Avonex (interferon beta 1a)

Avonex (interferon beta 1a) is a disease modifying drug (DMD) for relapsing remitting MS.

You self-inject Avonex into a muscle once a week to reduce the number and severity of relapses. It reduces the number of relapses by about one third (30%).

Common side effects include flu-like symptoms and injection site reactions.

What is Avonex used for in MS?

Avonex is a disease modifying drug (DMD) for relapsing remitting MS. You have fewer relapses than you might have had with no treatment and any relapses you do have should be less severe. 

Avonex is a moderately effective (category 1.1) DMD; in clinical trials people taking Avonex had about 30% fewer relapses than people taking placebo. In clinical trials, MRI scans showed people taking Avonex had fewer, smaller or no new areas of active MS (lesions). Avonex may also slow down the build-up of disability associated with MS.

Who can take Avonex?

Avonex can be prescribed for adults with active relapsing remitting MS.

Avonex has been approved for use on the NHS since 2002. It can only be prescribed by a neurologist.


It's important that you tell your MS team if you have any health problems or are taking other medicines. Avonex may not be suitable if you have severe depression or suicidal thoughts.

Conception and pregnancy

The prescribing advice for Avonex during pregnancy and breastfeeding is appropriately cautious.  However, pregnancy registers and clinical experience have shown no increased risk to the health of mother or baby with Avonex.

If you are trying for a family, talk to your MS nurse or neurologist about whether you should continue to take Avonex until you are pregnant.

If you become pregnant while on Avonex, your neurologist or MS nurse may recommend you stop taking it, depending on how active your MS has been and the benefits and risks to you and your baby. Women usually experience fewer relapses during pregnancy, so continuing to take Avonex may not be necessary.  However, if Avonex is stopped during pregnancy, it will take several months to reach full effectiveness when restarted and may not reduce relapses during the first few months after childbirth.

Pregnancy guidelines consider that it is safe to take Avonex during breastfeeding if required.

How do I take Avonex?

You self-inject Avonex into a muscle once a week. Avonex is supplied in several different formats; your MS nurse can explain the different injection devices and help you choose the most appropriate one for you.

Your MS nurse will show you how to do the injections, discuss the practicalities and offer advice or training and ongoing support if you should need it.

To give your body a chance to get used to Avonex and reduce the impact of side effects, your doctor or MS nurse may suggest you start on a lower dose (dependent on injection device).

What side effects could I get with Avonex?


Common side effects include:

  • Flu-like symptoms, such as headache, muscle ache and stiffness, chills or fever, following an injection
  • Injection site reactions

You are more likely to have these side effects when you first start taking Avonex. Most people have mild to moderate side effects which tend to go away over time.

A neurologist or MS nurse may suggest ways to reduce these side effects including:

  • To help with flu-like symptoms, it may be helpful to change the time of day of injection in order to sleep through the worst of the side effects
  • To help reduce fever, paracetamol or ibuprofen can be taken before the injection and at four to six hour intervals after the injection, as required

Common side effects (affecting more than 1 person in 100)

  • flu-like symptoms
  • headache
  • injection site reactions
  • blood cell abnormalities
  • feeling weak or tired
  • difficulty sleeping
  • diarrhoea, nausea and vomiting
  • muscular or joint pain
  • depression

Less common side effects (affecting less than 1 person in 100)

  • changes in menstruation (periods)
  • liver abnormalities
  • allergic reactions
  • heart problems and high blood pressure
  • hair thinning or loss
  • damage to small blood vessels leading to kidney problems

A full list of side effects is included in the manufacturer's Patient Information Leaflet.

Assessment before treatment

Before starting Avonex, you should have blood tests to measure blood cell counts and check liver function.

Assessment during treatment

Once you've started treatment, you'll have blood tests to measure blood cell counts and monitor liver function, generally every three months for the first year, then less frequently. Depending on local practice, the tests may be carried out at a local GP surgery or it may be necessary to attend a hospital clinic.

During the coronavirus outbreak, you may find that your regular blood tests happen less frequently, may take place in a different location or may temporarily stop. The Association of British Neurologists has assessed the risks and benefits of blood monitoring for people taking Avonex, and has recommended a safe minimum schedule for all DMDs during this period. The recommendation for Avonex is that the first blood test, 3 months after starting to take the drug, is essential, but that further tests can be halted temporarily. If you notice any new or worsening MS symptoms, you should contact your MS team.

How does Avonex work?

Interferons are proteins that occur naturally in the immune system. It is thought that beta interferon acts by reducing both inflammation and the immune response that is attacking the body's own myelin.

Avonex research

Avonex has been studied extensively since the early 1990s. The following study first demonstrated the effectiveness of Avonex:

  • MSCRG Study: Multiple Sclerosis Collaborative Research Group Study - 1996 - Avonex compared to placebo

In this clinical trial, 301 people with relapsing remitting MS took either Avonex or placebo for 2 years. The main measure of the study was the time it took for participants to have an increase in disability of 1 point on the EDSS scale which lasted for at least 6 months. The group taking Avonex were significantly less likely to have this increase in disability. At the end of 2 years it was estimated that approximately 35% of those taking placebo and 22% of those taking Avonex had increased disability, a 37% reduction compared to placebo. Avonex reduced the relapse rate by 32% compared to placebo and also reduced the number of brain lesions on MRI.

  • Risk Sharing Scheme - 2018 - long-term effectiveness of beta interferons and Copaxone

This study was set up in 2002 to evaluate the long-term benefits of beta interferons and Copaxone (glatiramer acetate). More than 5000 people taking either Avonex, Betaferon, Copaxone or Rebif were recruited at study centres around the UK and their disability levels (EDSS) monitored for 10 years. At the end of the study, changes in disability levels were compared to those recorded from an untreated group.

The study found that the DMDs delayed the need to use a walking stick (EDSS 6.0) by four years.

Find out more

National Institute for Health and Care Excellence (NICE).
Beta interferons and glatiramer acetate for treating multiple sclerosis.
NICE technology appraisal guidance TA527.
Full guideline (link is external)
Jacobs LD, et al.
Intramuscular interferon beta-1a for disease progression in relapsing multiple sclerosis. The Multiple Sclerosis Collaborative Research Group (MSCRG).
Annals of Neurology 1996;39:285-94.
Summary (link is external)
Palace J, Duddy M, Lawton M, et al.
Assessing the long-term effectiveness of interferon-beta and glatiramer acetate in multiple sclerosis: final 10-year results from the UK multiple sclerosis risk-sharing scheme.
J Neurol Neurosurg Psychiatry. 2019 Mar;90(3):251-260.
Full article (link is external)
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