Avonex (interferon beta-1a) is a prescription biologic drug that’s used to treat certain types of multiple sclerosis. Avonex can cause side effects that range from mild to serious. Examples include chills and fever.

Specifically, Avonex is used in adults to treat the following types of multiple sclerosis (MS):

The active ingredient in Avonex is a biologic medication called interferon beta-1a. Biologic medications are made from living cells. The drug comes as a liquid for injecting into a muscle.

Keep reading to learn about the common, mild, and serious side effects that Avonex can cause. For a general overview of the drug, including details about its uses, see this article.

Some people may experience mild to serious side effects during their Avonex treatment. Examples of the drug’s commonly reported side effects include:

  • weakness
  • muscle aches
  • fever
  • chills

The following sections include more side effects of Avonex. Each person’s experience with side effects may differ.

Mild side effects have been reported with Avonex. These include:

In most cases, these side effects should be temporary. And some may be managed easily. But if you have symptoms that are ongoing or bothersome, talk with your doctor or pharmacist. And do not stop using Avonex unless your doctor recommends it.

Avonex may cause mild side effects other than those listed above. See the drug’s prescribing information for details.

To learn more about mild allergic reaction, see the “Side effects explained” section below.

Serious side effects have been reported with Avonex. These include:

If you develop serious side effects while using Avonex, call your doctor right away. Some side effects may seem life threatening, or you may think you’re having a medical emergency. If so, immediately call 911 or your local emergency number.

To learn more about the following serious side effects, see the “Side effects explained” section below:

  • liver damage
  • depression and suicide
  • severe allergic reaction

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with Avonex, visit MedWatch.

Get answers to some frequently asked questions about Avonex’s side effects.

Does Avonex cause long-term side effects?

It’s possible. Long-term side effects include those that may start at any time you’re using a drug, even if you’ve used it for a long time. They also include side effects that may not go away, even after you stop using the drug.

Examples of long-term side effects reported in Avonex’s studies include:

You can learn more about the side effects of liver damage, depression, and suicide below. See the “Side effects explained” section.

If you’re concerned about possible long-term side effects from Avonex, talk with your doctor or pharmacist.

Will I experience certain side effects after stopping Avonex?

No, you shouldn’t experience any side effects from stopping Avonex treatment. Studies show it might take about 4 days after your last dose for your body to remove Avonex completely. After then, you may experience worsening of your MS symptoms. But some people may not notice a difference in their symptoms.

If you’re considering stopping Avonex, talk with your doctor or pharmacist first.

Help is out there

If you or someone you know is in crisis and considering suicide or self-harm, please seek support:

If you’re calling on behalf of someone else, stay with them until help arrives. You may remove weapons or substances that can cause harm if you can do so safely.

If you are not in the same household, stay on the phone with them until help arrives.

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Learn more about some of the side effects Avonex may cause.

Depression and suicide

Depression was a common side effect reported in studies of Avonex. Depression may cause symptoms such as:

  • hearing or seeing things that are not real
  • having aggressive thoughts or performing aggressive actions
  • feeling anxious or nervous
  • feeling hopeless
  • having negative thoughts about yourself
  • getting upset easily
  • hurting yourself or having thoughts of hurting yourself

Some factors can increase your risk of depression or suicide during Avonex treatment. These include:

  • having a history of mental health disorders, depression, or suicide
  • having a substance misuse history
  • being isolated or lacking social support

What might help

If you have a history of depression, mental health disorders, or suicide, talk with your doctor before starting Avonex. They may suggest mental health assistance before, during, and after Avonex treatment. This might include counseling, medication, and group support.

Talk with your doctor right away if you develop depression or suicidal thoughts during Avonex treatment. They’ll likely recommend an emergency medical evaluation. If your situation is life threatening, call 911 or your local emergency number immediately.

Liver damage

In studies of Avonex, liver damage was a rare side effect. Liver damage may cause symptoms such as:

  • confusion
  • yellowing of the skin or the whites of the eyes
  • nausea
  • vomiting
  • lack of appetite
  • dark urine the color of cola or pale stool the color of clay
  • fatigue (lack of energy)
  • bruising or bleeding easily
  • stomach pain

Factors that can increase the risk of liver damage when using this drug include:

  • active liver disease such as hepatitis
  • taking other medications that can affect the liver

What might help

If you have concerns about liver problems with Avonex treatment, talk with your doctor or pharmacist. They can review your medications and medical history to determine whether Avonex is right for you.

Allergic reaction

Like most drugs, Avonex can cause an allergic reaction in some people.

Symptoms can be mild to serious and can include:

  • skin rash
  • itchiness
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin (usually in your eyelids, lips, hands, or feet)
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms. Examples include:

  • an oral antihistamine, such as Benadryl (diphenhydramine)
  • a product you apply to your skin, such as hydrocortisone cream

Your doctor can confirm whether you’ve had a mild allergic reaction to Avonex. If they do, they’ll decide whether you should continue using it.

If you have symptoms of a severe allergic reaction, such as swelling or difficulty breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

Your doctor can confirm whether you’ve had a serious allergic reaction to Avonex. If they do, they may have you switch to a different treatment.

Keeping track of side effects

During your Avonex treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you first start a new drug or combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how your symptoms affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Keeping notes and sharing them with your doctor will help them learn more about how Avonex affects you. They can then use this information to adjust your treatment plan if needed.

Below is important information you should consider before using Avonex.

Warnings

Avonex can sometimes cause harmful effects in people who have certain conditions. This is known as a drug-condition interaction. Other factors may also affect whether Avonex is a good treatment option for you.

Talk with your doctor about your health history before using Avonex. Be sure to tell them if any of the following factors apply to you:

Alcohol and Avonex

You should not drink excessive amounts of alcohol while using Avonex. Severe liver damage, including liver failure, has been reported with the use of Avonex. So the use of alcohol can increase this risk.

Talk with your doctor or pharmacist if you have questions about drinking alcohol during Avonex treatment. They can tell you how much is safe to drink.

Pregnancy and breastfeeding with Avonex

Before starting Avonex treatment, it’s important to talk with your doctor if you’re pregnant, breastfeeding, or planning either.

Pregnancy

It’s not known whether it’s safe to use Avonex during pregnancy. Research has studied fetal development problems and the use of Avonex or similar drugs during early pregnancy.

The research has shown no connection between these problems and the use of these drugs. But information about the potential risk of low birth weight or pregnancy loss isn’t consistent. So no conclusions can be made.

If you’re pregnant or can become pregnant, talk with your doctor before starting treatment with Avonex.

Breastfeeding

It isn’t known whether Avonex is safe to use while breastfeeding.

Avonex and other drugs similar to it have been found to pass into breast milk at low levels. But information on how this affects the production of milk and a child’s who breastfed is limited.

If you’re breastfeeding or planning to do so, talk with your doctor before starting this medication.

Like most drugs, Avonex can cause a number of side effects that range from mild to serious. But most are temporary and go away after a few days to weeks. If you have questions about side effects that Avonex can cause, talk with your doctor.

Examples of questions to help get you started include:

  • What should I do if I develop an injection site reaction with Avonex?
  • How will you monitor me for liver or blood problems from Avonex?
  • Is muscle weakness or pain from Avonex something that will go away over time?
  • If I get flu-like symptoms from Avonex, will over-the-counter drugs treat them?
  • What is my risk of severe side effects from Avonex?

To learn more about Avonex, see these articles:

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Disclaimer: Healthline 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.