Vivitrol stays in your system for roughly 30 to 50 days after a single injection, though its strongest effects occur during the first four weeks. The drug has an elimination half-life of 5 to 10 days, meaning it takes several cycles of that window before naltrexone and its breakdown products fully clear your body.
How Vivitrol Releases Into Your Body
Unlike a pill that hits your bloodstream all at once, Vivitrol is a 380 mg injection of naltrexone packed inside tiny polymer microspheres. These microspheres break down slowly in muscle tissue, releasing the drug over weeks. That gradual erosion is what makes it a once-a-month treatment rather than a daily one.
After the injection, your blood levels follow a distinctive pattern. There’s an initial small spike about 2 hours after the shot, caused by naltrexone on the surface of the microspheres dissolving quickly. Then a second, larger peak appears 2 to 3 days later as the polymer begins to erode in earnest. Starting around day 7, levels slowly decline but remain in a therapeutic range for a full four weeks. This steady release avoids the daily peaks and valleys you’d get with oral naltrexone.
Half-Life and Total Clearance Time
The elimination half-life of naltrexone from a Vivitrol injection is 5 to 10 days. Its main metabolite, a compound the liver produces when it processes naltrexone, has the same 5 to 10 day half-life. That half-life is much longer than oral naltrexone’s (which clears in hours) because the rate-limiting step isn’t your liver’s processing speed. It’s how fast the polymer microspheres dissolve and release more drug into your bloodstream.
A general pharmacology rule is that a drug is considered fully eliminated after about 4 to 5 half-lives. With a half-life range of 5 to 10 days, that means complete clearance takes roughly 25 to 50 days after your last injection. For most people, naltrexone will be undetectable or negligible somewhere in the 30 to 50 day range, though the exact timeline depends on individual metabolism and how many monthly injections you’ve received.
How Long the Blocking Effect Lasts
What most people really want to know is how long Vivitrol actively blocks opioid receptors, since that determines when opioid-based medications (like those used for pain management or anesthesia) would work normally again. The blocking effect closely tracks blood levels but isn’t identical to total clearance. Therapeutic blockade is maintained for about 28 days, which is why the drug is dosed once a month.
After four weeks, receptor blockade begins to weaken as drug levels drop. But “weakening” isn’t the same as “gone.” Some degree of blocking activity can persist beyond day 28, tapering off gradually rather than shutting off like a switch. If you need opioid-based pain relief for a medical procedure, your care team will need to account for this lingering effect and may need to use higher doses or alternative pain management strategies.
What Happens With Repeated Injections
If you’ve been receiving Vivitrol monthly for several months, each new injection arrives before the previous dose has fully cleared. The drug reaches a steady state by the end of the first dosing interval, meaning your baseline blood levels stabilize after that initial month. This doesn’t dramatically change how long it takes to clear after your final injection, but it does mean your body has a slightly higher starting concentration compared to someone who received only one shot. Expect clearance to fall on the longer end of the 30 to 50 day range.
The Opioid-Free Window Before Starting
The clearance timeline matters in the other direction too. Before receiving a Vivitrol injection, you need to be free of all opioids for a minimum of 7 to 10 days. Naltrexone aggressively displaces opioids from receptors in the brain, and if opioids are still present when the shot is given, it can trigger precipitated withdrawal. This is a sudden, intense version of withdrawal that can be severe enough to require hospitalization. The 7 to 10 day window gives your body time to clear residual opioids so the naltrexone can bind to empty receptors without conflict.
How Your Liver Processes Vivitrol
Once naltrexone reaches the bloodstream, the liver breaks it down through a process called enzymatic reduction. The primary product of this breakdown is a metabolite called 6-beta-naltrexol, which is itself pharmacologically active, meaning it also contributes to opioid receptor blockade. This metabolite clears at the same rate as naltrexone itself (5 to 10 day half-life), so even as the parent drug diminishes, its breakdown product extends the overall duration of activity in your system.
People with significant liver impairment may process the drug differently. Because the liver handles the bulk of naltrexone metabolism, compromised liver function can alter both how effective the drug is and how long it lingers.
Drug Testing and Detection
Naltrexone is not an opioid, not a controlled substance, and not part of standard drug screening panels. A routine urine drug test at work or in a treatment program will not flag Vivitrol. Specialized laboratory tests can detect naltrexone and its metabolites if specifically ordered, but this is uncommon outside of clinical research settings. If you’re concerned about a drug test, Vivitrol itself won’t cause a positive result for opioids, benzodiazepines, or any other commonly screened substance.