Oral naltrexone clears your system within roughly 72 hours of your last dose. The drug itself has a short half-life of about 4 hours, but your body converts it into an active breakdown product that lasts considerably longer, with a half-life of about 13 hours. That active byproduct is what keeps naltrexone working between doses and what determines how long the drug’s effects actually linger. For the extended-release injection (Vivitrol), the timeline is much longer: about 30 days.
Oral Naltrexone: The Two-Part Timeline
When you take an oral naltrexone tablet, blood levels of the drug peak within about one hour. From there, naltrexone itself is broken down quickly, with a half-life of roughly 4 hours. That means half the original drug is gone in 4 hours, and naltrexone itself is essentially undetectable after about 24 hours.
But that’s only part of the story. Your liver converts naltrexone into a metabolite called 6-beta-naltrexol, which also blocks opioid receptors. This metabolite peaks in your blood within the same first hour but hangs around much longer, with a half-life of about 13 hours. Using the standard pharmacology rule that it takes roughly five half-lives to fully eliminate a substance, 6-beta-naltrexol takes about 65 hours (close to 3 days) to clear your body after a single dose.
So while naltrexone technically leaves fast, its effects persist for up to 72 hours because of this longer-lasting metabolite. That distinction matters if you’re timing a transition to or from another medication.
The Extended-Release Injection Lasts Much Longer
The monthly naltrexone injection (Vivitrol) follows a completely different timeline. After the shot, blood levels spike within about 2 hours, then rise again to a second peak 2 to 3 days later. Starting around day 7, levels begin a slow, steady decline that maintains therapeutic concentrations for a full 4 weeks.
This design avoids the daily peaks and valleys of the oral form. The trade-off is that once the injection is given, you can’t remove it from your system early. The drug’s effects last approximately 30 days from the injection date, and residual levels may linger beyond that as the last traces are absorbed from the injection site and cleared by the body.
Why the Washout Period Matters
Most people searching this question have a practical concern: how long do they need to wait before opioids (whether prescribed pain medication or opioid-based addiction treatment like buprenorphine or methadone) will work normally again? As long as naltrexone or its active metabolite is occupying your opioid receptors, those medications will be partially or fully blocked.
For oral naltrexone, the general recommendation is to wait at least 72 hours after your last dose. For the Vivitrol injection, prescribing guidelines recommend a full 30-day waiting period. In the reverse scenario, where someone is transitioning from an opioid to naltrexone, the standard practice involves a 7- to 10-day opioid-free washout period before the first naltrexone dose to avoid triggering sudden withdrawal.
These timelines aren’t just suggestions. Taking opioids while naltrexone is still active won’t produce the expected pain relief or therapeutic effect. And starting naltrexone too soon after opioid use can cause intense, rapid-onset withdrawal symptoms.
Factors That Can Slow Clearance
The 72-hour estimate for oral naltrexone applies to most adults, but several factors can push that timeline longer. Because naltrexone is processed almost entirely by the liver, any degree of liver impairment can slow how quickly your body breaks down and eliminates the drug. People with liver disease or reduced liver function may retain naltrexone and its metabolite for longer than average.
Other variables that influence clearance speed include age, body weight, kidney function, and overall metabolism. Older adults and people with kidney problems tend to eliminate drugs more slowly in general. If you’ve been taking naltrexone daily for a long period, the metabolite can also accumulate to slightly higher baseline levels than what you’d see after a single dose, which may add a modest amount of time to full clearance.
Detection on Drug Tests
Naltrexone is not an opioid and is not a controlled substance. Standard drug screening panels (the kind used for employment or legal purposes) do not test for naltrexone, and taking it will not cause a false positive for opioids or any other drug class. Specialized laboratory tests can detect naltrexone and its metabolite in blood or urine, but these are rarely ordered outside of clinical research settings or specific treatment monitoring programs. If you’re concerned about a routine drug test, naltrexone will not appear on it.