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Opioid Receptor Antagonist Pregnancy: Avoid — insufficient safety data; opioid dependence in pregnancy managed with supervised methadone/buprenorphine (safer options with established safety profile)

Naltrexone

Brand names: Nalorex, Vivitrol (extended-release IM)

Adult dose

Dose: Opioid dependence: 25 mg day 1, then 50 mg daily; Alcohol dependence: 50 mg once daily
Route: Oral (daily tablet) or IM (Vivitrol 380 mg monthly — not widely used in UK)
Frequency: Once daily
Max: 50 mg/day orally
Opioid dependence: MUST be opioid-free for ≥7 days (≥10 days for methadone) before starting — precipitates severe withdrawal; alcohol dependence: start after abstinence is established; not a substitute for counselling/support

Paediatric dose

Dose: Not established N/A/kg
Route: N/A
Frequency: N/A
Max: N/A
Not licensed in paediatrics

Dose adjustments

Renal

Use with caution in severe renal impairment

Hepatic

Avoid in acute hepatitis or hepatic failure; LFTs must be normal before starting

Paediatric weight-based calculator

Not licensed in paediatrics

Clinical pearls

  • Mechanism: pure mu/kappa/delta opioid receptor antagonist — blocks euphoric and reinforcing effects of alcohol (via endogenous opioid system) and exogenous opioids; reduces craving by removing reward signal without producing dependence
  • Opioid washout mandatory: naltrexone precipitates immediate, severe withdrawal in opioid-dependent patients; 7-day washout from short-acting opioids, 10-day from methadone; confirm with naloxone challenge test (0.4 mg SC — if no withdrawal in 30 min, safe to start naltrexone)
  • Medical emergency card: patients on naltrexone cannot receive effective opioid pain relief — must carry a card informing medical staff; in genuine emergency, higher opioid doses may override blockade but with respiratory depression risk; regional anaesthesia preferred for surgery
  • COMBINE trial (Anton et al. JAMA 2006): naltrexone ± acamprosate ± behavioural intervention in alcohol dependence — naltrexone reduced heavy drinking days; NICE CG115 recommends as first-line with psychological support
  • Low-dose naltrexone (LDN, 1.5–4.5 mg): off-label use in fibromyalgia, Crohn's disease, multiple sclerosis — proposed TLR4 antagonism mechanism; not licensed but emerging evidence; distinctly different dose regimen from standard use

Contraindications

  • Current opioid dependence without adequate washout (precipitates withdrawal)
  • Acute opioid withdrawal (COWS score ≥12)
  • Hepatic failure
  • Positive urine opioid screen at initiation

Side effects

  • Nausea (most common — take with food)
  • Headache
  • Abdominal pain
  • Fatigue
  • Insomnia
  • Hepatotoxicity (high doses — monitor LFTs)
  • Precipitated opioid withdrawal (if inadequate washout — emergency)

Interactions

  • Opioid analgesics — blocked by naltrexone; patients cannot receive effective opioid analgesia during treatment; medical alert card required
  • Thioridazine — additive lethargy and somnolence

Monitoring

  • LFTs (baseline, at 3 months, then annually)
  • Urine opioid screen (before starting and if adherence questioned)
  • Alcohol intake (AUDIT-C score)
  • Mood and mental health (depression can emerge)
  • Opioid withdrawal symptoms (first dose — COWS score)

Reference: BNFc; BNF 90; COMBINE trial (Anton et al. JAMA 2006); NICE CG115 (Alcohol Use Disorders); NICE CG52 (Opioid Dependence); MHRA SPC Nalorex; BNF Addiction chapter. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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