A single Vivitrol injection blocks the effects of opioids for up to 30 days. The shot is given once every four weeks (or once a month) as a 380 mg intramuscular injection into the gluteal muscle, and each dose maintains a therapeutic level of naltrexone in the bloodstream throughout that entire period.
How Vivitrol Works Over 30 Days
Vivitrol is an extended-release form of naltrexone, a drug that attaches to the same receptors in the brain that opioids target. By occupying those receptors, it prevents opioids from producing a high or pain relief. It also blocks the effects of the body’s own natural opioid-like chemicals, which plays a role in reducing alcohol cravings for people being treated for alcohol dependence.
After the injection, naltrexone levels in the blood follow a distinct pattern. The concentration spikes within about two hours of the shot, then rises to a second peak roughly two to three days later. Starting around day seven, levels begin a slow, steady decline that keeps the drug at effective concentrations for the full four weeks. This gradual release is what distinguishes Vivitrol from the daily oral tablet version of naltrexone, which causes blood levels to spike and drop every 24 hours.
What Happens Near the End of Each Dose
Vivitrol’s blocking effect is strongest in the first two to three weeks. As the fourth week approaches, naltrexone levels are at their lowest point in the cycle. This matters for a specific and serious reason: people who use opioids near the tail end of a dosing interval, or after missing a scheduled injection, face a heightened overdose risk.
There are two layers to that danger. First, the fading blockade may allow some opioid effects to break through, which can tempt a person into using more. Second, and more critically, tolerance drops significantly while on Vivitrol. A dose of heroin or prescription opioids that the person could once handle may now cause life-threatening respiratory depression. This reduced tolerance persists even after Vivitrol is fully discontinued, not just between injections.
If You Miss or Delay a Dose
The official guidance is straightforward: if an injection is late, get the next one as soon as possible. There is no “loading dose” to make up for lost time. The shot should not be given sooner than four weeks after the previous one, and the dose should not be increased beyond the standard 380 mg. During any gap in coverage, the opioid blockade weakens, and the overdose risks described above apply.
How Vivitrol Enters the Body
Vivitrol is injected into the muscle of the buttock, alternating sides each month. It is not a subcutaneous (under-the-skin) shot and cannot be self-administered at home. Each injection must be given by a healthcare provider. Because the medication is suspended in tiny polymer particles that dissolve slowly over weeks, the injection site can sometimes become sore, hard, or tender. These reactions are among the more common side effects and typically resolve on their own within days.
How Long People Stay on Vivitrol Overall
The question of how long each shot lasts is different from how long someone stays on Vivitrol as part of a treatment plan. There is no fixed stopping point. Treatment duration depends on individual recovery progress, relapse risk, and the condition being treated (opioid use disorder or alcohol dependence). Many providers recommend staying on the medication for at least several months, and some people continue for a year or longer. Stopping too early is one of the most common reasons for relapse, particularly with opioid use disorder, because the protective blockade disappears entirely once injections stop.
Opioid-Free Period Before Starting
Before the first injection, you need to be completely free of all opioids for a minimum of 7 to 14 days, depending on whether the opioids were short-acting (like heroin or oxycodone) or long-acting (like methadone). If any opioids are still bound to your receptors when Vivitrol is injected, the drug will rip them off those receptors and trigger sudden, severe withdrawal symptoms. This is called precipitated withdrawal, and it can be intensely uncomfortable and medically dangerous. A urine drug screen or a short test with oral naltrexone is often used to confirm opioid clearance before the first shot.
Vivitrol vs. Daily Oral Naltrexone
Both forms contain the same active drug, but the monthly injection solves the biggest problem with the daily pill: people stop taking it. Oral naltrexone requires a conscious decision every morning, and skipping a dose restores the ability to feel opioid effects within a day or two. Vivitrol removes that daily decision entirely. Once the shot is in, the medication works whether you want it to or not for roughly 28 to 30 days. The extended-release formulation also avoids the daily peaks and valleys in blood concentration that come with oral dosing, providing a more stable level of receptor blockade throughout the month.