The most common side effects of naltrexone are nausea, headache, dizziness, tiredness, and trouble sleeping. They are usually mild and tend to fade within the first week or two. Serious side effects are rare, and most people take naltrexone with little trouble once they get past the first few days.
Common naltrexone side effects
The side effects most people notice are mild and happen early. Nausea is the most common, especially in the first few days, and it usually settles quickly. Other common effects include headache, dizziness, feeling tired, trouble sleeping, and some stomach upset. Some people also feel a little anxious or notice aches, a lower appetite, or vivid dreams. For the great majority of people, these fade within one to two weeks as the body adjusts.
How to manage the early side effects
A few simple steps make the first weeks easier. Taking your pill with food helps calm nausea. Many providers start you at 25 mg for a few days, then move up to the full 50 mg, so your body eases into it. Staying hydrated and taking the dose at a consistent time each day help too. If trouble sleeping is a problem, ask your provider about the best time of day to take it. Most side effects do not need any treatment and simply go away on their own.
Injection-specific side effects (Vivitrol)
The monthly Vivitrol injection can cause the same effects as the pill, plus a reaction at the injection site. That usually means soreness, redness, swelling, or a small lump where the shot was given, and it typically clears on its own. Rarely, the injection site can develop a more serious reaction with significant pain, hardening, or tissue damage. Tell your provider if injection-site pain is severe or does not improve, since it may need attention.
Rare but serious side effects
Serious side effects are uncommon, but it helps to know the signs. At very high doses, naltrexone can irritate the liver, so watch for stomach pain that does not go away, dark urine, or yellowing of the skin or eyes. Allergic reactions are rare but can cause rash, swelling, or trouble breathing. Some people may notice mood changes or low mood, so tell your provider if you feel unusually down. And because naltrexone blocks opioids, starting it too soon after opioids can cause sudden withdrawal.
Common vs. serious side effects
Here is a quick side-by-side of the everyday effects most people feel and the rare ones that mean you should call a provider.
| Common (usually mild, fade in 1 to 2 weeks) | Rare but serious (contact a provider) |
|---|---|
| Nausea, stomach upset | Liver irritation (stomach pain, dark urine, yellowing) |
| Headache, dizziness | Allergic reaction (rash, swelling, trouble breathing) |
| Tiredness, trouble sleeping | New or worsening depression or mood changes |
| Anxiety, aches, lower appetite | Severe injection-site reaction (Vivitrol) |
For related reading, see our naltrexone for alcohol guide, what to avoid on naltrexone, drug interactions, and dosage guide.
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Daniel R. Karlin, MD, MA
Board-certified psychiatrist and addiction medicine specialist. Author of 21+ peer-reviewed publications (1,355 citations) including recent work in JAMA (2025) and the British Journal of Psychiatry (2023).
- Sources.
- U.S. Food and Drug Administration. Prescribing information for naltrexone (ReVia) and naltrexone extended-release injectable (Vivitrol), including adverse reactions and warnings.
- American Academy of Family Physicians. Alcohol Use Disorder: Medication Options. American Family Physician, 2024 (common adverse effects: nausea, headache, insomnia).
- Singh D, Saadabadi A. Naltrexone. StatPearls. National Library of Medicine.
This page is for education only and is not medical advice. Naltrexone is a prescription medication. Talk with a licensed provider about your specific situation.
