Avonex is used for treatment of relapsing forms of multiple sclerosis. It is the first drug to have proven, in a blinded clinical trial, that it slows the progression of disability as well as reduces the frequency of exacerbations.

In a phase III clinical trial, 40% fewer subjects on Avonex progressed one or more units on the Expanded Disability Status Scale than patients on placebo. The EDSS is the standard measure of disease progression in multiple sclerosis. In addition, subjects receiving Avonex in the trial experienced 32% fewer exacerbations of the disease than subjects on placebo. Furthermore, MRI studies showed a statistically significant reduction in the number and volume of active lesions in treated patients compared to placebo patients.

The side effects observed in the trial were modest and largely involved flu-like symptoms characteristic of all interferon drugs. There were minimal injection-site reactions.