How Long Does Naltrexone Stay in Your System?

At a glance

Three different questions

"How long does it stay in your system?" can mean three things, and each has a different answer.

1. How long is it in your blood? This is what half-life describes. Neither label says when naltrexone is completely gone. A common "5 half-lives" rule of thumb is not in either label, so we don't use it here.
2. How long does it keep working? For naltrexone, "working" means blocking opioids. The labels give blockade times that are longer than the tablet's half-life.^1,2 When it starts to help with drinking is a different question, answered on how long naltrexone takes to work.
3. How long would a test find it? That depends on what the test looks for. See drug tests.

Naltrexone tablets (oral)

Getting in. About 96% of a dose is absorbed from the gut, and blood levels of naltrexone and 6-β-naltrexol peak within an hour.¹

First pass through the liver. Much of each dose is broken down on its first pass through the liver, so only an estimated 5% to 40% reaches the bloodstream unchanged. The main breakdown product, 6-β-naltrexol, is active too, and the label attributes naltrexone's effect to both compounds.¹

Half-life. "The mean elimination half-life (T-1/2) values for naltrexone and 6-β-naltrexol are 4 hours and 13 hours, respectively."¹ These are two different compounds. Naltrexone's own half-life is about 4 hours, not "4 to 13 hours."

Leaving the body.

• More than 98% of naltrexone is metabolized (changed into other compounds).¹
• Naltrexone and its metabolites leave mainly through the kidneys (53% to 79% of the dose). Less than 2% of a dose comes out in urine as unchanged naltrexone. About 43% comes out in urine as 6-β-naltrexol (unchanged or conjugated).¹
• Naltrexone and its metabolites "may undergo enterohepatic recycling," meaning they may be passed into the gut and reabsorbed.¹

How long the block lasts. "Clinical studies indicate that 50 mg of naltrexone hydrochloride will block the pharmacologic effects of 25 mg of intravenously administered heroin for periods as long as 24 hours." Other data cited in the label suggest a double dose blocks for 48 hours and a triple dose for about 72 hours.¹ Those longer schedules (such as 100 mg every other day or 150 mg every third day) are options in supervised settings, but the label warns that the block "may be reduced by these extended dosing intervals."¹ Dosing is your prescriber's call; see naltrexone dosage.

Vivitrol (monthly injection)

Vivitrol is an extended-release form of naltrexone in tiny polymer microspheres, injected into the buttock muscle every 4 weeks or once a month.²

Levels over the month. Blood levels peak briefly about 2 hours after the shot and again about 2–3 days later. From about day 14 they slowly decline, "with measurable levels for greater than 1 month."²

Half-life. The half-life of naltrexone after a Vivitrol shot is 5 to 10 days and depends on how fast the polymer breaks down. The half-life of 6β-naltrexol is also 5 to 10 days.²

Blockade. Vivitrol "blocks the effects of exogenous opioids for approximately 28 days after administration."²

More total drug, less metabolite. One 380 mg shot gives 3 to 4 times more total naltrexone exposure than 28 days of 50 mg tablets. Repeat shots build up only minimally (<15%). Because less of the dose is broken down on a first pass through the liver, much less 6β-naltrexol is made than with tablets.²

It can't be removed. The label tells prescribers to explain that "once VIVITROL is injected, it is not possible to remove it from the body."²

For more on the shot, see Vivitrol (naltrexone injection) · Naltrexone vs Vivitrol · Vivitrol side effects.

What can change how long it lasts

If you have liver or kidney disease, ask your prescriber what this means for you. See also Naltrexone and liver tests (ALT/AST).

Will naltrexone show up on a drug test?

Not on the standard federal panel, because that panel doesn't look for it. The federal workplace drug-testing panel, which HHS republished in March 2026, covers marijuana, cocaine, a set of opioids (codeine, morphine, hydrocodone, hydromorphone, oxycodone, oxymorphone, heroin's marker 6-acetylmorphine, and fentanyl), PCP, and amphetamines including MDMA. Naltrexone and 6-β-naltrexol are not on it.³ That panel is set for federal workplace programs. If your test is for another employer, a clinic or a court, ask what it covers.

A false positive for oxycodone has been reported, rarely. Both labels hedge: tablets "may or may not interfere" with enzyme-based opioid tests, depending on the test, and Vivitrol "may be cross-reactive" with certain urine opioid immunoassays, so check the test maker's instructions.^1,2 In a 2024 case report, a man taking 50 mg tablets screened positive for oxycodone three times while his standard opiates screen stayed negative. Confirmatory testing found no oxycodone, but did find noroxymorphone, a minor naltrexone breakdown product that is also made when the body breaks down oxycodone.⁵ An earlier case report described the same pattern.⁶ One pathology lab's guide to its opiate screen lists high naltrexone levels as a possible cause of false positives.⁹ Confirmatory testing, which identifies specific drugs, is what sorts these results out, and the federal panel pairs each screen with a confirmatory test for named drugs.^3,5

A lab can find naltrexone when it's asked to. Mass-spectrometry methods can measure naltrexone and 6-β-naltrexol in urine below 1 ng/mL. One was developed to check whether people on court-ordered alcohol treatment in South Korea were taking their medicine.¹⁰

No published detection window. We found no study that says how many hours or days naltrexone stays detectable in urine, blood, saliva or hair. The nearest data are small studies from the 1970s. In patients taking 125 mg of naltrexone three times a week, 34% of a dose appeared in urine within 24 hours and 3% in the following 24 hours, mostly as 6-β-naltrexol.¹¹ In 4 people given 100 mg, low blood levels of naltrexone were still present at 72 hours.⁸ Those numbers describe how the drug leaves the body. They do not tell you how long any particular test stays positive.

Carry ID. Both labels advise carrying identification or documentation that shows you take naltrexone, so medical staff know in an emergency.^1,2

Stopping naltrexone before surgery

Why it matters. While naltrexone is blocking, opioid pain medicine may not work as expected. "In a situation requiring opioid analgesia, the amount of opioid required may be greater than usual, and the resulting respiratory depression may be deeper and more prolonged."¹ Opioid-containing cough, cold and diarrhea medicines may not work either.¹ After naltrexone is stopped, the opposite can happen: an anesthesia review notes that people may become more sensitive to opioids.¹²

Guidance agrees on stopping, not on exact timing. A 2024 scoping review by the Perioperative Pain and Addiction Interdisciplinary Network found 13 guideline documents. All recommended stopping naltrexone before surgery, whatever the dose, reason or form, but none was built on a systematic literature search or a formal expert-consensus method.⁷ Two examples:

Sources: SAMHSA⁴; Whately and Stead.¹² The Australian review adds that the gap may need to be longer with kidney or liver impairment, and that staying on naltrexone can be reasonable for procedures expected to cause little pain, such as cataract surgery or colonoscopy, decided case by case with the prescriber.¹²

Stopping has its own risk. A 2024 review for anesthesiologists says that patients who continue or stop naltrexone around surgery "are at an increased risk for undertreated pain or substance use relapse."¹³ The scoping review concluded that, for people at risk of returning to opioids or alcohol, stopping "should have a strong rationale."⁷ Plan the timing with both your surgical team and your naltrexone prescriber.

Emergency surgery or injury. For people on Vivitrol, the label suggests regional anesthesia or non-opioid pain medicine. If opioids are needed, the person should be continuously monitored in an anesthesia care setting.² SAMHSA's options for people on naltrexone include acetaminophen or ibuprofen for mild pain, regional anesthesia, and general anesthesia without opioids.⁴

Low-dose naltrexone is handled separately. The Australian review gives LDN, an off-label use at a fraction of the 50 mg dose, its own advice.¹² See surgery on LDN.

When the block wears off

"As the blockade of exogenous opioids provided by naltrexone hydrochloride wanes and eventually dissipates completely, patients who have been treated with naltrexone hydrochloride may respond to lower doses of opioids than previously used."¹ For Vivitrol, the label flags the end of the month after a shot, or a missed dose, as risky times.² Both labels tell prescribers to discuss opioid overdose reversal agents such as naloxone.^1,2 See Naltrexone vs naloxone.

Starting naltrexone after opioids. This is the reverse question: how long opioids need to be out of your system before naltrexone. The tablet label recommends at least 7 to 10 opioid-free days after short-acting opioids, and up to two weeks for people switching from buprenorphine or methadone.¹

What about low-dose naltrexone (LDN)?

See low-dose naltrexone. Side effects at LDN doses, and what trials saw when people stopped, are on low-dose naltrexone side effects.