Naltrexone Side Effects: Expected Effects and Urgent Safety Signals

First, identify the exact product

Naltrexone may mean a daily oral tablet, Vivitrol (a monthly extended-release injection), or a different product that contains naltrexone with another medicine. The products do not have identical adverse-event tables, ingredients, routes, or warnings.

This article summarises current US labels for oral naltrexone and Vivitrol. Whether a symptom is caused by the medicine, withdrawal, alcohol or opioid use, another condition, or a different medicine is for your clinician to diagnose.

Effects commonly reported with oral naltrexone

In an open-label safety study in people treated for alcohol dependence, the oral-tablet label reports nausea, headache, dizziness, nervousness, fatigue, insomnia, vomiting, anxiety, and somnolence among events newly reported by at least 2% of participants. The label also notes that adverse events can be difficult to distinguish from opioid withdrawal symptoms when a person has not been fully opioid-free.

For opioid-dependence studies, the oral label lists sleep difficulty, anxiety or nervousness, abdominal pain or cramps, nausea or vomiting, low energy, joint or muscle pain, and headache among events reported in more than 10% of participants. Those figures are from particular studies; they do not predict an individual’s experience.

Vivitrol: injection-site reactions deserve special attention

Vivitrol is an intramuscular injection, so pain, tenderness, firmness, swelling, redness, bruising, or itching may occur where it was given. In clinical trials, many reactions were mild or moderate. The Vivitrol label’s injection-site warning also reports serious postmarketing reactions including cellulitis, abscess, haematoma, sterile abscess, tissue necrosis, and cases requiring surgical treatment.

Contact the treating clinic promptly if the area gets more painful, swollen, hard or red, or if it blisters, opens, darkens or otherwise gets worse. Do not try to drain the area or inject it yourself. Our Vivitrol administration guide explains why this medicine must be injected by a healthcare professional.

When to seek urgent help

The current US labels call for prompt medical assessment of symptoms that may signal a serious reaction. These include:

• signs of a severe allergic reaction, such as hives, swelling of the face or throat, wheezing, or trouble breathing;
• symptoms compatible with acute hepatitis, such as persistent abdominal pain, dark urine, pale stools, yellowing of the skin or eyes, or unusual fatigue;
• progressive shortness of breath, cough, or wheeze after Vivitrol, which can need assessment for eosinophilic pneumonia;
• new or worsening depression, suicidal thoughts, or behaviour; and
• severe withdrawal symptoms after starting naltrexone, especially if there was recent opioid use.

Call emergency services if you are in immediate danger, have severe trouble breathing, suspect an overdose, or are at imminent risk of self-harm. For any other serious or worsening symptom, contact the prescribing team promptly. Do not change a dose or stop a long-acting injection on your own.

Can side effects be managed without speaking to a clinician?

The first step is to find out what is causing the symptom and how serious it is, and a list of home remedies cannot do that. Your clinician may need to review the product, dose timing, recent opioid exposure, other medicines, alcohol use and liver symptoms, and decide whether you should continue the medicine. Vivitrol cannot be removed once injected.

Bring a record of when the symptom began, the product and dose, all medicines and supplements, recent opioid use, alcohol use, and any changes at the injection site. This makes it easier for the clinician to distinguish a routine effect from a potentially serious problem.

Sources

1. DailyMed: Vivitrol prescribing information. US label, revised December 2025; injection-site, hepatic, respiratory, hypersensitivity, and mental-health warnings.
2. DailyMed: naltrexone hydrochloride tablet prescribing information. US oral-tablet label; reported adverse events and hepatic and mental-health warnings.