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Opioid Receptor Antagonist Pregnancy: Avoid — limited data

Naltrexone

Brand names: Naltrexene, Opizone, Vivitrol (extended-release IM — US)

Adult dose

Dose: Opioid dependence: 25 mg OD on day 1, then 50 mg OD. OR supervised regime: 100 mg on Mon/Wed, 150 mg on Friday. Alcohol dependence: 50 mg OD.
Route: Oral
Frequency: OD (or 3× weekly supervised)
Max: 150 mg on supervised dose days; 50 mg OD for alcohol
CRITICAL: patient must be opioid-free for at least 7–10 days before starting (longer for methadone, 10+ days) — confirm with naloxone challenge or urine drug screen. Precipitates severe acute opioid withdrawal if opioids present.

Paediatric dose

Route: N/A
Frequency: N/A
Max: Not licensed in children
Not established in paediatric patients

Dose adjustments

Renal

No dose adjustment required

Hepatic

Caution in hepatic disease — hepatotoxic in high doses; monitor LFTs

Clinical pearls

  • Emergency surgery: if patient on naltrexone needs opioid analgesia urgently — use regional anaesthesia if possible; higher opioid doses needed, with risk of respiratory depression when naltrexone wears off; contact addiction team
  • Naloxone challenge: before starting naltrexone, give naloxone 0.8 mg IM — if no withdrawal within 20 min, safe to start
  • Alcohol: naltrexone reduces craving and relapse — COMBINE study shows efficacy with counselling
  • Do not give naltrexone in patients actively using opioids — severe precipitated withdrawal within minutes

Contraindications

  • Current opioid use (precipitates severe withdrawal)
  • Acute hepatitis or liver failure
  • Concurrent opioid analgesics (patient will be resistant to pain relief)

Side effects

  • Nausea/vomiting (early — take with food)
  • Abdominal pain
  • Headache
  • Hepatotoxicity (at high doses >300 mg/day — not therapeutic doses)
  • Insomnia
  • Anxiety

Interactions

  • Opioids — precipitates severe acute withdrawal if patient is opioid-dependent; blocks analgesic effect of opioids (medical emergency if opioid analgesia needed)
  • Thioridazine — additive sedation

Monitoring

  • LFTs (baseline and periodic)
  • Urine opioid screen before starting
  • Signs of opioid withdrawal on initiation
  • Adherence and relapse monitoring

Reference: BNFc; BNF; NICE NG58 Opioid Dependence; COMBINE Study (Anton et al, JAMA 2006). Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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