Opioid Receptor Antagonist Pregnancy: No clinical data on use in pregnancy; animal data have shown reproductive toxicity and the potential risk for humans is unknown. Should only be given to pregnant women when, in the judgement of the attending physician, the potential benefits outweigh the possible risk. Use in pregnant alcoholic patients receiving long-term or substitution treatment with opiates, or in pregnant opioid-dependent patients, creates a risk of acute withdrawal syndrome with serious consequences for mother and foetus. Administration must be suspended if opiate analgesics are prescribed. Breast feeding is not recommended during treatment.

Naltrexone

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

Naltrexone is a long-acting opioid antagonist used as an adjunct to maintain abstinence in formerly opioid-dependent people and in alcohol dependence.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 50 mg (one tablet), after an initial dose of 25 mg (half a tablet)

Route: Oral (tablets should be taken with a liquid)

Frequency: Once daily

Max: A dose of over 150 mg on any single day is not recommended, since this can lead to a higher incidence of side effects

Source: Naltrexone hydrochloride 50 mg film-coated tablets (UK SPC). Treatment must begin with low doses according to the treatment induction schedule: the recommended initial dose is 25 mg (half a tablet) followed by 50 mg per day (one tablet). Treatment should be initiated and supervised by suitably qualified physicians. Naltrexone administered to opioid-dependent persons can cause life-threatening withdrawal symptoms: administration must not be started before a naloxone challenge test is performed and a negative result obtained, and treatment should be considered only in patients who have remained opioid-free for a minimum of 7-10 days; before starting treatment this test must be confirmed by urine screening. The dosage regimen can be modified to improve compliance to a three-times-a-week schedule: 2 tablets (100 mg) on Monday and on Wednesday and 3 tablets (150 mg) on Friday. A missed dose can be managed by providing 1 tablet per day until the next regular dose administration. Duration: as naltrexone is an adjunctive therapy and the full recovery process in opioid-dependent patients is individually variable, no standard duration of treatment can be stated; an initial period of three months should be considered, though prolonged administration may be necessary. Paediatric population: not recommended in children and adolescents below 18 years old; safe use in children has not been established. Elderly: safe use for the treatment of opiate dependence in the elderly has not been established. Liver function tests should be carried out both before and during treatment. During treatment, painful conditions should be treated with non-opioid analgesia only; in an emergency requiring opioid analgesia a higher than usual dose may be needed and the resulting respiratory depression may be deeper and more prolonged, requiring careful monitoring by trained personnel in a hospital centre.

Dose adjustments

Renal

Contraindicated in severe renal failure. Naltrexone is excreted predominantly in the urine, so caution should be observed in patients with impaired renal function.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to naltrexone hydrochloride or to any of the excipients
Acute hepatitis or liver failure
Severe renal failure
Patients currently dependent on opioids, since an acute withdrawal syndrome may ensue
Any patient with a positive screen for opioids or who has failed the naloxone provocation test
Use in conjunction with an opioid-containing medication; and in combination with methadone

Side effects

Very common: nervousness, anxiety, insomnia; common: irritability, affective disorders; rare: suicidal ideation, attempted suicide
Very common: headache, restlessness; common: dizziness; uncommon: tremor, somnolence
Very common: abdominal pain, nausea and/or vomiting; common: diarrhoea, constipation, decreased appetite
Very common: arthralgia and myalgia, asthenia; very rare: rhabdomyolysis
Common: tachycardia, palpitations, electrocardiogram change, chest pain; uncommon: liver disorder, blood bilirubin increased, hepatitis (transaminases may increase during treatment and decrease to baseline within several weeks after discontinuation)

Interactions

Opioid-containing medicines (cough and cold preparations, antidiarrhoeal preparations, opioid analgesics) — patients may not benefit from them; combination with methadone is contraindicated per the UK SPC
In an emergency requiring opioid analgesia, the amount of opioid required may be greater than usual and the resulting respiratory depression may be deeper and more prolonged
Disulfiram — safety and efficacy of concomitant use unknown; concomitant use of two potentially hepatotoxic medications is not ordinarily recommended unless probable benefits outweigh known risks (US labelling)
Thioridazine — lethargy and somnolence have been reported following doses of naltrexone and thioridazine (US labelling)
Studies to evaluate possible interactions with drugs other than opiates have not been performed; caution is advised with concomitant administration of other drugs (US labelling)

Clinical monograph

How it works

It competitively blocks opioid receptors, abolishing the euphoric and other effects of exogenous opioids and modulating alcohol-related reward pathways.

Prescribing in practice

It can precipitate acute, severe opioid withdrawal in opioid-dependent patients, so the patient must be opioid-free for an adequate period (confirmed where appropriate) before starting.
Because it blocks opioid analgesia, plan alternative pain management and warn that attempts to override the block with large opioid doses risk fatal overdose, especially after treatment stops when tolerance is lost.
Assess liver function before and during treatment, as hepatotoxicity has been reported at higher doses.

Monitoring

Monitor liver function and adherence, and review ongoing abstinence and psychosocial support during treatment.

Counselling the patient

Carry information that you are taking an opioid blocker so emergency teams can plan pain relief.
Do not try to overcome the blockade with opioids, and remember your tolerance falls after stopping, raising overdose risk.

Evidence & guidelines

Naltrexone as part of a relapse-prevention programme is supported by NICE guidance on alcohol-use and opioid-dependence management.

Reference: COMBINE trial (Anton et al. JAMA 2006); NICE CG115 (Alcohol Use Disorders); NICE CG52 (Opioid Dependence); MHRA SPC Nalorex; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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