▴ Intramuscular interferon-β-1a, a recombinant interferon-β approved in the US for the treatment of relapsing forms of multiple sclerosis (MS), has also been evaluated in the treatment of patients with a first clinical demyelinating episode and brain lesions consistent with MS confirmed by magnetic resonance imaging (MRI).▴ In a randomised, double-blind trial, patients at high risk of developing clinically definite MS who received intramuscular interferon-β-1a 30μg once weekly had a 44% reduction in the cumulative probability of developing MS, compared with placebo recipients (rate ratio 0.56; p = 0.002), over a 3-year period after a first, MRI-confirmed demyelinating event.▴ These results were supported by MRI findings that showed significantly smaller increases in the volume of brain lesions and the number of new/enlarging and gadolinium-enhancing lesions in interferon-β-1a recipients than in those receiving placebo.▴ A nonblind extension of this trial demonstrated that early treatment with interferon-β-1a significantly reduced the probability of developing MS by 35% (p = 0.03), compared with delayed treatment, over a 5-year period.▴ Intramuscular interferon-β-1a was generally well tolerated; however, influenza-like syndrome was documented in >50% of patients at high risk of developing clinically definite MS who received the drug.Table. Features and properties of intramuscular interferon-β-1a (Avonex®)