Naltrexone is a generic drug that’s prescribed for opioid use disorder and alcohol use disorder in adults. As with other drugs, naltrexone can cause side effects, such as nausea, anxiety, and headache.

Naltrexone oral tablet is not available in a brand-name version.

Naltrexone comes as an oral tablet. Naltrexone also comes in forms that are injectable and topical, but this article does not cover them.

Read on to learn about potential common, mild, and serious side effects of naltrexone. For a general overview of the drug, including details about its uses, see this article.

Naltrexone can cause certain side effects, some of which are more common than others. These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

These are just a few of the more common side effects reported by people who took naltrexone in clinical trials. These side effects vary depending on which condition the drug is being used to treat.

More common side effects in people taking naltrexone for opioid use disorder include:

More common side effects in people taking naltrexone for alcohol use disorder include:

Mild side effects can occur with naltrexone use. This list doesn’t include all possible mild side effects of the drug. For more information, you can refer to naltrexone’s prescribing information.

Mild side effects that have been reported with naltrexone include:

These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. If you develop a side effect while taking naltrexone and want to tell the FDA about it, visit MedWatch.

* An allergic reaction is possible after using naltrexone. However, this side effect wasn’t reported in clinical trials. To learn more, see the “Naltrexone: Side effect specifics” section below.

Although not common, naltrexone may cause serious side effects. The list below may not include all possible serious side effects of the drug. For more information, you can refer to naltrexone’s prescribing information.

If you develop serious side effects while taking naltrexone, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Serious side effects that have been reported and their symptoms include:

  • Liver problems, such as hepatitis. Symptoms can include:
    • fever
    • nausea and vomiting
    • abdominal pain
    • dark-colored urine or light-colored stools
    • jaundice
  • Depression. Symptoms can include:
    • fatigue
    • changes in appetite
    • loss of interest or pleasure in activities and hobbies
    • changes in sleep, such as oversleeping or difficulty sleeping
    • difficulty concentrating or making decisions
  • Risk of opioid overdose if you take opioids during treatment with or after stopping naltrexone.*
  • Risk of opioid withdrawal symptoms if you take an opioid in the 7–10 days before starting naltrexone.*
  • Severe allergic reaction.†

* For more information about this side effect, see “Naltrexone: Side effect specifics” below.
† An allergic reaction is possible after using naltrexone. However, this side effect wasn’t reported in clinical trials. To learn more, see the “Naltrexone: Side effect specifics” section below.

Naltrexone may cause several side effects. Here are some frequently asked questions about the drug’s side effects and their answers.

Can lower doses of naltrexone cause fewer side effects than higher doses?

It’s possible. Naltrexone side effects can vary depending on the dose and condition that’s being treated.

Naltrexone is sometimes prescribed off-label at low dosages (also referred to as low dose naltrexone) to treat a number of different conditions. These conditions include fibromyalgia and Crohn’s disease. With off-label use, a drug is prescribed for a use not approved by the Food and Drug Administration (FDA). At lower dosages, naltrexone works differently in the body and has been reported to cause fewer side effects.

If you have questions or concerns about side effects of naltrexone, talk with your doctor.

Do side effects of naltrexone go away?

Most side effects of naltrexone are mild and will go away as your body adjusts to the drug. They tend to be temporary, lasting a few days to weeks.

It’s possible for naltrexone to cause serious side effects that may require medical attention. For more information, see the “Naltrexone: Serious side effects” section above.

If you have questions about naltrexone and side effects, talk with your doctor.

Does naltrexone cause changes in weight?

Possibly. In clinical trials, weight loss and weight gain were rare side effects in people taking naltrexone for opioid use disorder. However, weight changes weren’t reported in people taking the drug for alcohol use disorder.

Naltrexone alone isn’t typically prescribed for weight loss. However, there is a combination drug containing naltrexone and bupropion (Contrave) that is FDA-approved for weight loss. Talk with your doctor if you are interested in learning more about Contrave.

If you have weight changes with naltrexone, talk with your doctor.

Learn more about some of the side effects that naltrexone may cause. To find out how often side effects occurred in clinical trials, see the prescribing information for naltrexone.

Risk of opioid overdose

Taking opioid medications during treatment with naltrexone or after stopping naltrexone can increase your risk of opioid overdose. Although they weren’t common, cases of opioid overdose have been reported in people who took opioids during or after treatment with naltrexone in clinical trials.

Symptoms of opioid overdose include:

  • shallow breathing
  • slow breathing or heart rate
  • pinpoint pupils
  • skin feeling cold or clammy
  • decreased responsiveness

What you can do

You should avoid all opioids while taking naltrexone. During and after treatment with naltrexone, your body will likely be more sensitive to opioids. Taking any amount of opioids can put you at a higher risk of accidental overdose.

If you need pain medication, such as after surgery or a procedure, it’s important to let your doctor know that you’ve been taking naltrexone. Your doctor will recommend alternative pain medications that don’t contain opioids.

Risk of opioid withdrawal symptoms

It’s possible to have symptoms of opioid withdrawal when starting naltrexone if you still have opioids in your body. These withdrawal symptoms can be severe and may require immediate medical attention. This side effect was rare in clinical trials in people who were opioid-free. Your doctor may give you a naloxone challenge test or a urine test for opioids before prescribing naltrexone to confirm you are opioid-free.

Common withdrawal symptoms include:

What you can do

If you have current symptoms of opioid withdrawal, you should not take naltrexone. You should not take any opioids or opioid-containing medications for a minimum of 7–10 days before starting naltrexone. If you’ve been taking buprenorphine or methadone, you should wait 14 days before starting naltrexone.

It’s important to tell your doctor if you have recently taken opioids or opioid-containing medications. You can ask your doctor if you don’t know whether the medication that you’ve been taking is an opioid. Talk with your doctor about when it’s safe for you to start taking naltrexone.

Allergic reaction

As with most drugs, naltrexone can cause an allergic reaction in some people. However, this side effect wasn’t reported in clinical trials.

Symptoms can be mild or serious and can include:

  • skin rash
  • itching
  • flushing
  • swelling under your skin, typically in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What you can do

For mild symptoms of an allergic reaction, call your doctor right away. They may recommend ways to ease your symptoms and determine whether you should keep taking naltrexone. However, if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Suicide prevention

If you know someone at immediate risk of self-harm, suicide, or hurting another person:

  • Ask the tough question: “Are you considering suicide?”
  • Listen to the person without judgment.
  • Call 911 or the local emergency number, or text TALK to 741741 to communicate with a trained crisis counselor.
  • Stay with the person until professional help arrives.
  • Try to remove any weapons, medications, or other potentially harmful objects if it’s safe to do so.

If you or someone you know is having thoughts of suicide, a prevention hotline can help. The 988 Suicide and Crisis Lifeline is available 24 hours a day at 988. During a crisis, people who are hard of hearing can use their preferred relay service or dial 711 then 988.

Find more links and local resources.

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Be sure to talk with your doctor about your health history before you take naltrexone. This drug may not be the right treatment for you if you have certain medical conditions or other factors that affect your health. These include:

Current opioid withdrawal symptoms: If you currently have opioid withdrawal symptoms, taking naltrexone may make your symptoms worse. Your doctor will advise you not to take naltrexone if you are experiencing withdrawal symptoms. They will determine when it’s safe for you to start taking naltrexone.

Currently or recently taking opioid medications: Starting naltrexone with opioids in your system can cause severe withdrawal symptoms. This risk applies if you’re currently taking opioids or have taken any in the last 7–10 days. If you are switching from buprenorphine or methadone to naltrexone, you may be at risk of withdrawal symptoms for 2 weeks after your last dose. Talk with your doctor if you don’t know whether a medication that you are taking is an opioid. They will determine when it’s safe for you to start taking naltrexone.

Liver problems: If you have or have had liver disease, such as hepatitis, taking naltrexone can worsen your condition or put you at higher risk of liver problems. If you start having liver problems or your liver problems worsen during treatment, your doctor may have you stop taking the drug.

Kidney problems: After you take a dose of naltrexone, your kidneys help remove the drug from your system. If you have kidney problems, this may be harder for your body to do. If you already have a kidney condition, such as chronic kidney disease, you may be at higher risk of side effects. Talk with your doctor about taking naltrexone to determine whether it’s the right treatment option for you.

Allergic reaction: If you’ve had an allergic reaction to naltrexone or any of its ingredients, your doctor will likely recommend that you do not take naltrexone. Ask your doctor what other medications may be better options for you.

Pregnancy or breastfeeding: It’s not known whether it’s safe to take naltrexone if you’re pregnant or breastfeeding. Naltrexone has not been studied in pregnant people or people who are breastfeeding. Please talk with your doctor before taking naltrexone.

Alcohol consumption: There are no known interactions between alcohol and naltrexone. If you take naltrexone for alcohol use disorder, you should not drink alcohol. This medication should lessen your desire to drink alcohol. Drinking alcohol during treatment with naltrexone can increase your risk of liver problems and should be avoided. Talk with your doctor if you have any concerns about avoiding alcohol while taking naltrexone.

Like most medications, naltrexone can cause side effects. Most are mild and usually go away on their own after a few days to weeks. Naltrexone can also cause some serious side effects. You should talk with your doctor if you have any side effects that last, are bothersome, or are serious.

If you’d like to learn more about naltrexone, talk with your doctor or pharmacist. They can help answer any questions you have about side effects from taking the drug.

Besides talking with your doctor, you can do some research on your own. These articles might help:

Disclaimer: Medical News Today has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.