Vivitrol is designed to block the pleasurable effects of alcohol for about 28 days, or four weeks. It’s given as a single injection once a month, and the medication slowly releases from the injection site over that entire period. The blocking effect isn’t permanent, though, and understanding how it changes throughout the month can help you plan around your treatment schedule.
How the 28-Day Cycle Works
Each Vivitrol injection delivers 380 mg of naltrexone into the gluteal muscle. The drug is embedded in tiny polymer particles that gradually erode, releasing naltrexone into your bloodstream over approximately four weeks. This slow-release design is what distinguishes Vivitrol from the daily oral naltrexone pill: you get one shot and don’t have to think about taking medication every day.
The elimination half-life of naltrexone from a Vivitrol injection is 5 to 10 days, meaning it takes that long for the drug level to drop by half once the polymer stops releasing it. By the time you’re due for your next injection at the four-week mark, the medication is still present in your system, but at its lowest concentration. In clinical studies, trough levels at day 28 measured around 0.75 to 1.3 ng/mL depending on the formulation and dose studied. That’s significantly lower than peak levels earlier in the cycle, which is why staying on schedule matters.
What Vivitrol Actually Blocks
Vivitrol doesn’t stop you from getting drunk. It blocks the reward pathway that makes drinking feel good. Naltrexone and its active breakdown product attach to opioid receptors in the brain, particularly the mu-opioid receptor. When you drink alcohol, your brain normally releases its own natural opioids, which create that warm, euphoric “buzz.” Vivitrol prevents those opioids from activating their receptors, so the pleasurable reinforcement doesn’t happen.
If you drink while on Vivitrol, you’ll still experience the physical effects of alcohol: impaired coordination, slower thinking, reduced reaction time, and nausea if you drink too much. What you won’t feel is the rewarding high that drives the cycle of craving and compulsive drinking. Over time, this disconnect between drinking and pleasure helps reduce cravings and makes it easier to cut back or stop.
Vivitrol also influences stress-related hormonal pathways that play a role in alcohol consumption, adding another layer to how it reduces the drive to drink.
How Quickly It Starts Working
Vivitrol begins releasing naltrexone shortly after injection, and the medication reaches effective blood levels within the first few days. The polymer particles release the drug at a fairly steady rate through most of the month, with the highest concentrations occurring in the first two weeks. By the third and fourth weeks, levels are gradually declining but still therapeutically active.
The Final Days Before Your Next Shot
The lowest point in your medication cycle is the stretch just before your next injection. Because naltrexone levels are at their trough around day 28, some people notice a slight return of cravings or a subtle shift in how they feel about alcohol during this window. This doesn’t mean the medication has completely worn off, but the blockade is weaker than it was at mid-cycle.
This is one reason why keeping your injection appointments on time is important. If you delay your next shot by several days or a week, you’re extending that low-coverage window and giving cravings more room to build. Alternating injection sites between the left and right buttock each month helps ensure consistent absorption.
What Happens After You Stop
If you decide to stop Vivitrol or miss an appointment, the blocking effect doesn’t disappear overnight. Because of the 5-to-10-day half-life, naltrexone will remain in your system at gradually decreasing levels for several weeks after your last injection. Most of the drug clears within about 30 to 50 days after the final shot, though this varies from person to person based on factors like metabolism and body composition.
During this washout period, the alcohol-blocking effect weakens progressively. You may start to notice that alcohol feels more rewarding again, which can be a vulnerable time for relapse. Having a plan in place for this transition, whether that means switching to oral naltrexone, increasing therapy sessions, or other support strategies, can make a significant difference.
Why the Monthly Schedule Matters
The every-four-weeks dosing schedule is specifically designed so that each new injection overlaps with the tail end of the previous one, maintaining continuous receptor blockade. When you stay on schedule, there’s no gap in coverage. The fresh injection starts releasing naltrexone while the remnants of the last dose are still circulating, keeping blood levels in the therapeutic range without interruption.
For people who struggled with the daily oral pill, whether because of forgetting doses or choosing to skip them before drinking, the monthly injection removes that daily decision point. You can’t un-take the shot, which is part of what makes it effective. One large clinical review found that the extended-release injection format helps maintain consistent medication exposure in a way that daily pills often don’t achieve in real-world use, simply because adherence is built into the delivery method.