The Main Study was a 4-week treatment period which consisted of 1 Avonex manual injection using a prefilled syringe, followed by 3 Avonex injections using the single-use autoinjector. The Extension Study was designed to provide continuation of treatment with the Avonex single-use autoinjector to eligible participants who completed the Main Study for up to an additional 12 weeks, and to obtain additional safety and preference data for the Avonex single-use autoinjector.