Naltrexone

Key facts

• Naltrexone is an opioid antagonist used to help prevent relapse in opioid use disorder and to reduce heavy drinking in alcohol use disorder; it is also combined with bupropion for weight management.
• It comes as a daily tablet or a monthly injection, and it works by blocking opioid receptors rather than by sedating you.
• You must be completely opioid-free, typically 7 to 10 days off short-acting opioids and longer after methadone or buprenorphine, before your first dose; taking it too soon triggers sudden, severe opioid withdrawal.
• If you relapse and use opioids after stopping naltrexone or missing doses, your tolerance will have dropped, so a previously normal dose of opioid can cause a fatal overdose.

What naltrexone treats

Naltrexone treats opioid use disorder, helping prevent relapse once you have completed withdrawal, and alcohol use disorder, reducing cravings and heavy drinking. A lower-dose combination with bupropion (Contrave) is used for weight management. It does not treat opioid withdrawal itself, and it is not a pain medicine.

How naltrexone works

Naltrexone binds tightly to opioid receptors in the brain without activating them, blocking opioids from producing their effects and dulling the rewarding signal alcohol produces through the same pathway. Because the receptors are occupied, opioids taken while on naltrexone have little or no effect, and the urge to keep drinking or using is reduced.

Before you take it

• Do not take naltrexone if you currently use opioids, are in opioid withdrawal, or have not confirmed you are opioid-free; your prescriber may test this before your first dose.
• Do not take it if you have acute hepatitis or liver failure; tell your prescriber about any liver disease.
• Avoid opioid-containing medicines while on naltrexone, including some cough and diarrhoea medicines, since they will not work for pain relief and may be blocked entirely in an emergency.
• Tell your prescriber about pregnancy, breastfeeding, or a history of depression or self-harm.

Side effects

Common effects include nausea, headache, dizziness, fatigue, and joint or muscle aches.

Stop and seek urgent medical care for any of these:

• Yellowing of the skin or eyes, dark urine, or abdominal pain, which can signal liver injury.
• Agitation, vomiting, diarrhoea, or muscle aches shortly after a dose if you have used opioids recently, which can signal precipitated withdrawal.
• Severe allergic reaction: rash, facial swelling, difficulty breathing.
• New or worsening low mood or thoughts of self-harm.

Safety essentials

• Confirm you are fully opioid-free before starting; starting too soon precipitates sudden, severe withdrawal that can begin within minutes of the dose.
• After any gap in treatment or a relapse, your opioid tolerance will be lower than before. Using your previous dose of opioid can cause a fatal overdose; this is a well-documented cause of death after stopping naltrexone.
• Carry a card or wear a bracelet noting you take naltrexone, since it will block opioid pain relief, including in an emergency, and tell every doctor, dentist, and pharmacist you see.
• Periodic liver function tests are recommended, particularly at higher doses or with pre-existing liver disease.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.