD is a chronic, relapsing disorder, but mortality and functional outcomes are strongly modifiable by sustained MOUD.^[1,4,14]

Natural history

• Without treatment, annual mortality in OUD is approximately 1-2% per year, an order of magnitude higher than age-matched peers.^[2,14]
• Overdose is the leading cause of death; in the fentanyl era, the time from first use to fatal overdose has shortened.^[2]

Treatment response and retention

• Six-month retention on methadone or buprenorphine is approximately 50-60% in routine care; retention is the strongest predictor of survival.^[6-7]
• All-cause mortality is reduced by roughly 50% during active MOUD compared with out-of-treatment periods.^[14]

Relapse and recovery

• Relapse rates after medically supervised withdrawal without MOUD exceed 80% within 12 months, with elevated overdose risk in the post-detoxification window.^[1,14]
• Long-term recovery is achievable; many patients require multiple treatment episodes before sustained remission.^[1]

Acute opioid overdose is a respiratory emergency, and the field response is naloxone plus airway support.^[5]

Opioid overdose recognition and reversal

• Classic triad: respiratory depression, miosis, and depressed level of consciousness.^[5]
• Naloxone is administered intranasally (4 mg) or intramuscularly (0.4 mg), repeated every 2-3 minutes until adequate respiration is restored.^[5]
• Fentanyl-era overdoses often require repeat naloxone dosing and prolonged observation due to fentanyl's long terminal half-life relative to naloxone.^[2,5]

Acute withdrawal management

• Opioid withdrawal is rarely life-threatening in healthy adults but is severely uncomfortable and drives relapse.^[1]
• Buprenorphine induction in the emergency department or office is feasible once moderate withdrawal is established (COWS at or above 8-12), with same-day linkage to ongoing care.^[1,15]
• Symptomatic agents (clonidine, loperamide, ondansetron, NSAIDs) are adjuncts only, not substitutes for MOUD.^[1]

Hospitalization and safety planning

• Hospitalization is indicated for severe medical complications (aspiration, anoxic injury), serial overdoses, or co-occurring suicidal ideation.^[1]
• Every patient with OUD should be offered take-home naloxone, education for household contacts, and a same-day or next-day MOUD plan; "warm handoff" from the ED to addiction care reduces overdose mortality.^[1,5,15]

OUD treatment policy is in active flux, and several clinical questions remain unsettled.

• Buprenorphine induction in the fentanyl era: traditional induction protocols based on a COWS threshold of 8-12 produce more precipitated withdrawal in patients using fentanyl; low-dose ("micro-induction") protocols are increasingly used but have a thin RCT evidence base.^[15]
• Methadone access: SAMHSA loosened take-home methadone rules during and after the COVID-19 pandemic without increased diversion or overdose, raising questions about whether daily observed dosing should remain the default.^[4]
• Extended-release naltrexone vs buprenorphine: an open-label RCT found extended-release naltrexone non-inferior to buprenorphine once induction was completed, but a larger fraction never reached the start of the naltrexone arm.^[11]
• Involuntary commitment for OUD: several US states authorize civil commitment for substance use disorders; outcome data are limited and ethical concerns about coercion are unresolved.^[1]
• Safer-supply and supervised consumption programs: implemented in Canada and several European countries; observational data suggest reduced overdose mortality, but high-quality comparative trials are lacking.^[1]
• Long-acting injectable buprenorphine vs sublingual: monthly injectable formulations improve adherence in selected patients; comparative effectiveness data with sublingual buprenorphine are still maturing.^[7]

• OUD in DSM-5-TR is diagnosed by 2 or more of 11 criteria within a 12-month period, with severity graded as mild (2-3), moderate (4-5), or severe (6 or more).^[3]
• Tolerance and withdrawal do not count toward an OUD diagnosis when opioids are taken solely under appropriate medical supervision.^[3]
• Methadone, buprenorphine, and extended-release naltrexone are the three FDA-approved medications for OUD; all reduce illicit opioid use, and methadone and buprenorphine reduce all-cause and overdose mortality.^[1,4,7,14]
• Methadone is a full mu-opioid agonist with no ceiling effect; buprenorphine is a partial agonist with a ceiling effect on respiratory depression.^[4,7]
• Buprenorphine has very high mu-receptor affinity and can precipitate withdrawal if started while a full agonist still occupies the receptor; wait until moderate withdrawal (COWS at or above 8-12) before induction.^[7,15]
• Methadone prolongs the QTc interval and can cause torsades, especially at doses above 100 mg/day or with concomitant QT-prolonging drugs.^[9]
• Naloxone reverses opioid-induced respiratory depression; intranasal 4 mg or IM 0.4 mg is standard, repeated until respiration is restored.^[5]
• Naloxone has a shorter duration of action than methadone, buprenorphine, and many synthetic opioids; observe for re-sedation.^[5]
• Extended-release naltrexone requires 7-14 days of opioid abstinence before induction; missing this window precipitates severe withdrawal.^[11]
• Buprenorphine and methadone are both first-line in pregnancy; buprenorphine is associated with shorter neonatal opioid withdrawal syndrome than methadone.^[16]
• Discontinuation of MOUD without a clear plan is followed by sharply elevated overdose mortality compared with continued treatment.^[14]
• In the US, the X-waiver requirement for buprenorphine prescribing was eliminated in 2023; any DEA-registered prescriber may now prescribe buprenorphine for OUD.^[10]
• The opioid overdose classic triad is respiratory depression, miosis, and depressed level of consciousness.^[5]
• Take-home naloxone distribution to patients and their household contacts reduces community overdose deaths and is recommended for every patient with OUD.^[1,5]

No external funding. No conflicts of interest declared. Peer-review status: pending.