Avonex (interferon beta-1a) for multiple sclerosis

Last updated May 19, 2025, by Andrea Lobo, PhD
Fact-checked by Inês Martins, PhD

What is Avonex for MS?

Avonex (interferon beta-1a) is an approved injectable medication used to treat adults with relapsing forms of multiple sclerosis (MS), including clinically isolated syndrome (CIS), relapsing-remitting MS (RRMS), and active secondary progressive MS (SPMS).

Its active ingredient, interferon beta-1a, is a lab-made version of natural substances called interferons that help the body fight infections. While the exact mechanism through which Avonex works in MS is not fully understood, it is believed to calm the immune system and reduce inflammatory damage in the brain and spinal cord.

The therapy was developed and is marketed by Biogen.

Therapy snapshot

Who can take Avonex?

Avonex is approved in the U.S. for adults with CIS, RRMS, and active SPMS.

In Europe, the medication is indicated for people with relapsing forms of MS, but it should be discontinued in those who develop progressive forms of the disease.

Avonex is contraindicated for people who have experienced harmful immune reactions (such as an allergic reaction) to natural or lab-made interferon beta, albumin, or any other component of the medication. If an allergic reaction occurs, treatment should be stopped.

How is Avonex administered?

Avonex is given via intramuscular, or into-the-muscle, injections once a week, and is available as a prefilled syringe or an autoinjector called the Avonex Pen. It can be self-administered by patients or caregivers after appropriate training.

The recommended dosage is 30 micrograms (mcg) once a week. However, to minimize potential flu-like symptoms, patients typically begin the medication at a lower, 7.5 mcg dose in the first week, increasing it gradually to the full 30 mcg dose by week four. A titration kit, called AVOSTARTGRIP, is available for use with the prefilled syringe to aid in accurate dosing during this initial period.

Avonex injections should be given into the upper, outer thigh or the upper arm, rotating injection sites weekly to avoid injection site reactions. It should be taken out of the refrigerator about 30 minutes before injection to reach room temperature.

Avonex in clinical trials

Avonex’s approval in the U.S. was largely based on data from two Phase 3 clinical trials involving more than 600 adults with relapsing forms of MS. Both tested Avonex against a placebo.

• The MSCRG trial enrolled people who had experienced at least two relapses in the past three years and treated them for up to two years. Results showed a significant reduction in the risk of sustained disability progression, as well as significantly fewer relapses and brain lesions in people given Avonex compared with those on a placebo. In a 15-year follow-up study (NCT00525343), patients who continued treatment with Avonex had lower disability levels and better quality of life than those who stopped the medication.
• The CHAMPS trial enrolled people with CIS who had experienced one relapse and had lesions suggestive of MS. Avonex significantly reduced the risk of progression to clinically definite MS after three years in the main trial, and benefits were sustained for up to 10 years in a long-term study called CHAMPIONS (NCT00179478).

The open-label Phase 3b ASPECT trial (NCT00828204) also supported the approval of the Avonex Pen and the dose titration regimen.

Common side effects of Avonex

The most common side effects of Avonex reported in clinical trials include flu-like symptoms such as:

• chills
• fever
• aching muscles
• generalized weakness.

Some patients receiving Avonex have experienced depression, suicidal thoughts, hallucinations, and other behavioral problems. A healthcare provider may decide to stop treatment if patients experience mood and behavioral problems such as:

• irritability
• depression
• nervousness
• anxiety
• aggressive behavior
• thoughts of hurting themselves or suicide
• hearing or seeing things that are not there.

In rare cases, the medication may lead to severe liver injury that results in liver failure and death. Patients should inform their healthcare provider if they experience symptoms such as nausea, loss of appetite, tiredness, and changes in the color of their urine or stools. Liver health should also be routinely monitored during treatment with Avonex.

Serious allergic reactions and skin reactions can also happen while taking Avonex. Symptoms may include itching, swelling, shortness of breath, rash, hives, or skin blisters. Emergency help should be sought right away if any of these symptoms develop and patients should speak to their doctor before taking another dose of the medication.

Treatment may be used with caution in patients who are pregnant or breastfeeding, but they should discuss the benefits and risks of receiving treatment in those circumstances with their healthcare team.

Multiple Sclerosis News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.