How Does Naltrexone Work? All About Its Mechanism Of Action

In 2022, almost 49 million people in the U.S. reported having a substance use disorder. This is a health condition where you drink alcohol or use drugs even when these substances are causing personal or health problems. Substance use disorder can affect your personal relationships and the ability to keep up at work, school, or home. 

Fortunately, there are many treatment options available for substance use disorder. This includes therapy, support groups, and medications. Naltrexone (Vivitrol) is one medication that treats alcohol use disorder and opioid use disorder. But how does naltrexone work and what exactly does it do? Let’s discuss.

How does naltrexone work?

To understand how naltrexone works, it helps to know about the body’s natural opioid system. Your body naturally releases opioids in certain situations. These are called endogenous opioids, and they have many functions. For example, endogenous opioids help relieve pain after an injury. They’re also released when we do pleasurable activities, like eat a good meal.

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When you drink alcohol or take opioids, your body releases endogenous opioids and a chemical called dopamine. Dopamine makes us feel good. It’s sometimes referred to as the “happy hormone”. But repeated use of substances to activate this dopamine release in the body can lead to addiction and dependence.

So what does naltrexone do? It’s an opioid blocker (antagonist). It blocks the rewarding effects of alcohol and opioids by blocking opioid receptors (areas in the body that opioids attach to). This makes substances like opioids and alcohol less enjoyable and lowers cravings for them. This may help you have an easier time avoiding these substances.

Good to know: Naltrexone isn’t the same as naloxone (Narcan, Zimhi). Although both medications are opioid antagonists, naloxone is used to quickly treat opioid overdoses. Naltrexone can’t treat an overdose. It’s a good idea to keep naloxone on hand if you or a loved one has opioid use disorder, even if you’re taking naltrexone. It can save someone’s life if they’re experiencing an opioid overdose. You should be able to get naloxone over the counter at any pharmacy. You may also be able to get it for free through distribution programs.

How and when should you take naltrexone?

Naltrexone comes in an oral and an injectable form.

The oral form comes as a 50 mg tablet that’s typically taken once a day. It’s only available as a generic. The usual dose for alcohol use disorder and opioid use disorder is 50 mg per day. When to take naltrexone depends on what’s best for you. But taking it at the same time each day will help you remember to take it.

Naltrexone also comes as an intramuscular (IM) injection. It comes as a 380 mg vial that’s injected into your buttock muscle by a healthcare professional. The usual dose is 380 mg every 4 weeks. It’s available as a brand-name product only (Vivitrol).

Good to know: Some research suggests that taking naltrexone “as needed” is effective. This is sometimes called “targeted naltrexone” dosing, where you take naltrexone only before situations where you’re likely to binge drink or experience intense cravings. This may be especially true for heavy drinkers and people who don’t want to quit alcohol completely. But this dosing method isn’t FDA approved. Talk to your prescriber before taking naltrexone this way.

How effective is naltrexone?

In general, oral naltrexone is a better option for alcohol use disorder than opioid use disorder. But IM naltrexone may be a good option for both. However, there’s no one-size-fits-all approach to treating these conditions. Whether naltrexone is a good option for you depends on several personal factors.

Alcohol use disorder

Either form of naltrexone is considered a first-choice treatment option for moderate to severe alcohol use disorder. 

Still, research on naltrexone for alcohol use disorder is mixed. For example, an analysis of six studies found that oral naltrexone helped people drink less alcohol versus placebo (a pill with nothing in it). They were also more likely to quit alcohol entirely. However, another study of over 200 men showed that oral naltrexone didn’t reduce the amount of alcohol they drank or increase the likelihood of quitting alcohol.

If taking oral naltrexone daily is something you struggle with, IM naltrexone may be a better option. One small study found that people receiving IM naltrexone were slower to return to drinking than those taking oral naltrexone. In another study comparing IM naltrexone to placebo, IM naltrexone led to a 25% reduction in heavy drinking days.

Good to know: Along with naltrexone, acamprosate is considered a first-choice treatment option for alcohol use disorder. It works on the brain to lower cravings. Disulfiram is also an option, but it’s not typically tried first. It works by causing unpleasant symptoms when you drink alcohol. Check out other GoodRx Health articles to learn about the differences between acamprosate versus naltrexone, and disulfiram versus naltrexone.

Opioid use disorder

Oral naltrexone isn’t recommended as a first-choice medication for opioid use disorder. This is supported by research. For example, a large review of over 1,000 people with opioid use disorder found that naltrexone wasn’t better than placebo at preventing relapse to opioids.

Injectable naltrexone may be considered a treatment option for certain people with opioid use disorder. It's best for people who have completed the detox process, and are no longer physically dependent on opioids. In these cases, it may help prevent a lapse in opioid use. A study of 250 people found that people receiving injectable naltrexone were more likely to stay away from opioids than people receiving placebo.

Good to know: Even though oral naltrexone isn’t a first-choice option for opioid use disorder, they may be situations where it’s a good choice for you. This is because the best medication for opioid use disorder depends on several factors, including your treatment goals and your medical history. Other medication options besides naltrexone include methadone (Methadose), buprenorphine (Subutex), and buprenorphine / naloxone (Suboxone).

Can you take naltrexone during a relapse?

If you use alcohol or opioids while taking naltrexone, there are a few things to consider.

If you relapse to alcohol

It’s likely OK to continue taking naltrexone. But talk to your prescriber for more guidance. Naltrexone is absorbed by the liver and can cause liver damage in rare cases. So naltrexone may not be the best option for people with liver problems.

It’s important to note that although naltrexone can lower the urge to drink alcohol, it doesn’t lessen the effects alcohol has on the body. Drinking alcohol can still worsen coordination, response time, and thinking. So even if you’re taking naltrexone, alcohol can still have negative effects if you continue to drink.

If you relapse to opioids

Discuss whether you should continue naltrexone with your prescriber. After taking naltrexone, you might be more sensitive — and have a lower tolerance — to opioids. This means that you’re at greater risk of an opioid overdose, even with opioids doses that are lower than what you’ve tolerated in the past.

On the other hand, starting naltrexone while you’re still taking opioids (or too soon after stopping opioids) can cause opioid withdrawal. Withdrawal symptoms include sweating, irritability, and diarrhea. This is why you shouldn’t take any opioids for 7 to 10 days before starting naltrexone.

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Written by:

Kevin Le, PharmD, BCPS, BCPPS

Kevin Le, PharmD, BCPS, BCPPS is a clinical pharmacy specialist in solid organ transplant at Ann & Robert H. Lurie Children’s Hospital of Chicago. He has been working as a pediatric pharmacist since 2016.

Edited by:

Amy B. Gragnolati, PharmD, BCPS

Amy Gragnolati, PharmD, BCPS, is a pharmacy editor for GoodRx. Amy currently holds her pharmacist license in Georgia and California.

Reviewed by:

Katie E. Golden, MD

Katie E. Golden, MD, is a board-certified emergency medicine physician and a medical editor at GoodRx.

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If you or someone you know struggles with substance use, help is available. Call SAMHSA’s National Helpline at 1-800-662-4357 to learn about resources in your area.