How Long Does Naltrexone Take to Work?

Asking how long it lasts after a dose, or how long a drug test can find it? That is covered on how long naltrexone stays in your system.

Timeline: first dose to month 6

No build-up phase: the block starts with the first dose

Naltrexone is absorbed almost completely from the gut and blocks opioid receptors on the day you take it. Daily tablets do not pile up in the body either: blood levels did not accumulate on 100 mg a day.¹

Because the block is immediate, anyone with opioids in their body can go into sudden withdrawal on the first dose. After short-acting opioids, the label asks for an opioid-free gap of at least 7 to 10 days.¹ The details are on naltrexone dosage.

For alcohol, the label says the mechanism "is not understood," though preclinical data point to the body's own opioid system.¹ In a 1992 trial of 70 men, the difference showed up mainly after a first slip (the relapse figures).⁹ J.D. Sinclair, after whom the Sinclair method is named, proposed that each drink taken while the reward is blocked weakens the urge a little, which he called extinction.¹⁰ That is a hypothesis, not a proven mechanism. The Sinclair method page covers it.

Your drinking log shows whether it is working

The trials on this page measured heavy drinking days, drinks per day and days with any drinking.^2,7 In the Vivitrol trial, a heavy drinking day meant 5 or more standard drinks for men and 4 or more for women.⁵ So the most useful check is a written count: note each day's drinks from the first dose and bring the log to follow-up visits.

A 6-day lab study suggests what to look for. The first drink felt the same on naltrexone as on placebo; the difference came after it, with fewer and slower drinks and less craving.⁶ A first drink that feels unchanged does not mean the drug has failed.

Daily pill-taking matters too. In two trials the benefit was larger in people who took their tablets reliably, though those groups were not randomized (pill-taking).^11,12 MedlinePlus adds that naltrexone helps only while you keep taking it.¹³

The fastest data come from the shot, in one trial

The early-response figures come from a post hoc analysis of the 24-week Vivitrol trial: a look back at data the trial was not set up to test.^2,14 The trial gave monthly shots plus counseling to 624 alcohol-dependent adults, most of them still drinking at the first shot, and the 380 mg dose cut the rate of heavy drinking days by 25% compared with placebo over the whole trial.¹⁴

Three limits apply to the early finding:

• It is one trial. A second paper on the same trial, looking at 82 people who had already stopped drinking for at least 4 days, is the same evidence, not a new study.¹⁵
• The label narrowed who it applies to. The Vivitrol label reports greater drops in drinking days and heavy drinking days among the 8% who had stopped drinking the week before the first shot, and says the same effects were not evident in the 92% who were actively drinking. The label says people should not be actively drinking at the first injection.⁵
• It does not transfer to tablets. After a shot, naltrexone stays measurable for more than a month, and one shot delivers 3 to 4 times the naltrexone of 28 days of 50 mg tablets.⁵

How the shot and the tablets compare on cost and use is on naltrexone vs Vivitrol, and the shot itself is covered on Vivitrol.

Guidelines judge it at 4 to 6 weeks or 3 months

The US tablet label sets no checkpoint. Its alcohol trials used 50 mg a day for up to 12 weeks, and other durations were not tested.¹ The two guidelines that do give one differ:

• NICE (England, 2011): stop oral naltrexone if drinking persists 4 to 6 weeks after starting. NICE wrote this for people starting after assisted withdrawal.³
• VA/DoD (US, 2021): stop and consider other options if there is no detectable benefit after 50 mg a day for 3 months. For the shot, the cut-off is no detectable benefit within 3 months.⁴

Stopping is not the only choice. Raising the dose, more check-ins, different counseling or a switch are covered in what to do if you are still drinking heavily. The other medicines are compared on naltrexone vs acamprosate and naltrexone vs disulfiram. Whether it works at all, and for whom, is on does naltrexone work?

Side effects tend to come in the first weeks

In a 12-week open-label study of about 570 people taking the tablets for alcohol, 10% had new nausea and 7% new headache.^1,8 Nausea was the most common reason the 15% who stopped for side effects gave up the drug.⁸ In a study of 120 people that rated side effects after 1 week, 15% had moderate or severe nausea in that week. It was tied to missed doses and heavier drinking, and was less likely in people who had been sober longer before starting.¹⁶ In a 128-person trial, people with more or worse side effects in their first week, especially nausea and fatigue, were more likely to drop out early.¹⁷

With the shot, nausea after the first injection tends to be mild and to ease within a few days.⁵ Rates compared with placebo are on naltrexone side effects.

Early side effects, mostly nausea and headache, typically settle within 1 to 2 weeks, and the VA/DoD guideline suggests taking each dose with food.⁴ The opioid check, starting dose, timing and when to call are in starting naltrexone: the first two weeks.

Weight loss and low-dose naltrexone run on other clocks

Weight loss. The 50 mg tablet's label covers alcohol and opioid use only.¹ The approved weight-loss product, Contrave, combines naltrexone with bupropion. Its label says to check the result after 12 weeks on the full dose and to stop if weight has not dropped by at least 5%.¹⁸ Naltrexone and bupropion (Contrave) covers it.

Low-dose naltrexone (LDN). LDN uses doses far below the 50 mg tablet and is off-label. Its evidence, and its own timing questions, are on low-dose naltrexone.

Your experience: If you've started naltrexone, you can tell us what your first weeks were like. Stories appear on naltrexone reviews only with permission.

How we checked

• Timing figures come from the current tablet, Vivitrol and Contrave labels, NICE CG115 and the VA/DoD 2021 guideline, checked against saved copies on October 17, 2026.^1,3-5,18
• Trial and lab figures come from the PubMed abstracts of 11 papers, listed in the references; we haven't read the full texts, so day-by-day drink counts are not shown.
• Ciraulo 2008 and O'Malley 2007 both analyze the Garbutt 2005 trial and count as one trial.