How Long Does Naltrexone Stay in Your System
A clear guide to how long naltrexone stays in the body and how long its opioid block lasts, covering oral half-life, the monthly Vivitrol shot, and the opioid-free window needed before the first dose.
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How Long Naltrexone Stays in Your System
For most people, oral naltrexone clears the body in about 1 to 2 days, but its opioid block lasts roughly 24 to 48 hours per dose—which is why the pill is taken daily. The monthly Vivitrol shot is a different animal: one injection keeps the receptors blocked for about a month.
The piece people mix up is the difference between how long the drug stays measurable and how long it keeps working. Naltrexone leaves fast, yet the block outlasts the parent drug because its main breakdown product hangs around longer. Below is the full picture, with the numbers laid out, and the one timing rule that matters most before you start: you have to be fully opioid-free first, or the first dose throws you into sudden, severe withdrawal.
- Oral naltrexone has a short half-life of about 4 hours, and its active breakdown product about 13 hours, so the pill mostly clears in 1 to 2 days[1].
- The opioid block from one oral dose lasts roughly 24 to 48 hours, while the monthly Vivitrol shot blocks opioids for about 28 days[2].
- You must be off all opioids before your first dose—usually 7 to 10 days—or naltrexone sets off precipitated withdrawal.
How Long Naltrexone Stays in Your Body
Naltrexone moves through the body on a faster clock than people expect for a medicine this powerful. Taken as the 50 mg oral tablet, the parent drug has a half-life of only about 4 hours[1].
The part that lasts longer is what the liver turns it into. Naltrexone breaks down into an active metabolite called 6-β-naltrexol, which still blocks opioid receptors and sticks around with a half-life of roughly 13 hours[1]. So the drug doing the early work fades within hours, while its offspring carries the effect into the next day.
Clearing a drug almost completely takes about five half-lives. Run that math on the metabolite and the parent drug, and oral naltrexone mostly clears in 1 to 2 days for most people. That is far quicker than buprenorphine (Suboxone), which lingers for the better part of a week.
How Long the Opioid Block Lasts
Here is where the working clock and the clearing clock split apart. Even though oral naltrexone is mostly gone in a day or two, a single 50 mg pill keeps opioids blocked for roughly 24 to 48 hours. That is the whole reason it is dosed daily, and it is why some clinics give a larger dose every two or three days instead.
While the block is up, a dose of heroin, fentanyl, or a painkiller meets a closed door. Naltrexone and its active metabolite sit on the mu-opioid receptors and keep other opioids from attaching, so using on top produces little or no high. The block fades as the dose wears off rather than switching off at a clean hour.
The monthly shot stretches that same blockade across weeks. Extended-release injectable naltrexone—Vivitrol—blocks opioids for about 28 days from one injection, which is exactly why it is given once a month[2].
| Form | How long one dose blocks opioids |
|---|---|
| Oral naltrexone (50 mg pill) | About 24 to 48 hours |
| Vivitrol (monthly injection) | About 28 days |
What this means in practice. The pill asks for a decision every day, and a missed day reopens the door faster than people realize. The shot takes that daily decision off the table for four weeks at a time—the main reason it tends to hold people in treatment better than the tablet does.
How Long Until Naltrexone Starts Working
Once naltrexone reaches the receptors, it starts blocking opioids quickly—within an hour or so of an oral dose. There is no slow build-up to wait through the way there is with some medicines. The receptors get occupied, and from that point a high is off the table.
The Vivitrol injection reaches a protective level fast as well, then holds it steady across the month. People do not spend days waiting for the shot to “kick in” before they are covered.
So the catch with naltrexone is never how fast it works once it is in you. The catch is entirely about timing the start, because of one hard rule.
Why You Must Wait to Be Opioid-Free
This is the part that decides whether naltrexone can be your starting point. Because naltrexone shoves opioids off their receptors, taking it while any opioid is still in your body sets off precipitated withdrawal—a fast, brutal version of dope sickness that hits harder than withdrawal coming on naturally.
To avoid it, you have to be genuinely off opioids before the first dose, usually 7 to 10 days, and longer for fentanyl, which stores in the body’s fat and clears more slowly than heroin or pills[1].
What this means for starting. That waiting window, not the medicine, is the real barrier. Getting through a week or more of withdrawal before you are even allowed to begin is hard, and it is the reason many people start on buprenorphine or methadone instead—those can be started while you still have opioids on board. With naltrexone, the honest path usually runs through a medical detox first, where medications make the withdrawal far gentler than the agony you are bracing for. Detox is the on-ramp, not a dead end. Once you are through it, the door to naltrexone is open.
Getting Help
If you are reading this to understand the timeline, that is a good sign—it usually means you are taking recovery seriously, whether for yourself or someone you love. Naltrexone leaves the body fast, blocks opioids for a day or a month depending on the form, and carries no high and nothing to get hooked on. The only real hurdle is the opioid-free window at the very start, and a medical detox clears it.
The medicine works, and the life on the other side of those first opioid-free days is steadier than the fear holding you in place. If you want the fuller picture, learn how naltrexone treats opioid addiction, compare the daily pill with the monthly Vivitrol shot, or see what side effects to expect.
When you are ready to find a program or a prescriber near you, find a treatment center →.
If you or someone you love is in crisis, call or text 988 any time to reach the Suicide and Crisis Lifeline, or call 911 for a medical emergency.
If any of this lands, the next step doesn’t have to be a big one. You can find treatment now and get matched with someone who can help you find the right care and take the next step. Reaching out today is a real step forward — and one you can make right now.
Frequently asked questions
Does naltrexone show up on a drug test?
Standard workplace and clinical drug panels do not screen for naltrexone, because it is a blocker with no abuse potential and no street value, so there is no reason for routine tests to look for it. A lab can detect it with a specific test if asked, but that is rarely done. If you ever do need to explain a result, naltrexone is a prescribed medicine for opioid or alcohol use disorder, which is a medical fact, not a mark against you.
How long after stopping naltrexone do opioids work again?
After your last oral dose, the opioid block fades over about 24 to 48 hours, so normal opioid effects return within a day or two. After a Vivitrol shot it takes much longer, because the block lasts about 28 days and tapers off near the end of the month[2]. This is exactly the window to be careful in: your tolerance has dropped while you were on naltrexone, so a dose that once felt normal can be enough to overdose.
Can you overdose trying to override the naltrexone block?
Yes, and it is dangerous. Some people take larger and larger amounts of opioids trying to feel something through the block, which is one of the most lethal things you can do on naltrexone. As the dose wears off, all of that opioid can hit the receptors at once and stop your breathing. Trying to override the block is a known cause of overdose death, not a way around it. Keep naloxone (Narcan) on hand and never test the block.
How long does the Vivitrol shot last?
One Vivitrol injection blocks opioids for about a month, which is why it is given every four weeks[2]. The medicine releases slowly from the muscle over that time, so detectable drug is present the whole month rather than clearing in a day or two like the pill. The block is strongest in the first few weeks and tapers toward the end, which is why staying on schedule for the next shot matters.
Why does naltrexone block opioids longer than it stays in your system?
Because the block does not come only from naltrexone itself. The liver turns naltrexone into an active product called 6-beta-naltrexol, which also occupies opioid receptors and lasts longer than the parent drug, with a half-life of roughly 13 hours versus about 4[1]. So the original drug fades within hours while its breakdown product keeps the receptors covered, which is how one oral dose holds the block for 24 to 48 hours.
How long do you have to be off opioids before starting naltrexone?
Usually 7 to 10 days off all opioids, and longer for fentanyl, which clears more slowly because it stores in body fat[1]. Starting before you are clear triggers precipitated withdrawal, a sudden and severe form of dope sickness. A provider confirms you are opioid-free, sometimes with a small naloxone challenge, before giving the first dose. If you are still using, a medical detox is the safe way to clear that window.
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