Naltrexone injectable (Vivitrol)

What it treats

Vivitrol is approved by the U.S. Food and Drug Administration for alcohol use disorder and for the prevention of relapse to opioid dependence after opioid detoxification. In alcohol use disorder, it's used to reduce heavy drinking days and to blunt the reward from alcohol. In opioid use disorder, it holds abstinence after detox by making opioids ineffective.

Clinicians reach for it when daily pill-taking is the main barrier, when someone wants a non-controlled option, or when a structured monthly visit is a useful part of care.

How it works

Naltrexone binds tightly to opioid receptors but doesn't activate them. If alcohol is used, the reward pathway (which relies partly on the body's own opioid-like chemicals) is quieter. If an opioid is used, the drug can't produce its usual effect.

Vivitrol is a slow-release form. After the injection, naltrexone levels rise over a few days and hold steady for about 4 weeks before tapering off. This removes the daily adherence problem that limits oral naltrexone.

Receptor mechanism (detail)

Naltrexone is a competitive antagonist at the µ-opioid receptor, with weaker activity at κ and δ receptors. In the Vivitrol formulation, the drug is embedded in biodegradable polymer microspheres that release naltrexone gradually after intramuscular injection. Plasma levels are high enough to block opioid effects for roughly 4 weeks, though the tail is longer than the peak.

Potency and typical dosing pattern

Ranges are typical framework only, not a prescription for any individual.

The dose is 380 mg intramuscularly into the gluteal muscle, given once every 4 weeks. There's no titration and no partial dose. The injection is deep and needs a long needle; some clinicians alternate sides to reduce local reactions.

Before the first dose, an opioid-free window of 7 to 10 days is required (longer for methadone or buprenorphine). A urine drug screen and a careful history help confirm. Some clinicians use a naloxone challenge test in higher-risk cases.

Safety monitoring

• Liver function tests (LFTs) at baseline and periodically.
• Opioid-free confirmation before the first dose. A urine drug screen plus a careful history of last opioid use.
• Injection site. Reactions are common (soreness, induration, swelling) and usually settle. Serious site reactions, including cellulitis, abscess, and rare tissue necrosis, do happen and need urgent attention.
• Overdose risk when the shot wears off or after stopping. Tolerance to opioids drops. If someone relapses, a familiar dose can be fatal.
• Mood. Some people report low mood or feeling flat.
• Adherence to the schedule. Late shots leave a gap in coverage.

What to expect

Common side effects

• Injection site soreness, redness, or induration.
• Nausea, especially early.
• Headache.
• Fatigue.
• Trouble sleeping or vivid dreams.
• Low mood or feeling flat in some people.
• Dizziness.

Nausea is more common with the first shot and tends to fade with later doses.

Serious side effects and warnings

• Hepatotoxicity. Naltrexone can cause liver injury. LFTs at baseline and periodically catch most problems early. New right-upper-quadrant pain, jaundice, or dark urine needs prompt attention.
• Injection site reactions. Most are mild. Severe reactions, including cellulitis, abscess, sterile inflammation, and rare tissue necrosis, have been reported. New warmth, spreading redness, drainage, or a hard mass that isn't improving needs urgent evaluation.
• Precipitated opioid withdrawal. If Vivitrol is given while opioids are still on board, it can trigger severe, rapid withdrawal that's hard to manage and doesn't respond to the usual opioid rescue.
• Loss of tolerance and overdose. After the shot wears off, or after stopping, opioid tolerance is lower. A relapse dose that was once tolerated can be fatal. A naloxone kit at home matters.
• Depression and suicidality. Uncommon but reported.
• Eosinophilic pneumonia. Rare but reported with any naltrexone formulation.

Sexual and relational effects

Sexual side effects aren't a prominent feature of naltrexone, though some people report reduced libido or delayed orgasm. More often the relational conversation is about recovery: partners, family, and sponsors as part of a support system. Being on Vivitrol doesn't remove the work of building relationships without alcohol or opioids at the center. If sexual function shifts after starting, that's worth raising with the prescriber.

Weight, appetite, and sleep

Weight change on naltrexone is usually modest. Appetite can dip early because of nausea. Sleep can be disrupted in the first weeks, sometimes with vivid dreams. Most of this settles.

Starting and dosing basics

This section is general background, not a dosing instruction for any individual.

Before the first shot, a full opioid-free window matters: 7 to 10 days from short-acting opioids and longer for methadone or buprenorphine. A urine drug screen and a careful history are standard. Some clinicians do a naloxone challenge in higher-risk cases. The injection is deep IM into the gluteal muscle and needs the supplied long needle. Trying to give it in the deltoid or subcutaneously will fail.

Missed doses and interactions

Missing an appointment leaves a gap. Once the shot has worn off, opioid tolerance recovers, and relapse in that window carries higher overdose risk. Getting back on schedule matters.

The big interaction is with opioids of any kind. Prescription pain medications, cough syrups with codeine, and street opioids will either not work or will need to be given at much higher risk. Any surgery or dental work needs the treating team to know that naltrexone is on board. A medical alert card can help in an emergency.

Alcohol interactions aren't dangerous the way disulfiram is, but at high doses alcohol still causes its usual problems (impaired driving, overdose, aspiration), so this isn't a free pass.

Stopping and tapering

Naltrexone doesn't cause physical dependence, so there's no medical withdrawal from stopping the shots. The bigger issue is what happens if opioid use resumes after the coverage wears off. Tolerance is reduced, and the risk of overdose from a previously familiar dose is real. Any plan to stop should include a relapse-prevention plan and a naloxone kit at home.

Pregnancy and breastfeeding

Data on naltrexone in pregnancy is limited. It does cross the placenta and enters breast milk in small amounts. The decision belongs with a clinician who knows the situation, weighing the risks of continued substance use against those of the medication.

Anyone who is pregnant, planning a pregnancy, or breastfeeding should talk it through with a prescriber.

Cost and generic availability

Vivitrol is branded and expensive. There is no generic depot naltrexone. Patient assistance programs and manufacturer support programs are commonly used to cover cost. Oral naltrexone (generic, inexpensive) is the alternative when the shot isn't accessible, though daily adherence is the trade-off.

Common questions

How is this different from the pill? The medication is the same. The shot removes the daily dose problem and covers about 4 weeks per injection. That's a real advantage for anyone who struggles with daily pills.

What if I want pain medication for surgery? Opioid pain medications won't work while Vivitrol is on board, and the shot's effects last longer than the calendar month suggests. Elective surgery is best planned with the treatment team well in advance. Non-opioid approaches are the default.

What if I relapse to opioids while covered? The effect is blocked. Using much larger doses to try to override the block can cause dangerous respiratory depression. And once the shot wears off, tolerance is lower, so a familiar dose can be fatal.

Do I have to keep getting it forever? No. How long depends on the person, the substance, and the plan built with the prescriber. Many people stay on it for a year or more; some stop sooner. The stopping conversation matters.

Questions to ask your prescriber

• What are we hoping this treats, and how will we know it's working?
• How long should I plan to stay on the monthly shot?
• What's the plan if I have pain or need surgery?
• What signs would tell you it isn't the right medication for me?
• Should I have naloxone at home in case of a relapse?

Sources

This guide draws on current prescribing information and public health references. It is reviewed for clinical accuracy and updated as guidance changes, and current as of June 8, 2026.

• U.S. Food and Drug Administration. Naltrexone extended-release injectable suspension (Vivitrol) prescribing information.
• MedlinePlus, U.S. National Library of Medicine. Naltrexone injection.
• DailyMed, U.S. National Library of Medicine. Naltrexone extended-release injectable suspension.
• American Society of Addiction Medicine. National Practice Guideline for the Treatment of Opioid Use Disorder.
• National Institute of Mental Health. Mental health medications.