Substance use & recovery

How Naltrexone and Vivitrol Block the Opioid High

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Naltrexone works by blocking, not replacing. It sits on the opioid receptors so opioids, and to a degree alcohol, lose their pull, which removes the reward that drives relapse. Because it blocks rather than fills those receptors, a person has to be fully off opioids before the first dose, or it triggers sudden withdrawal.

Last updated: July 2026

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How does naltrexone work?

Naltrexone is an opioid antagonist, meaning it binds to the mu-opioid receptors and occupies them without activating them 1. While it is on the receptor, opioids that reach the brain cannot dock and cannot produce a high or ease withdrawal, so the reward that drives relapse is simply not there. Naltrexone produces no opioid effect of its own: no high, no sedation, no physical dependence, and no withdrawal when it is stopped. It is one of the three FDA-approved medications for opioid use disorder, alongside methadone and buprenorphine, and it is the only one of the three that works by blocking rather than occupying the receptor with an opioid 1.

What is Vivitrol, and how is it different from the pill?

Vivitrol is the extended-release injectable form of naltrexone, given as a shot about once a month, while naltrexone also exists as a daily oral tablet 2. The medicine and the mechanism are identical; only the delivery differs. The monthly injection removes the daily decision to take a pill, which is why many people and clinicians prefer it, since a single missed day cannot undo the protection. The blocker is present steadily for weeks at a time. The trade-off is that the injection cannot be taken back once given, so the timing of the first dose matters a great deal. With the daily tablet, by contrast, a person can stop at any point, which makes it easier to start but also easier to skip, and skipping reopens the window in which opioids could work again. Neither form is more of a real medication than the other; they are two ways of delivering the same blocker, and the choice usually comes down to whether daily adherence or a monthly commitment fits a person's life better.

Why do you have to be fully off opioids before starting?

Because naltrexone knocks opioids off the receptors, giving it while any opioid is still in the body forces the person into sudden, severe withdrawal all at once. That is why a completed detox is required first. This requirement is also naltrexone's main hurdle: extended-release naltrexone is harder to start than buprenorphine-naloxone precisely because it requires finishing detox before the first dose, which leads to more early induction failures and relapse during that gap 2. The waiting period before the first injection is the vulnerable window, and it is managed carefully by the treating clinician.

Is naltrexone the same as naloxone or Narcan?

No, and the two are easy to confuse because both are opioid blockers with similar names. Naloxone, sold as Narcan, is a short-acting antagonist used to reverse an overdose in an emergency; its effect lasts only about 30 to 90 minutes, so an overdose can return and 911 should always be called 3. Naltrexone is long-acting and is used to prevent relapse over days or weeks, not to rescue someone who is overdosing right now. One is an emergency rescue; the other is ongoing relapse prevention. They are not interchangeable.

Does naltrexone work as well as methadone or buprenorphine?

Once a person is successfully started on it, extended-release naltrexone and buprenorphine-naloxone were similarly safe and effective in a head-to-head trial 2. The catch is getting started: because naltrexone requires completing detox first, more people relapse before they ever receive the first dose, which lowers its real-world effectiveness compared with medications that can be started sooner. It is also worth being straight that the largest population studies linking a medication to fewer overdoses point specifically to buprenorphine and methadone 4. Naltrexone is a genuine, evidence-based option, and it is the right one for some people, particularly those who cannot or do not want to take an opioid-based medication.

How does naltrexone help with alcohol?

Naltrexone is also one of three FDA-approved medications for alcohol use disorder, and it works on the same reward system 5. By blunting the pleasurable release that alcohol triggers, it reduces craving and the reinforcing pull of drinking. Like the other alcohol medications, it is not addictive and can be used with or without counseling 5. This dual use is why the same injection, Vivitrol, is prescribed for both opioid and alcohol use disorder. Some approaches to naltrexone for alcohol, such as the Sinclair method, use the pill in a specific timed way, which this library covers separately.

The overdose risk to understand

There is one safety point that matters more than any other with naltrexone. Because starting it requires being fully detoxed, and because the blocker eventually wears off, a person's opioid tolerance is lowered during and after treatment. Returning to a previous amount of an opioid after tolerance has dropped sharply raises the risk of a fatal overdose 6. Trying to override the blockade with large doses of opioids is especially dangerous for the same reason. Lowered tolerance means a past-normal amount of opioids can be deadly, which is why relapse after any opioid-free period is the highest-risk moment.

Who naltrexone tends to fit best

Naltrexone is a genuine, evidence-based standard of care, and it fits some situations better than others 7. It appeals to people who cannot or do not want to take an opioid-based medication, who are already past the detox stage and stable enough to complete the opioid-free waiting period, or who are in settings that favor a non-opioid approach. One group worth naming carefully is people leaving incarceration: their tolerance is low after enforced abstinence, and the first weeks after release carry a sharply elevated overdose risk 8, so having any effective medication in place matters a great deal. Naltrexone can be that medication, though the requirement to be fully detoxed first makes the timing around release especially important. The right choice among methadone, buprenorphine, and naltrexone is made with a clinician who weighs a person's history, tolerance, access, and preference, not by ranking the medications in the abstract.

Common questions

Vivitrol is an extended-release injection designed to keep naltrexone active for roughly a month between doses. During that window the opioid receptors stay blocked, so a single missed pill cannot undo the protection the way it can with the daily tablet. The exact timing and schedule are set by the clinician, and the injection cannot be removed once it is given.

Most people do not feel naltrexone itself, because it produces no high, no sedation, and no opioid effect. What people notice is the absence of the reward: opioids no longer produce a high, and for many, the urge to drink lessens. Because it is not an opioid, it causes no physical dependence and no withdrawal when it is stopped.

Because naltrexone displaces opioids from the receptors, taking it while any opioid is still in the body forces sudden, severe withdrawal. A completed detox and an opioid-free waiting period are required first. That waiting window is naltrexone's main drawback, since some people relapse before the first dose, and the timing is managed carefully by the treating clinician.

No. Naltrexone is a blocker, not an opioid, so it produces no high and no physical dependence, and stopping it causes no withdrawal. This is different from methadone or buprenorphine, which are opioids taken in a steady, controlled way. For alcohol use disorder, naltrexone is likewise non-addictive and can be used with or without counseling.

No. Narcan is naloxone, a short-acting blocker used to reverse an overdose in an emergency, lasting only about 30 to 90 minutes. Naltrexone is long-acting and prevents relapse over days or weeks. They are not interchangeable: naltrexone is not an overdose rescue, and naloxone is not a maintenance treatment.

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When to get help fast

  • —Signs of an opioid overdose in someone who has used: slow, shallow, or stopped breathing, blue-gray lips or fingertips, pinpoint pupils, or being unresponsive and impossible to wake
  • —A relapse to opioids after a detox or an opioid-free period, when tolerance is low and even a past-normal amount can be fatal
  • —Sudden, severe withdrawal within minutes to hours of a first naltrexone dose, meaning opioids were still in the body
  • —Thoughts of suicide or self-harm

If someone may be overdosing, call 911 now and give naloxone if it is available; the naltrexone blockade does not remove the need to call for help. For thoughts of suicide, call or text 988.

This article is health education, not medical advice, and it includes no dose. It cannot replace an evaluation by a licensed clinician, who decides whether and when naltrexone or Vivitrol is appropriate and safe to start.

References

  1. 1.Substance Abuse and Mental Health Services Administration (2021). TIP 63: Medications for Opioid Use Disorder — Full Document. SAMHSA Treatment Improvement Protocol 63. link ✓Naltrexone is one of the three FDA-approved medications for opioid use disorder; describes the medications and their mechanisms, with naltrexone acting as the antagonist option.
  2. 2.Lee JD, Nunes EV, Novo P, et al. (2018). Comparative effectiveness of extended-release naltrexone versus buprenorphine-naloxone for opioid relapse prevention (X:BOT): a multicentre, open-label, randomised controlled trial. The Lancet. doi:10.1016/S0140-6736(17)32812-X ✓Extended-release naltrexone is harder to initiate than buprenorphine-naloxone because it requires completed detox first, causing more early induction failures and relapse; once successfully initiated, both were similarly safe and effective.
  3. 3.National Institute on Drug Abuse (2024). Naloxone DrugFacts. National Institute on Drug Abuse (NIDA), NIH. link ✓Naloxone (Narcan) is a short-acting opioid antagonist that reverses overdose, with an effect lasting only about 30 to 90 minutes, so overdose can recur and 911 should always be called.
  4. 4.Wakeman SE, Larochelle MR, Ameli O, et al. (2020). Comparative Effectiveness of Different Treatment Pathways for Opioid Use Disorder. JAMA Network Open. doi:10.1001/jamanetworkopen.2019.20622 ✓In a study of more than 40,000 adults with OUD, only buprenorphine or methadone was associated with reduced overdose and serious opioid-related acute care.
  5. 5.National Institute on Alcohol Abuse and Alcoholism (2024). Recommend Evidence-Based Treatment: Know the Options. National Institute on Alcohol Abuse and Alcoholism (NIAAA), NIH. link ✓Naltrexone is one of three FDA-approved medications for alcohol use disorder; these medications are non-addictive and can be used with or without counseling.
  6. 6.Strang J, McCambridge J, Best D, et al. (2003). Loss of tolerance and overdose mortality after inpatient opiate detoxification: follow up study. BMJ. doi:10.1136/bmj.326.7396.959 ✓Detoxification lowers opioid tolerance, and returning to previous doses after an opioid-free period raises the risk of fatal overdose.
  7. 7.National Institute on Drug Abuse (2024). Medications for Opioid Use Disorder. National Institute on Drug Abuse (NIDA), NIH. link ✓Medications for opioid use disorder, including naltrexone, are an evidence-based standard of care.
  8. 8.Binswanger IA, Nowels C, Corsi KF, et al. (2012). Return to drug use and overdose after release from prison: a qualitative study of risk and protective factors. Addiction Science & Clinical Practice. doi:10.1186/1940-0640-7-3 ✓Overdose risk is sharply elevated in the first weeks after release from incarceration, driven in part by lowered tolerance during enforced abstinence.

8 sources, numbered by first appearance. General health information, not medical advice. AI-assisted editorial content — every citation independently verified. Editorial policy

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