Substance guides

Vivitrol shot side effects: What to expect

Published May 25, 2025 · 6 min read · Updated April 2026

Reviewed for accuracy by licensed clinical professionals.

Vivitrol (extended-release naltrexone injection) is given monthly for opioid and alcohol use disorders. Understanding side effects helps you stay on treatment.

Common side effects

Injection site reactions (pain, hardness, or nodules at injection site) in up to 70% of patients. Nausea (first few days). Headache. Fatigue. Decreased appetite. Dizziness. These typically improve after the first 1-2 injections.

Serious side effects

Injection site reactions requiring medical attention (severe pain, large hard lumps, or signs of infection). Liver injury (rare; liver function should be monitored). Severe allergic reaction (rare). Precipitated withdrawal if given while opioids are still in the system.

Important warnings

Vivitrol blocks opioid effects. If you attempt to override the block with large doses of opioids, you risk fatal overdose. After Vivitrol wears off, your opioid tolerance is reduced, making overdose risk extremely high if you return to previous doses.

Authoritative sources

This article references guidelines from: SAMHSA · NIDA · ASAM

Frequently asked questions

Does the Vivitrol shot hurt?

The injection can cause pain, hardness, or nodules at the injection site in up to 70% of patients. This typically improves with subsequent injections.

What are the worst side effects of Vivitrol?

Severe injection site reactions and the risk of overdose if attempting to use opioids on top of the block.

How long do Vivitrol side effects last?

Most common side effects resolve within 3-5 days. Injection site reactions may persist for 1-2 weeks.

Disclaimer: Informational only. Not medical advice. SAMHSA: 1-800-662-4357.