Naltrexone
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History
Naltrexone (originally EN-1639A) was synthesized at Endo Laboratories (later acquired by DuPont) in 1965 by Matthew Fishman, Harold Blumberg, and colleagues, in a structure-activity exploration of naloxone analogs intended to extend duration of action and improve oral bioavailability.[citation needed]
The National Institute on Drug Abuse (NIDA) funded extensive clinical development through the 1970s. FDA approval for opioid use disorder followed in 1984 under the trade name Trexan (later renamed ReVia). The initial clinical reception was muted: oral naltrexone proved disappointing in OUD treatment, with adherence as low as 20-30% at 6 months in unsupervised outpatient populations, leading to relapse rates comparable to no medication treatment in some cohorts.
The pivot to alcohol use disorder came from work by Joseph Volpicelli, Charles O'Brien, and colleagues at the University of Pennsylvania, who hypothesized that endogenous opioid release during alcohol consumption contributed to the reinforcing properties of drinking and that naltrexone blockade would reduce drinking quantity and craving. The 1992 Archives of General Psychiatry paper (Volpicelli et al PMID 1417505) established efficacy in a placebo-controlled trial.[citation needed] FDA approval for AUD followed in 1994. The COMBINE trial (Anton RF et al, JAMA 2006 PMID 16670409) is the largest AUD pharmacotherapy trial; it confirmed modest naltrexone efficacy without an obvious benefit from combining naltrexone with acamprosate or with intensive counseling, consistent with subsequent meta-analyses showing modest but real treatment effects.
Vivitrol (extended-release injectable naltrexone) was approved by FDA for AUD in 2006 and for OUD in 2010, addressing the oral-adherence problem that had limited oral naltrexone's OUD utility. Vivitrol use in OUD remains controversial because of the induction challenge (7-14 day opioid-free interval), but it is the dominant modern naltrexone formulation in MOUD practice.
Contrave (naltrexone 8 mg + bupropion 90 mg sustained-release combination) was approved by FDA for weight management in 2014, based on the hypothesis that naltrexone-mediated blockade of POMC-neuron feedback inhibition enhances bupropion's effect on hypothalamic energy balance.
Low-dose naltrexone (LDN) at 1-4.5 mg/day, far below the 50 mg therapeutic dose for AUD/OUD, gained popularity in the 2000s for off-label use in fibromyalgia, chronic pain, Crohn disease, and autoimmune conditions, primarily through patient-advocacy networks rather than formal regulatory channels. The evidence base for LDN has expanded gradually; modest trial evidence supports use in fibromyalgia and possibly in Crohn disease, but most LDN indications remain investigational.Experience
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Problems
Titration strategies
- Short-acting opioids (heroin, oxycodone, hydrocodone): at least 7 days - Long-acting opioids (methadone, sustained-release morphine, buprenorphine): at least 10-14 days - Confirm absence of withdrawal symptoms before challenge
Naloxone challenge (recommended before first naltrexone dose): 0.4 mg naloxone IV or SC; observe 20-30 minutes for precipitated withdrawal. If no withdrawal, proceed with naltrexone. If withdrawal emerges, wait longer and retest.
Oral induction: 25 mg test dose; if tolerated, 50 mg/day starting next day.
Vivitrol induction: 380 mg IM after confirming opioid-free status. The 4-week duration of action means a single induction error commits the patient to weeks of inappropriate antagonist exposure.Effects
- <effect>: unknown ref "opioid-blockade"
- <effect>: unknown ref "precipitated-withdrawal"
- <effect>: unknown ref "reduced-alcohol-craving"
- Nausea no reports yet no reports yetCommon at initiation; usually self-limited over 1-2 weeks.Did you experience this?How often have you seen this?How was it? (-100 worst, +100 best)How was it? (-100 worst, +100 best)
- Headache no reports yet no reports yetCommon.Did you experience this?How often have you seen this?How was it? (-100 worst, +100 best)How was it? (-100 worst, +100 best)
- <effect>: unknown ref "fatigue"
- Dizziness no reports yet no reports yetCommon.Did you experience this?How often have you seen this?How was it? (-100 worst, +100 best)How was it? (-100 worst, +100 best)
- Insomnia no reports yet no reports yetReported; conversely some patients report improved sleep.Did you experience this?How often have you seen this?How was it? (-100 worst, +100 best)How was it? (-100 worst, +100 best)
- Anxiety no reports yet no reports yetReported, especially early; may reflect blockade of endogenous opioid tone.Did you experience this?How often have you seen this?How was it? (-100 worst, +100 best)How was it? (-100 worst, +100 best)
- Depression no reports yet no reports yetMixed signal; some reports of dysphoria from endogenous-opioid blockade; other studies do not show consistent depressive signal. The 2010s data from Vivitrol OUD trials show no excess suicide risk.Did you experience this?How often have you seen this?How was it? (-100 worst, +100 best)How was it? (-100 worst, +100 best)
- <effect>: unknown ref "hepatotoxicity"
- <effect>: unknown ref "injection-site-reaction"
- <effect>: unknown ref "opioid-analgesia-failure"
Pharmacokinetics
Absorption
Oral peak plasma 1 hour; bioavailability variable (~5-40%) with mean ~5-10% for parent due to extensive first-pass; the 6-beta-naltrexol metabolite reaches higher systemic exposure and contributes most of the clinical effect. Food does not significantly affect absorption. Vivitrol IM produces an early plasma peak at 2-3 days followed by sustained microsphere-mediated release over the 4-week dosing interval, bypassing first-pass entirely.[1]Distribution
Plasma protein binding ~21% (relatively low). Volume of distribution ~16 L/kg. Crosses blood-brain barrier readily (required for therapeutic effect). Crosses placenta in small amounts; excreted in breast milk at low concentrations.[1]Metabolism
Primary metabolism via cytoplasmic dihydrodiol dehydrogenase to 6-beta-naltrexol (active mu-antagonist, longer half-life than parent and major contributor to clinical effect). Minor CYP-mediated metabolism produces 2-hydroxy-3-methoxy-6-beta-naltrexol and other inactive metabolites. Importantly, naltrexone is NOT a significant CYP3A4 or CYP2D6 substrate, so the medicine has a notably clean drug-interaction profile - distinguishing from methadone and buprenorphine. Both parent and metabolite are conjugated with glucuronic acid for excretion.[1]Elimination
Predominantly renal as glucuronide conjugates (~70%); fecal ~30%. Half-life parent ~4 hours, 6-beta-naltrexol ~13 hours after oral dosing. Vivitrol exhibits a terminal half-life of 5-10 days with sustained microsphere-mediated release producing therapeutic plasma concentrations throughout the 4-week dosing interval.[1]Pharmacodynamics
Naltrexone is a pure opioid receptor antagonist (no intrinsic activity at any opioid receptor) with high mu-opioid receptor affinity (Ki ~0.1-1 nM) and substantial kappa-opioid receptor affinity (Ki ~5-10 nM); delta affinity is lower. The mu-receptor blockade is the basis for both OUD efficacy (blocks reinforcing effect of subsequent opioid use) and AUD efficacy (blocks the endogenous-opioid contribution to alcohol-induced reward). The kappa-receptor antagonism may contribute additional efficacy in alcohol craving and is the subject of ongoing research in stress-induced relapse.
6-beta-naltrexol, the major active metabolite, has similar receptor pharmacology to the parent and is the dominant contributor to net pharmacological effect given its higher systemic exposure after oral dosing.
In the LDN paradigm, the proposed mechanism is transient mu-receptor blockade producing rebound elevation in endogenous opioid tone; supplementary proposed mechanisms include glial cell modulation, TLR4 signaling effects, and other non-opioid actions. These mechanisms remain hypothetical; the LDN evidence base is modest and growing.Interactions
The clinically important interactions for prescribers:
- Opioid analgesics (mu-agonists). Naltrexone BLOCKS the analgesic and respiratory-depressant effects of all mu-opioid analgesics at therapeutic doses. Patients on naltrexone who require opioid analgesia for acute pain, post-surgical care, or trauma management need specialist consultation; options include holding naltrexone (only feasible for oral; Vivitrol cannot be reversed), using very high opioid doses to overcome blockade (risks delayed respiratory depression as naltrexone wears off), or using non-opioid analgesia. Patients should wear medical alert ID indicating naltrexone treatment.
- Mixed agonist-antagonists (buprenorphine, nalbuphine, butorphanol). Naltrexone precipitates withdrawal in patients on these medications. Buprenorphine in particular must be discontinued and a 7-14 day washout completed before naltrexone induction.
- Antidiarrheal opioids (loperamide, diphenoxylate). Therapeutic effect blocked; less clinically critical.
- Cough/cold medications containing codeine, hydrocodone, dextromethorphan. Effects blocked; less critical except for DXM-containing products where dissociative effect would also be blocked.
- Disulfiram + naltrexone. Sometimes co-prescribed in AUD; LFT monitoring more frequent due to combined hepatotoxicity signal.
- Acamprosate + naltrexone. Sometimes co-prescribed in AUD; COMBINE trial showed no clear synergy but no antagonism either.
- No significant CYP-mediated interactions. Naltrexone is metabolized by a non-CYP pathway (dihydrodiol dehydrogenase), so the typical CYP3A4 and CYP2D6 inhibitor/inducer lists do not significantly affect naltrexone exposure - a clinically distinguishing feature from methadone and buprenorphine.
Pregnancy and lactation
Pregnancy data on naltrexone are limited but accumulating. Observational signals do not show clear teratogenicity or major-malformation excess in either oral or depot-naltrexone-exposed pregnancies. The Australian naltrexone-implant cohort (which differs from Vivitrol but is the largest registry of depot-naltrexone-in-pregnancy data) showed comparable maternal-fetal outcomes to background pregnancy populations.
The clinical decision in pregnancy is challenging: - Untreated OUD or AUD carries substantial maternal-fetal risk (overdose, premature labor, fetal alcohol spectrum disorder) - Abrupt discontinuation of naltrexone-MOUD can precipitate opioid relapse with overdose risk in patients whose tolerance has decreased during MOUD treatment - Continuing naltrexone-MOUD through pregnancy avoids those risks but commits to monitoring without strong safety data - Switching to methadone or buprenorphine MOUD requires precipitated-withdrawal navigation in the opposite direction
ACOG (Committee Opinion 711) and ASAM guidance has historically favored methadone or buprenorphine as MOUD in pregnancy. Recent guidance recognizes naltrexone-MOUD as an acceptable option for patients already maintained on it. Specialist consultation is appropriate.[citation needed]
Breast milk: naltrexone and 6-beta-naltrexol both pass into breast milk at low concentrations; M/P ratio approximately 0.7; relative infant dose <2% of maternal weight-adjusted dose. Breastfeeding is generally considered acceptable in stable maintained patients, though specific guidance varies.Monitoring
Baseline before initiation:
- Liver function tests (the boxed hepatotoxicity warning at supratherapeutic doses; clinical caution in active hepatitis or hepatic failure)
- Pregnancy status in females of reproductive potential
- Opioid-free interval confirmation (7-14 days, see Induction titration block) - the central OUD safety pivot
- Naloxone challenge (recommended for OUD induction): 0.4 mg IV/SC naloxone; observe 20-30 min for precipitated withdrawal
- Acute hepatitis or hepatic failure: contraindicated until stabilized
- Documentation of medical alert ID requirement for the patient
Ongoing:
- LFTs at baseline + at 3 months + every 6-12 months (less intensive than the boxed warning might suggest at therapeutic 50 mg/day dose)
- Adherence assessment (oral): self-report + collateral; consider switching to Vivitrol if oral adherence problematic
- Injection-site assessment at each Vivitrol injection
- Pregnancy testing in females of reproductive potential at intake + as clinically indicated
- PDMP review at intake + regularly per state requirements (the antagonist mechanism means PDMP review is less acute than for agonists but still part of comprehensive OUD care)
- For AUD: drinking-pattern assessment using validated tools (AUDIT-C, TLFB)
Patient counseling
Opioid blockade and acute pain. While you are on naltrexone, opioids will not work for pain relief. If you have surgery, injury, or any acute pain that would normally require opioid analgesia, your provider needs to know you are on naltrexone immediately. WEAR MEDICAL ALERT ID indicating naltrexone treatment.
Opioid-free interval before starting (OUD). You must be completely off opioids for 7-14 days before your first naltrexone dose, longer for methadone or buprenorphine. Starting naltrexone too soon will cause severe acute withdrawal that lasts hours.
Reduced opioid tolerance. While on naltrexone you lose tolerance to opioids. If you stop naltrexone and resume opioid use, the dose that previously felt routine may now be a fatal overdose dose. Keep naloxone (Narcan) available.
Vivitrol depot is one-way (one month). Once Vivitrol is injected, the blockade lasts approximately 4 weeks and cannot be reversed. Plan ahead for surgical or dental procedures.
Hepatotoxicity. Although less concerning at the therapeutic dose, naltrexone can affect the liver. Report yellow skin or eyes, dark urine, right upper quadrant pain, or unusual fatigue. We will check liver tests periodically.
Alcohol use (AUD). Naltrexone reduces craving and the rewarding effect of alcohol, but does NOT prevent intoxication or make alcohol "safe." Driving impaired remains illegal and dangerous. The medicine works best paired with behavioral treatment, support groups, and lifestyle change.
Pregnancy and breastfeeding. If you become pregnant on naltrexone, contact your prescriber promptly. The decision to continue naltrexone vs switch to methadone/buprenorphine in pregnancy is individualized.
Naloxone awareness. Even with naltrexone, household members should have naloxone available; tolerance loss during treatment means accidental overdose risk if patient deviates.
Vivitrol injection-site care. The injection site may be sore or swollen for several days; this is expected. Spreading redness, fever, severe pain, or drainage requires immediate evaluation - rare cases of cellulitis, abscess, and tissue necrosis have been reported.Relevant anecdote
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