Interferons (Avonex, Rebif and Betaferon or Betaseron) belong to a class of messengers that play a role in regulating the immune system. The interferons in general have anti-viral actions and effects on the immune system, and they inhibit tumor growth. They have been hailed as an exciting new therapy, but the benefit they offer is small and may not significantly prevent the progression of disability. Most important, side effects are common and can be severe. They are taken via injection, weekly, three times a week or every other day.