Multiple sclerosis (MS) is the main nontraumatic cause of handicap in young adults. Immunomodulators and treatments limiting lymphocyte migration have been proven efficient to treat relapsing-remitting MS. Subcutaneous IFN-β1a improve relapse rate and MRI parameters in a series of double-blind, placebo-controlled trials in relapsing-remitting MS patients. Similar results, and with a greater extent, were obtained when treating patients with a first demyelinating event suggestive of MS. Except for the rare liver toxicity, the drug is well tolerated and has no severe adverse reaction. When compared with intramuscular IFN-β1a, both relapse rate and MRI parameters were modulated in favor of the subcutaneous administration. Although the effect of subcutaneous IFN-β1a on disability progression is limited, the good tolerance profile together with the efficiency of the drug explain why this treatment, as well as the other interferons and glatiramer acetate, is a first-line therapy for relapsing-remitting MS.