Naltrexone Side Effects: Expected Effects and Urgent Safety Signals
Nausea, headache, dizziness, and tiredness are reported with naltrexone. Vivitrol also has injection-site effects; certain symptoms need prompt clinical review.
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. It cannot diagnose whether a symptom is caused by the medicine, withdrawal, alcohol or opioid use, another condition, or a different medicine.
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 becomes increasingly painful, swollen, hard, red, blistered, open, darkened, or otherwise worse. Do not attempt to drain or inject the area 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 for immediate danger, severe trouble breathing, suspected overdose, or imminent risk of self-harm. For other serious or worsening symptoms, contact the prescribing team promptly rather than changing a dose or stopping a long-acting injection on your own.
Can side effects be managed without speaking to a clinician?
Safe management starts with identifying the cause and seriousness of the symptom. It is not a generic list of home remedies. A clinician may need to review the product, dose timing, recent opioid exposure, other medicines, alcohol use, liver symptoms, and whether the medicine should be continued. 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.
Primary sources
- DailyMed: Vivitrol prescribing information
US label, revised December 2025; injection-site, hepatic, respiratory, hypersensitivity, and mental-health warnings.
- DailyMed: naltrexone hydrochloride tablet prescribing information
US oral-tablet label; reported adverse events and hepatic and mental-health warnings.