Avonex is a brand-name injection that’s prescribed for relapsing forms of multiple sclerosis (MS). Avonex contains the active drug interferon beta-1a and belongs to the immunomodulator drug class.

Avonex is FDA-approved to treat the following relapsing forms of MS in adults:

Avonex also treats clinically isolated syndrome (CIS) in adults. With this condition, you have one episode of MS symptoms that lasts at least 24 hours. It may or may not lead to developing MS.

Drug details

You’ll find key information about Avonex below.

  • Drug form: Injection into a muscle
  • Generic or biosimilar available? No
  • Prescription required? Yes
  • Controlled substance? No
  • Year of FDA approval: 1996

Avonex can cause mild or serious side effects. The following lists contain some of the key side effects that may occur while taking Avonex. These lists do not include all possible side effects.

For more information on the possible side effects of Avonex, talk with your doctor or pharmacist. They can give you tips on how to manage any side effects that may be bothersome.

Note: The Food and Drug Administration (FDA) tracks side effects of drugs they have approved. If you would like to report to the FDA a side effect you’ve had with Avonex, you can do so through MedWatch.

Mild side effects

Below is a partial list of mild side effects of Avonex. To learn about other mild side effects, talk with your doctor or pharmacist, or view Avonex’s prescribing information.

Most of these side effects may go away within a few days or a couple of weeks. But if they become more severe or don’t go away, talk with your doctor or pharmacist.

* For more information about allergic reaction and Avonex, see “Allergic reaction” below.

Serious side effects

Serious side effects from Avonex aren’t common, but they can occur. Call your doctor right away if you have serious side effects. Call 911 if your symptoms feel life threatening or if you think you’re having a medical emergency.

Serious side effects and their symptoms can include:

  • Heart failure. Symptoms can include:
    • shortness of breath
    • swelling of your legs, ankles, or feet
    • sudden weight gain
  • Problems with your blood cells. Symptoms can include:
    • bruising or bleeding easily
    • fever
    • infections
    • feeling unusually weak or tired
  • Problems with your thyroid, such as an overactive or underactive thyroid. Symptoms can include:
    • feeling hot or cold
    • weight gain or weight loss
    • dry skin
    • fast or slow heartbeat
    • feeling restless, nervous, or shaky
    • feeling weak or tired
  • Severe injection site reactions, including necrosis (death of tissue). Symptoms can include:
    • pain
    • bruising
    • discoloration
    • lump or area of raised skin
  • Seizures.
  • Depression, suicidal thoughts, or other severe psychiatric symptoms.
  • Liver damage.
  • Severe allergic reaction.*

* For more information about allergic reaction and Avonex, see “Allergic reaction” 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 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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Allergic reaction

As with most drugs, some people can have an allergic reaction after taking Avonex. This was a rare side effect in clinical trials of this drug.

Symptoms of a mild allergic reaction can include:

A more severe allergic reaction is rare but possible. Symptoms of a severe allergic reaction can include:

  • swelling under your skin, typically in your eyelids, lips, hands, or feet
  • swelling of your tongue, mouth, or throat
  • trouble breathing

Call your doctor right away if you have an allergic reaction to Avonex, as the reaction could become severe. Call 911 or your local emergency number if your symptoms feel life threatening or if you think you’re having a medical emergency.

Avonex contains the active drug interferon beta-1a, which is a biologic drug. A biosimilar form of Avonex isn’t available.

A biosimilar is a drug that’s similar to a particular brand-name biologic drug. Biologics are made from living cells. So it’s not possible to copy them exactly. Generics, on the other hand, are drugs made from chemicals. They are an exact copy of the active drug in a brand-name medication.

Biosimilars are considered to be as safe and effective as their corresponding biologic drug. And as with generics, they tend to cost less than brand-name medications.

Another brand-name version of interferon beta-1a is available. It’s called Rebif. However, it can’t be substituted for Avonex.

The Food and Drug Administration (FDA) approves prescription drugs such as Avonex to treat certain conditions. Avonex may also be used off-label for other conditions. Off-label use is when a drug that’s approved to treat one condition is used to treat a different condition.

Avonex for MS and CIS

Avonex is FDA-approved to treat the following conditions in adults:

  • Relapsing forms of multiple sclerosis (MS), which include:
    • Relapsing-remitting MS. With this type of MS, there are periods of active disease that cause symptoms. These are followed by periods of remission where symptoms ease or go away completely.
    • Active secondary progressive MS. With this type of MS, the condition gradually gets worse all the time. But relapses can still occur, during which symptoms are noticeably worse for a while.

MS is a long-term condition that happens when your immune system mistakenly attacks the nerves in your brain and spinal cord. It causes inflammation (swelling) and scar tissue to develop around your nerve fibers. This makes it hard for your brain to communicate with the rest of your body. MS can have various symptoms, depending on which nerves are affected.

There’s currently no cure for MS. However, Avonex can slow the progression of the disease. It can help you have fewer relapses and reduce the number of lesions (damaged areas) in your brain. It slows down the development of disabilities (such as having trouble walking).

Avonex and children

Avonex is not approved for use in children. However, it’s sometimes used off-label to treat MS in children.

Research in children has shown that interferon beta (the active drug in Avonex) can:

  • slow the progression of disability
  • reduce the number of relapses
  • reduce brain lesions

More research is needed to know if Avonex is safe and effective for use in children.

The following information describes dosages that are commonly used or recommended. However, be sure to take the dosage your doctor prescribes for you. Your doctor will determine the best dosage to fit your needs.

Drug forms and strengths

Avonex comes in two forms for injection:

  • single-use prefilled autoinjector pen
  • single-use prefilled syringe

Avonex comes in one strength: 30 micrograms (mcg).

Dosage for relapsing-remitting MS and active secondary progressive MS

The dosing frequency for Avonex is once a week. You should have your injection on the same day each week.

When you begin treatment, your doctor may start you on a low dose of Avonex. This helps to minimize flu-like symptoms that can be a side effect of the injections. The dose is gradually increased over 4 weeks to help your body get used to the medication.

The recommended starting dosage schedule for Avonex is as follows:

  • Week 1: 7.5 mcg
  • Week 2: 15 mcg
  • Week 3: 22.5 mcg
  • Week 4: 30 mcg

When you start treatment, you’ll use Avonex prefilled syringes with an Avostartgrip titration kit. The titration kit contains three devices that you use to give the first three doses.

The full dosage of Avonex is typically 30 mcg once per week. This is also the maintenance dose you’ll continue to receive during treatment.

Dosage for clinically isolated syndrome

The dosage for clinically isolated syndrome is the same as it is for relapsing forms of MS. See the section just above for details.

How to use

You should inject Avonex according to your doctor’s or another healthcare professional’s instructions.

You’ll receive Avonex as an injection into a muscle. Your doctor will teach you or your caregiver how to prepare and inject Avonex.

You can find detailed instructions for using the Avonex pen and prefilled syringe on the manufacturer’s website. Talk with your doctor or pharmacist if you have questions on how to give the Avonex injection.

About using Avonex

Below you’ll find information about key dosage issues.

  • When to use: You should receive Avonex once per week.
  • If you miss a dose: If you don’t receive your Avonex injection on your usual day, you should receive it as soon as possible that week. Then continue with your usual schedule the following week. However, do not take Avonex 2 days in a row. If you miss a dose and don’t remember until just before you’re supposed to take your next dose, ask your doctor what to do.
  • Length of use: Avonex is meant to be used as a long-term treatment. If you and your doctor determine that Avonex is safe and effective for you, you’ll likely take it long term.


Do not use more Avonex than your doctor recommends. For some drugs, doing so may lead to unwanted side effects or overdose.

What to do in case you take too much Avonex

If you think you’ve taken too much of this drug, call your doctor. You can also call America’s Poison Centers at 800-222-1222 or use its online tool. However, if your symptoms are severe, call 911 or your local emergency number, or go to the nearest emergency room right away.

Other drugs are available that can treat multiple sclerosis (MS). Some may be a better fit for you than others. If you’re interested in finding an alternative to Avonex, talk with your doctor. They can tell you about other medications that may work well for you.

The following drugs are similar to Avonex:

You may wonder how Avonex compares with other medications that are prescribed for similar uses. The table below gives a brief comparison of Avonex and Rebif.

Active druginterferon beta-1ainterferon beta-1a
Formintramuscular injectionsubcutaneous injection
Usesrelapsing-remitting multiple sclerosis (MS)
• active secondary progressive MS
clinically isolated syndrome (CIS)
• relapsing-remitting MS
• active secondary progressive MS

Both Avonex and Rebif are only prescribed to adults for these uses.

Avonex and Rebif can cause similar side effects, but some different ones as well. To learn more about these drugs, including their potential side effects, see the prescribing information for Avonex and Rebif. Your doctor or pharmacist can also tell you more about these drugs.

Avonex and Gilenya are prescribed for similar uses. The table below gives a brief comparison of these drugs.

Active druginterferon beta-1afingolimod
Formintramuscular injectionoral capsules
Usesrelapsing-remitting multiple sclerosis (MS)
• active secondary progressive MS
clinically isolated syndrome (CIS)
• relapsing-remitting MS
• active secondary progressive MS

For these uses Avonex may only be prescribed to adults, while Gilenya may be prescribed to adults and children ages 10 years and older.

Avonex and Gilenya can cause similar side effects, but some different ones as well. To learn more about these drugs, including their potential side effects, see the prescribing information for Avonex and Gilenya. Your doctor or pharmacist can also tell you more about these drugs.

Avonex is not known to interact with other medications, herbs, supplements, foods, or alcohol. The manufacturer of Avonex didn’t look at interactions in clinical trials of the drug.

However, this doesn’t mean that interactions won’t be recognized in the future. For example, new drugs could be approved that interact with Avonex.

Before starting Avonex treatment, talk with your doctor and pharmacist. Tell them about all prescription, over-the-counter, and other drugs you take. Also, tell them about any vitamins, herbs, and supplements you take. Sharing this information can help you avoid potential interactions.

If you can become pregnant, consider the following information about pregnancy and breastfeeding.

Avonex and pregnancy

It’s not known whether Avonex should be used during pregnancy. If you’re planning a pregnancy or can become pregnant, talk with your doctor before using this medication.

Avonex and breastfeeding

It’s not known whether Avonex should be used while breastfeeding. If you’re currently breastfeeding or planning to do so, talk with your doctor before using this medication.

As with all medications, the cost of Avonex can vary. The actual price you’ll pay depends on your insurance plan, your location, and the pharmacy you use.

It’s important to note that you’ll have to get Avonex at a specialty pharmacy. This type of pharmacy is authorized to carry specialty medications. These are drugs that may be expensive or may require help from healthcare professionals to be used safely and effectively.

Financial and insurance assistance. If you need financial support to pay for Avonex, or if you need help understanding your insurance coverage, help is available.

Avonex has a support program you may be eligible for. For information, call or visit the program website.

To learn more about saving money on prescriptions, check out this article.

Generic or biosimilar version. Avonex isn’t available in a biosimilar form. Like generics, biosimilar medications are considered to be as safe and effective as their parent drugs. And biosimilars tend to cost less than brand-name drugs.

Here are answers to some frequently asked questions about Avonex.

Does Avonex cause weight gain or weight loss?

It’s unlikely. Weight changes were not reported in people who took Avonex in clinical trials. However, weight changes have been reported by people taking other forms of interferon beta (the active drug in Avonex).

Tell your doctor if your weight changes while you’re using Avonex. This could be a sign of more serious side effects. For example, suddenly gaining a lot of weight could be a symptom of fluid retention caused by heart failure.

Gradual weight gain can also be a sign of an underactive thyroid gland. On the other hand, weight loss can be a symptom that your thyroid gland has become overactive.

If you have concerns about weight changes during Avonex treatment, talk with your doctor.

Can I take Avonex if I’m receiving treatment for depression?

Possibly, but this is something you should discuss with your doctor. Some people taking Avonex have developed depression. If you already have depression, it’s possible that taking Avonex could make it worse.

However, one review of studies didn’t find a clear relationship between depression and drugs that contain interferon beta (the active drug in Avonex). People who’d had depression in the past were more at risk for depression in the first 6 months of taking interferon compared with people without this history. However, the risk wasn’t increased enough to conclude that these people shouldn’t take interferon beta at all.

If you do take Avonex while receiving treatment for depression, it’s very important to tell your doctor right away if you feel your depression is getting worse. The same is true if you experience any other changes in your mood or behavior, such as extreme sadness or hopelessness, anxiety, irritability, aggression, hallucinations, or paranoia.

Will I get any side effects when I stop taking Avonex?

It’s not likely. Stopping Avonex is not known to cause any specific side effects. You don’t need to stop taking it gradually as you would with a drug that causes withdrawal symptoms. (Withdrawal symptoms are side effects that occur when you stop taking a drug your body is dependent on.)

If you stop treatment with Avonex, the most likely effect would be that your MS could start to get worse.

Before taking Avonex, talk with your doctor about your health history. Avonex may not be right for you if you have certain medical conditions or other factors affecting your health. Be sure to talk with your doctor if any of the following apply to you:

  • past allergic reaction to this or a similar drug
  • history of depression or other psychiatric illness
  • liver problems
  • heart problems, such as heart failure
  • low numbers of red blood cells, white blood cells, or platelets in your blood
  • history of seizures
  • thyroid problems
  • pregnancy
  • breastfeeding

Note: For more information about the potential negative effects of Avonex, see the “Avonex side effects” section above.

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.

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