Pharmacology · Opioids and antagonists
Naltrexone !
A long-acting oral opioid antagonist that helps prevent relapse in alcohol and opioid dependence. It precipitates withdrawal if opioids are still in the body.
Updated 2026-11-01
The figure shows naloxone, a very similar antagonist, in the inactive μ receptor; naltrexone occupies the same site. The available naltrexone structure is of the δ receptor.
Mechanism of action
Naltrexone is a competitive opioid-receptor antagonist, mainly at μ. It occupies the receptor without activating it and blocks the effect of opioids taken afterwards. In alcohol dependence it reduces the reinforcement mediated by endogenous opioids, and with it craving and relapse. Unlike naloxone, it is active by mouth and its effect lasts much longer.
- Occupies the μ receptor without activating it
- Blocks the effect of opioids that arrive later
- Reduces alcohol reinforcement mediated by endogenous opioids
- Less craving and fewer relapses
Pharmacokinetics
- Absorption
- Well absorbed by mouth, with extensive first-pass metabolism.
- Metabolism
- Almost all is metabolised, mainly to 6-β-naltrexol, an active metabolite.
- Elimination
- Kidney; enterohepatic recycling may occur.
- Half-life
- About 4 hours for naltrexone and about 13 hours for 6-β-naltrexol.
Indications
- Alcohol dependence, as part of a comprehensive treatment plan
- Blocking the effects of opioids after detoxification, to prevent relapse
Adverse effects
Common
- Nausea
- Headache
- Dizziness
- Anxiety and insomnia
- Abdominal pain and diarrhoea
Contraindications
- Current use of opioid analgesics
- Current opioid dependence, including patients on methadone or buprenorphine
- Acute opioid withdrawal
- A failed naloxone challenge or a urine screen positive for opioids
- Hypersensitivity to naltrexone
Key interactions
- Opioid analgesics, antidiarrhoeals and antitussives
- Lose their effect; in an emergency non-opioid alternatives or specialist management are needed.
- Opioid agonists in a dependent patient
- Precipitated withdrawal.
- Thioridazine
- Lethargy and drowsiness have been reported.
What is monitored
- No opioids before starting (negative urine screen or naloxone challenge)
- Liver function tests
- Mood and relapse risk
Administration and safety
- Before starting, the patient must be opioid-free (tramadol included) for the recommended period, which is longer after long-acting opioids.
- Patients are advised to carry identification stating that they take an opioid antagonist.
Nursing care
- Confirm that no opioids are being used before the first dose.
- Explain that opioid painkillers will not work and that trying to overcome the blockade can be fatal.
- Warn the family that after stopping it opioid tolerance is lower and overdose risk is higher.
- Watch for signs of liver injury: jaundice, dark urine, abdominal pain.
Pregnancy and breastfeeding
Data in pregnancy are limited; the benefit is weighed against the risk of relapse.
Test yourself
Think of the answer, then tap the question to see it.
Why must the patient be opioid-free before starting naltrexone?
Because naltrexone displaces opioids from the receptor and precipitates withdrawal that can be severe.
What overdose risk does a patient face after stopping naltrexone following detoxification?
Opioid tolerance is lower; taking the dose used before can cause a fatal overdose.
How does naltrexone differ from naloxone?
Both are opioid antagonists, but naltrexone is active by mouth and lasts much longer; it is used to prevent relapse, not to reverse an acute overdose.
Related drugs
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Naltrexone: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); NALTREXONE HYDROCHLORIDE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 23 (McGraw Hill, 2023); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.