What it is:Opioid use disorder (OUD) is a chronic, lifelong condition built from two intertwined problems: compulsive use despite harm, and a physically dependent body that goes into withdrawal without the drug. About 70% of US overdose deaths involve an opioid, whether that's a prescription pill, heroin, or illicit fentanyl, and death rates have reached epidemic levels.
The core problem:Nobody actually knows the exact cause. The working model is a triad: the right patient, the right genetic risk, exposed to opioids. Once exposure happens repeatedly, the brain's reward circuitry and the receptor-level physiology both adapt, and stopping isn't a willpower problem anymore, it's a neurobiological and physiologic one.
What you do about it:Medication for opioid use disorder (MOUD) is first-line, not a last resort after counseling fails. The three FDA-approved options, methadone, buprenorphine, and naltrexone, all work at the same mu-opioid receptor but from three completely different angles. Long-term medication is what actually lowers the risk of accidental overdose and relapse.
Every drug in this chapter is defined by what it does at one receptor, the mu-opioid receptor. Methadoneis a full agonist, buprenorphineis a partial agonist, naltrexoneis a full antagonist. Learn where each one sits on that agonist-to-antagonist spectrum and you can predict its safety profile, its induction rules, and why switching between them is dangerous if done wrong.
OUD is diagnosed clinically, not with a lab test. DSM-5 defines it as a problematic pattern of use causing clinically significant impairment or distress, shown by at least 2 of 11 criteriain the same 12-month period.
| Domain | What it captures |
|---|---|
| Impaired control | Using more or longer than intended, unsuccessful attempts to cut down, a lot of time spent obtaining/using/recovering, cravings |
| Social impairment | Failure to fulfill role obligations, continued use despite social/interpersonal problems, giving up activities |
| Risky use | Recurrent use in physically hazardous situations, continued use despite knowing it's causing a physical or psychological problem |
| Pharmacological criteria | Toleranceand withdrawal(criteria 10 and 11) |
| Severity | # criteria met |
|---|---|
| Mild | 2–3 |
| Moderate | 4–5 |
| Severe | 6 or more |
Tolerance and withdrawal are still counted as criteria even when they're the expectedphysiologic result of appropriate long-term opioid therapy for pain. Don't confuse "physically dependent" with "has OUD," but do remember DSM-5 still counts those two items toward the total.
| COWS score | Withdrawal severity |
|---|---|
| 5–12 | Mild |
| 13–24 | Moderate |
| 25–36 | Moderate/severe |
| >36 | Severe |
The etiology of OUD is genuinely unknown at the population level, but the receptor pharmacology of the treatments is well understood, and that's what actually matters for practice.
The circuit that matters most is the mesolimbic pathway, dopamine projecting from the ventral tegmental area (VTA) to the nucleus accumbens (NAc). Opioid use drives dopamine release here, which is what produces the euphoric "reward." With repeated use, something shifts: dopamine firing stops tracking the drug itself and starts firing in response to conditioned cues, the street corner, the paraphernalia, the people associated with use. That's why cravings can hit hard in a specific location years into recovery even without any drug in the room. The prefrontal cortexgoverns the preoccupation/anticipation phase (the obsessive planning and inability to focus on anything else), and dysfunction there is what drives compulsive, executive-function-defying behavior.
All three FDA-approved OUD medications act at the same mu-opioid receptorthat heroin, fentanyl, and oxycodone act on. What differs is how muchthey activate it and how stronglythey bind.
| Drug | Receptor behavior | What that buys you |
|---|---|---|
| Methadone | Full mu agonist, long half-life | Satisfies the receptor steadily all day without the peak-and-crash of a short-acting opioid, so cravings and withdrawal resolve without producing a "high" when dosed correctly |
| Buprenorphine | Partial mu agonist, very high binding affinity, low intrinsic activity | Occupies the receptor strongly (blocking other opioids from binding) but only partially activates it, which caps respiratory depression above a certain dose (the ceiling effect) - safer in overdose, but its high affinity means it can rip a full agonist off the receptor |
| Naltrexone | Full mu antagonist | Blocks the receptor completely: no analgesia, no euphoria if the patient uses on top of it, but also nothing to relieve withdrawal on its own |
Buprenorphine's affinity for the mu receptor is higherthan fentanyl's, but its intrinsic activity is lower. Give it to a patient whose receptors are currently occupied by a full agonist and it displaces that agonist, then activates the receptor far less than the full agonist did. Net effect: a sudden, steep dropin opioid effect, which the body reads as withdrawal, and reads it fast and hard. This is precipitated withdrawal, and it's the single biggest safety issue in starting buprenorphine.
Chronic opioid exposure downregulates the body's own opioid signaling, which is what produces tolerance (needing more drug for the same effect) and physical dependence (needing the drug just to feel normal). Take the drug away and the system that had adapted to its constant presence is suddenly unopposed, driving the autonomic surge of withdrawal, which is why alpha-2 agonists like clonidine, which dampen central sympathetic outflow, blunt withdrawal symptoms like tachycardia, hypertension, and diaphoresis even though they don't touch the opioid receptor at all.
| Intoxication | Withdrawal |
|---|---|
| Euphoria, dysphoria, slurred speech, miosis(pinpoint pupils), apathy, sedation, attention impairment | Lacrimation, mydriasis, piloerection, diaphoresis, diarrhea, yawning, muscle aches, insomnia |
| Severe/overdose: decreased breathing, pulmonary edema, loss of consciousness, death | Anxiety, tachycardia, hypertension, chills (the sympathetic surge alpha-2 agonists target) |
Pupils flip depending on which state you're looking at: miosis with intoxication, mydriasis with withdrawal.A pinpoint-pupil patient who's barely breathing is an overdose. A wide-pupil patient who's sweating, yawning, and crampy is withdrawing.
Opioid withdrawal is miserable but not typically life-threatening. If a patient in apparent withdrawal develops delirium, think about withdrawal from something else, classically alcohol, which can kill. Don't anchor on the opioid history and miss it.
Loperamide and dextromethorphanare OTC drugs that cause CNS depression and mild hallucinogenic effects at high doses. People misuse loperamide with agents that boost its ability to cross the blood-brain barrier: verapamil, methadone, cimetidine, or grapefruit juice.At supratherapeutic doses loperamide can cause QTc prolongation, torsades de pointes, ventricular dysrhythmias, syncope, and cardiac arrest.Dextromethorphan overdose, notably, canbe treated with naloxone.
Diagnosis is clinical: history and DSM-5 criteria. Labs support safety assessment, not diagnosis.
Goals of treatment:stop use of the substance, end substance-seeking behavior, and return to normal functioning. For acute withdrawal specifically, the goal is preventing progression to a life-threatening severity while keeping the patient comfortable and functional enough to engage with treatment. Because OUD is chronic, long-term medication is what reduces the risk of accidental overdose and relapse, it is not a short course you finish and stop.
Programs like Narcotics Anonymous and other 12-step models help many patients, but participation should not be requiredto receive medication. Behavioral therapy gets introduced if and when the patient is ready for it, not as a gatekeeping prerequisite.
Full agonist. Most potent analgesia of the three. Requires daily visits to a certified Opioid Treatment Program (OTP).
Partial agonist with a ceiling effect. Office-based, more accessible, safer overdose profile.
Full antagonist. Zero abuse potential, zero withdrawal relief. Requires full detox first.
This is a classic case-based decision. Methadonehas a "large" effect size for chronic pain because it's a full agonist, so it's the better call for a patient who also needs real analgesia, but it demands daily OTP visits, which is a brutal barrier for anyone rural or without reliable transportation. Buprenorphineis safer (ceiling effect on respiratory depression) and can be prescribed and dispensed through an outpatient pharmacy, which makes it the more practical choice when access and safety outweigh the need for potent pain control.
Getting a patient ontobuprenorphine safely is its own skill, because of the precipitated-withdrawal risk described above. There are three approaches, and picking the right one depends on what opioid the patient is using and how badly they want to avoid a withdrawal window.
| Approach | Ideal candidate | Physiologic goal | Typical protocol |
|---|---|---|---|
| Standard induction | Short-acting opioid users (heroin, oxycodone) able to tolerate some withdrawal | Let the full agonist wash out so receptors are empty before dosing | Wait for objective withdrawal (COWS generally ≥8, with buprenorphine REMS specifying ≥12 for the very first dose); 4 mg initial, repeat in 1–2 hours if COWS still high; target 8–16 mg day 1 |
| Low-dose (micro/Bernese) induction | Fentanyl users (lipophilic depot storage makes standard induction brutal), patients transitioning off high-dose methadone, or pain patients unwilling to stop their full agonist | Gradually replace the full agonist at the receptor level, no withdrawal required at all | Continue the full agonist while up-titrating tiny buprenorphine doses, e.g. 0.2–0.5 mg BID day 1, 1–2 mg BID day 2, increasing slowly over 4–7 days while tapering off the full agonist around day 7 |
| Macro-induction (rapid/high-dose) | High-tolerance patients (heavy fentanyl use) presenting in withdrawal, ED/inpatient settings | Saturate mu receptors immediately with sheer buprenorphine dose | 16–32 mg administered immediately or over a short interval (e.g. 8 mg ×3 over 1 hour); COWS >8 usually still required first |
Fentanyl is highly lipophilic and accumulates in fat tissue, a "depot effect" that means the drug keeps leaching back out long after the last use. That makes the standard "wait for withdrawal" approach unreliable for fentanyl users: they might still be too full of opioid to dose safely, or they might suffer for an unpredictably long time. Low-dose induction sidesteps the whole problem by never requiring a withdrawal window in the first place, at the cost of a complicated, multi-day dosing schedule.
| Symptom relief | Serum level (ng/mL) | Typical dose to get there |
|---|---|---|
| Craving reduction | 1–2 | ~4–8 mg/day |
| Withdrawal suppression | 2–3 | ~8 mg/day (serum ≈1.8 ng/mL) |
| Blockade of other opioids | ≥3 | 16–24 mg/day |
| Full stabilization | 3–5 | 16–24 mg/day (or monthly long-acting injection) |
8 mg/day feels like a reasonable maintenance dose, and it does relieve withdrawal, but it does notreliably reach the ≥3 ng/mL threshold needed to blockother opioids. A patient who says they're "not sick, but still have cravings and I'm worried I'll use if I see my dealer" needs a dose increase toward the 16–24 mg/dayrange, not reassurance that 8 mg is enough for everyone.
| Methadone (Schedule II, OTP only) | ||
| Initial dose | 10–30 mg once daily; reduce to 10–20 mgif age >60 or interacting medications identified | |
| Titration | Increase 5–10 mg no sooner than every 4–7 days, based on clinical response | |
| Maintenance | 60–120 mg/day | |
| Buprenorphine transmucosal products (Schedule III) | ||
| Buprenorphine SL tablet (generic) | 2 mg, 8 mg | Target 16 mg/day; range 4–24 mg/day |
| Suboxone (bup/naloxone) SL tablet | 2/0.5 mg, 8/2 mg | Target 16/4 mg/day; range 4/1–24/6 mg/day |
| Suboxone SL or buccal film | 2/0.5 to 12/3 mg | Target 16/4 mg/day; range 4/1–24/6 mg/day |
| Zubsolv SL tablet | 0.7/0.18 to 11.4/2.9 mg | Target 11.4/2.9 mg/day |
| Bunavail buccal film | 2.1/0.3 to 6.3/1 mg | Target 8.4/1.4 mg/day |
| Extended-release / depot products | ||
| Buprenorphine implant (Probuphine) | 4 implants subdermal, left in place 6 months; only for patients stable on ≤8 mg/day transmucosal equivalent for ≥3 months | |
| Buprenorphine SC injection (Sublocade) | 300 mgSC monthly ×2 months, then 100 mgmonthly; requires prior stability on 8–24 mg/day transmucosal | |
| Naltrexone XR injection (Vivitrol) | 380 mg IMin the gluteal area, alternating buttocks, every 4 weeks | |
| Naltrexone oral (Revia) | ||
| Initial | 25 mg daily with food after confirmed opioid-free period | |
| Titration | If no signs of precipitated withdrawal, increase to 50 mg daily | |
| Required opioid-free window before starting naltrexone (either formulation) | ||
| Short-acting opioids | 7–10 daysopioid-free | |
| Long-acting opioids | 10–14 daysopioid-free | |
| Adjunct for withdrawal symptoms (not MOUD) | ||
| Clonidine / lofexidine | Alpha-2 agonists, dosed to attenuate anxiety, tachycardia, hypertension, chills, piloerection | |
Methadone is a full mu-opioid agonistwith a long half-life that allows once-daily dosing, suppressing withdrawal symptoms and cravings for maintenance therapy. It's the only one of the three with genuinely potent analgesic activity, which is why it's the go-to for a patient who has both OUD and significant chronic pain.
Methadone's peak respiratory depressant effect occurs later and lasts longerthan its peak analgesic effect. That mismatch is exactly why initiation and dose titration are the highest-risk windows, and why converting a patient onto methadone from another opioid has to be done cautiously. Concomitant alcohol or benzodiazepines dramatically raise overdose risk and should always be assessed for.
Methadone has extensive cytochrome P450 interactionsand carries a real risk of QT prolongation, which climbs further when combined with other QT-prolonging drugs.
For OUD, methadone can only be dispensed through a federally certified Opioid Treatment Program (OTP), regulated jointly by the DEA and SAMHSA. It can, however, be given to a patient during a hospital admissionfor treatment of unrelated health conditions without that same restriction.
Buprenorphine is available alone or combined with naloxone (the naloxone is there to deter IV misuse; it's poorly absorbed sublingually but active if injected). Because it's a partial agonist it provides someintrinsic pain control and has a ceiling effecton respiratory depression, which is the core reason it's considered safer than a full agonist. It can still be misused, though, and sedation/intoxication is the main adverse effect seen.
Prescribing:a Schedule III drug that historically required a special federal waiver to prescribe outside an OTP. Regulations here are actively changing, so always check current DEA guidance rather than assuming the old X-waiver rules still apply.
Starting the first dose:the buprenorphine REMS recommends a COWS score of 12 or higherbefore the very first dose, specifically to avoid dosing someone who's still too full of a full agonist (precipitated withdrawal risk). Emergency department initiation is a recognized and encouraged option for patients presenting in withdrawal who are appropriate candidates.
Phases of treatment:inductionswitches the patient from the misused opioid to buprenorphine at the minimum dose that eliminates use, withdrawal, and craving with minimal adverse effects (initial doses should be observed); stabilizationbegins once withdrawal/cravings are controlled with minimal side effects; maintenancemay be indefinite and shifts focus to the psychosocial drivers of the disorder.
Sublingual tablet, sublingual film, and buccal film are not equivalentto each other. Switching a patient between products can require a dose adjustment, and it's worth monitoring after any product change.
Product-specific adverse effects:sublingual/buccal formulations can cause oral hypoesthesia, oral mucosal erythema, and glossodynia, on top of the class effects of constipation, vomiting, dizziness, sedation, insomnia, and blurred vision. Respiratory depression risk is highest with concurrent CNS depressants, especially benzodiazepines. Tobacco use before dosing decreases buprenorphine absorption, blunting its effect.
Naltrexone is a mu-opioid antagonistavailable as an oral tablet or an extended-release IM injectable (Vivitrol). It does not provide analgesia and does nothing to relieve cravings or withdrawal on its own, its entire value is blocking the receptor so that using an opioid on top of it does nothing.
Give naltrexone to a patient with current physiologic dependence or recent opioid useand you will precipitate severe withdrawal, because you're slamming a full antagonist onto receptors that are still occupied. Confirm the opioid-free window first, 7–10 daysfor short-acting opioids, 10–14 daysfor long-acting ones, before the first dose.
Extended-release injectable naltrexone is FDA-approved specifically for use following opioid detoxificationto help prevent relapse; it must be administered by a healthcare provider using the manufacturer-provided needle, sized to the patient at each visit, with the injection site monitored closely for pain or induration.
Adverse effects:swelling, bruising, and pruritus at the injection site with the XR formulation; elevated liver function tests can occur with either formulation, use with caution and monitor. Both naltrexone and its active metabolite are renally excreted, so use caution with renal impairment; the XR injection needs dose adjustment in mild-to-moderate hepatic impairment (no data in severe impairment).
Clonidine or lofexidine attenuate the sympathetic surgeof withdrawal, anxiety, tachycardia, hypertension, chills, and piloerection, by dampening central noradrenergic outflow. They aren't MOUD themselves and don't touch cravings the way buprenorphine or methadone do, but they're a useful adjunct for mild withdrawal and can help bridge a patient into buprenorphine initiation.
Adverse effects:hypotension, dizziness, sedation, exactly what you'd expect from central alpha-2 agonism, so watch blood pressure and mental status, especially if stacking with other sedating withdrawal-management medications.
Naloxone is a competitive mu-opioid receptor antagonistand the single most important tool for reducing opioid-related deaths, it can revive an unconscious patient with respiratory depression. Because it's an antagonist, it can precipitate withdrawal in a dependent patient, but that's a far better outcome than the alternative.
| Delivery form | Directions |
|---|---|
| IM injection(two 0.4 mg/mL vials) | Inject 1 mL IM in shoulder or thigh on signs of overdose. Call 911 immediately. May repeat once in 2–3 minutes if minimal/no response |
| Auto-injector(Evzio) | Apply to outer thigh, hold 5 seconds; voice prompts guide use. Call 911. May repeat once in 2–3 minutes |
| Nasal spray(Narcan, 4 mg) | Full spray contents in one nostril. Call 911. May repeat once in 2–3 minutes using the other nostril |
| Intranasal atomizer(2 mg/2 mL syringe) | Spray half the syringe into each nostril. Call 911. May repeat once in 2–3 minutes |
Naloxone does not cause meaningful adverse effectsif given to someone who isn't actually on opioids, so when in doubt, give it. It also still works on the opioid component of a mixed intoxication (opioid plus benzodiazepine or alcohol), it just won't reverse the non-opioid part, so the patient may still need further support even after the opioid effect is reversed.
Naloxone's duration of action is short, roughly 30–90 minutes. Fentanyl and other lipophilic opioids have a depot effect, stored in fat and slowly released, so their effective duration outlasts naloxone. A patient who's revived can become somnolent againonce the naloxone wears off, well after they seemed fine. This is why anyone reversed from an opioid overdose needs extended monitoring, not just a single dose and a pat on the back.
Longer-acting parenteral antagonists like nalmefeneexist, but longer isn't automatically better: nalmefene's extended duration risks precipitating a prolonged, severe withdrawal that's hard to reverseonce it's started. Also worth knowing: an opioid's binding affinity (Ki)matters for reversal. Drugs with very strong receptor binding, like fentanyl or carfentanil, are harder to displace and can require higher or repeated naloxone doses.
| Situation | What to know |
|---|---|
| Age >60 | Reduce methadone initial dose to 10–20 mg |
| Renal impairment | Use naltrexone (both formulations) cautiously, the drug and its active metabolite are renally excreted |
| Hepatic impairment | Naltrexone XR needs dose adjustment in mild-to-moderate impairment (no data in severe); buprenorphine implant and SC injection are not recommended in moderate-to-severe hepatic impairment |
| Pregnancy | Patients on buprenorphine should be counseled to inform their prescriber immediately if pregnant or planning pregnancy |
| Precipitated withdrawal | Can happen with buprenorphine dosed too early, or naltrexone given to a still-dependent patient. Both are avoidable by respecting the COWS/washout thresholds above |
| Parameter | When | Watching for |
|---|---|---|
| COWS/OOWS score | During induction and withdrawal management | Symptom severity and response to treatment |
| Liver function tests | Baseline, then periodically on buprenorphine or oral naltrexone | Hepatotoxicity, elevated LFTs |
| Renal function | Baseline and periodically on naltrexone | Impaired clearance of drug and active metabolite |
| ECG / QTc | Baseline and with methadone dose changes, especially if on other QT-prolonging drugs | QT prolongation, arrhythmia risk |
| Respiratory status/sedation | During methadone initiation and titration | The highest overdose-risk window in the whole chapter |
| Urine toxicology and PDMP | Regularly throughout maintenance | Adherence, non-prescribed substance use, diversion |
| Injection site | Each Sublocade or Vivitrol administration | Induration, abnormal pain, tampering |
| Pill/film count | Each buprenorphine maintenance visit | Adherence and diversion |
| Symptoms, cravings, treatment goals | Every visit | Relapse risk and engagement with the treatment plan |