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How long does naltrexone take to work?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on July 14, 2022.

Official answer


Naltrexone is available as a long-acting intramuscular injection (brand name: Vivitrol) or as oral tablets. Oral naltrexone is well-absorbed and will begin working within one hour after administration. After intramuscular injection, naltrexone reaches an initial peak at roughly 2 hours, followed by a second peak observed 2 to 3 days later.

How is naltrexone used?

Naltrexone is approved for use in the treatment plan of patients with opioid use disorder (OUD) or alcohol use disorder, along with group support and counseling. Naltrexone is classified as a full opioid antagonist that blocks euphoric actions (does not cause addiction or a "high"). When someone with OUD uses opiates like heroin or oxycodone, naltrexone blocks the pleasurable effects from the opioid.

Naltrexone is not a controlled substance and any doctor can prescribe naltrexone.

Naltrexone tablets are usually given once per day, although alternative maintenance dosing schedules may be used. It is available as a generic option from your pharmacy.

The intramuscular injection is an extended-release formulation and is given once every 4 weeks. There is no generic option available for Vivitrol to date, which contains 380 mg of naltrexone per vial. Vivitrol is only available at certified pharmacies and is shipped to your health care provider, who will give you the injections each month.

Oral naltrexone tablets have not been shown to be more effective than placebo for opioid use disorder due to poor patient adherence.

Do not start naltrexone therapy until you are opioid-free for at least 7 to 10 days after your last opioid use. This will help you to avoid an opioid withdrawal that may require hospitalization.

For treatment of alcohol dependence, you should not be actively drinking at the time you start treatment.

How long does naltrexone work?

The 50-mg oral tablets have an effect that lasts 24 to 36 hours. Higher doses have a longer duration, with 100 mg lasting 48 hours and 150 mg lasting 72 hours. Oral naltrexone tablets are usually given once a day.

Blood levels of intramuscular naltrexone will begin to slowly decrease 14 days after dosing, but levels will be measurable for about one month.

Bottom Line

  • Naltrexone is approved for use in the treatment of opioid use disorder and alcohol use disorder.
  • Oral naltrexone is well-absorbed and will usually begin working within one hour after a dose. Intramuscular naltrexone is an extended-release formulation and has two peaks, first at 2 hours then again 2 to 3 days later.
  • The 50-mg oral tablets typically last one day, while the intramuscular injection has an effect for one month, although blood levels will start to decline after 14 days.

This is not all the information you need to know about naltrexone for safe and effective use. Review the full naltrexone prescribing information here, and discuss this drug with your doctor or other health care provider.


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