Avonex (interferon beta-1a) is a brand-name injection that’s prescribed for relapsing forms of multiple sclerosis (MS) in adults. As with other drugs, Avonex can cause side effects, such as flu-like symptoms and injection site reactions.

Avonex 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 Avonex in clinical trials:

* For more information about this side effect, see “Side effect specifics” below.

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

Mild side effects that have been reported with Avonex 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 during treatment with Avonex and want to tell the FDA about it, visit MedWatch.

* For more information about this side effect, see “Side effect specifics” below.
† An allergic reaction is possible after injecting Avonex. To learn more, see the “Side effect specifics” section below.

Avonex 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 Avonex’s prescribing information.

If you develop serious side effects during treatment with Avonex, 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:

* This side effect wasn’t reported during clinical trials of Avonex. However, there have been reports of it in people during treatment with Avonex. It’s unclear how often this has occurred or if Avonex was the cause.
† For more information about this side effect, see “Side effect specifics” below.
‡ An allergic reaction is possible after injecting Avonex. To learn more, see the “Side effect specifics” section below.

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

Does Avonex cause long-term side effects?

It’s possible. However, the more common side effects of Avonex, such as flu-like symptoms, typically occur within hours to days after injecting a dose. These symptoms typically last for a day after each dose but may become less bothersome over time.

No long-term side effects were reported during the clinical trials of Avonex. However, long lasting side effects have been reported in people who’ve injected Avonex. Examples include:

It’s unclear how often the reports have occurred or whether Avonex was the cause. To learn more, see the “Serious side effects” section above.

Talk with your doctor if you have questions or concerns about long-term side effects that Avonex may cause.

Will I experience certain side effects when stopping Avonex?

Probably not. There are no known side effects from stopping Avonex treatment.

However, if you stop Avonex therapy, it’s likely that your MS symptoms will worsen. (Avonex is prescribed MS and clinically isolated syndrome.) Talk with your doctor before stopping Avonex treatment.

If you have questions about possible side effects after stopping Avonex, talk with your doctor.

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

Flu-like symptoms

Flu-like symptoms were the most common side effect reported with Avonex use in clinical trials. Symptoms typically occur within hours to days of the injection. Flu-like symptoms are more likely to occur when first starting therapy. They tend to ease or go away over time.

Flu-like symptoms may include:

What you can do

To decrease the risk and severity of this side effect, your doctor may prescribe a lower Avonex dose to start. Then they’ll slowly increase your dose over a few weeks. To learn more about Avonex and dosage, see this article.

On injection days, your doctor may recommend taking an over-the-counter (OTC) medication to relieve pain and fever. Examples include acetaminophen (Tylenol) and ibuprofen (Advil, Motrin). These medications can help prevent or reduce flu-like symptoms from your injection. It’s also important to drink plenty of water to stay hydrated.

Depending on how soon you experience flu-like symptoms after the injection, you could consider injecting your dose at different times of the day. If you have questions about dose timing, talk with your doctor for more information.

Injection site reactions

Avonex is given as an intramuscular injection. Injection site reactions were a common side effect in the drug’s clinical trials. In rare cases, a reaction at the injection site could become severe, including infection and necrosis (severe skin damage).

Symptoms of a reaction around the area where you receive your injection may include:

What you can do

You or your caregiver will give your Avonex injection once per week. To reduce the risk of injection site reactions, the injection site should be rotated with each injection. This means choosing a different spot for each Avonex injection. Avonex should not be injected into any area where the skin is irritated, bruised, infected, or scarred. To learn more about how Avonex is given, see this article.

Injection site reactions will typically go away on their own. But if the symptoms bother you, there are some ways to help relieve them. You may try applying a cold compress to help reduce swelling and pain. Your doctor may recommend an OTC pain medication such as ibuprofen (Advil, Motrin). This can help minimize pain and inflammation.

Talk with your doctor if you experience injection site reactions that don’t ease in a few days. You should also contact your doctor if the symptoms are getting worse instead of better.

Allergic reaction

As with most drugs, Avonex can cause an allergic reaction in some people. However, it’s not clear how often this side effect occurred 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 continue treatment with Avonex. However, if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency number.

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

Thyroid problems: If you have a thyroid problem, such as an underactive or overactive thyroid, it’s possible that Avonex could make your condition worse. Tell your doctor if you have a history of thyroid problems before starting Avonex. They’ll monitor your thyroid hormone levels regularly during treatment.

Depression or mood disorders: It’s possible that Avonex can cause depression, mood changes, or suicidal thoughts or behaviors. If you have a mental health condition, Avonex treatment could worsen it. Before starting Avonex, tell your doctor if you’ve ever had depression or other mood disorders. Your doctor can help determine whether Avonex is the right treatment option for you.

Heart problems: If you have heart failure or other heart problems, Avonex could worsen your condition. Tell your doctor about any heart problems you have before starting Avonex. Your doctor may monitor you more closely for signs of heart problems or may recommend a different treatment option for you.

Seizures: If you have a seizure disorder, Avonex treatment could increase your risk of seizures. Some people have developed seizures during treatment with Avonex. Be sure to tell your doctor if you have a seizure disorder or have had a seizure in the past. They may recommend a different treatment option for you.

Liver problems: If you have liver damage, Avonex therapy could make your condition worse. This is because Avonex may cause liver damage. If you have liver disease or a history of liver problems, talk with your doctor before starting treatment with Avonex.

Also be sure to tell your doctor what other medications you take. Combining Avonex with other medications that can damage the liver may increase your risk of liver problems. Your doctor may monitor you more closely for signs of liver problems during Avonex treatment.

Problems with low blood cell counts: Avonex may decrease your number of red blood cells, white blood cells, and platelets. If you already have low blood cell counts, Avonex therapy may lower them further. Your doctor will likely monitor your blood cell counts closely during treatment with Avonex.

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

Pregnancy or breastfeeding: It’s not known whether Avonex is safe to receive during pregnancy or while breastfeeding your child. Talk with your doctor before starting Avonex if you’re pregnant, planning to become pregnant, or breastfeeding.

Alcohol consumption: There are no known interactions between Avonex and alcohol. However, both Avonex and alcohol may cause liver damage. Consuming alcohol during Avonex treatment may further increase the risk. If you drink alcohol, talk with your doctor about whether it’s safe for you to consume it during treatment with Avonex.

As with most drugs, Avonex can cause side effects. Most side effects are mild and may go away as your body adjusts to the drug. However, serious side effects are possible. 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 Avonex, talk with your doctor or pharmacist. They can help answer any questions you have about side effects from treatment with the drug.

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

  • More information about Avonex: For details about other aspects of Avonex, refer to this article.
  • Dosage: For information about the dosage of Avonex, view this article.
  • A look at MS: For details about MS, see our MS hub.

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.