The EVIDENCE study was a direct comparative study of two dose regimens of interferon (IFN) beta- 1a used in the treatment of relapsing- remitting multiple sclerosis (RRMS): 30 mcg intramuscularly once weekly (qw; n=338) and 44 mcg subcutaneously three times weekly (tiw; n = 339). The study continued for an average of 64 weeks. The safety population consisted of all patients receiving at least one dose of study drug. Clinical assessments occurred every 4 weeks for 24 weeks and then every 12 weeks. Blood tests for safety were taken at baseline and at weeks 4 and 12, and every 12 weeks thereafter. Overall adverse events were more common with the 44 mcg tiw regimen (p = 0.007), and were due predominantly to differences in injection- site reactions. The majority of adverse events were rated mild by investigators. Hepatic and haematological adverse events and asymptomatic laboratory abnormalities were more common with 44 mcg tiw (p < 0.001), with no difference seen for severe events. Flu- like symptoms were more common with 30 mcg qw (p = 0.031), were more severe and persisted for longer. Serious adverse events were comparable for both groups, as were drug discontinuations. In conclusion, although adverse events were more common with high- dose, high- frequency IFN therapy, differences were primarily for mild events and did not affect treatment adherence. Based on superior clinical and magnetic resonance imaging outcomes over an average of 64 weeks, coupled with modest safety differences, the riskbenefit ratio for IFN therapy in RRMS favours the 44 mcg tiw regimen over this period of time. The EVIDENCE study was a direct comparative study of two dose regimens of interferon (IFN) beta-1a used in the treatment of relapsing-remitting multiple sclerosis (RRMS): 30 mcg intramuscularly once weekly (qw; n = 338) and 44 mcg subcutaneously The study continued for an average of 64 weeks. The safety population consisted of all patients receiving at least one dose of study drug. Clinical assessments occurred every 4 weeks for 24 weeks and then every 12 weeks. Blood tests for safety were taken at baseline and at weeks 4 and 12, and every 12 weeks thereafter. Overall adverse events were more common with the 44 mcg tiw regimen (p = 0.007), and were due due predominantly to differences in injection- The majority of adverse events were rated mild by investigators. Hepatic and haematological adverse events and asymptomatic laboratory abnormalities were more common with 44 mcg tiw (p <0.001), with no difference seen for severe events. Flu- li were more common with high dose of 30 mcg qw (p = 0.031), were more severe and persisted for longer. , based on superior frequency IFN therapy, differences were primarily mild events and did not affect treatment adherence. Based on superior clinical and magnetic resonance imaging outcomes over an average of 64 weeks, coupled with modest safety differences, the risk benefit ratio for IFN therapy in RRMS favors the 44 mcg tiw regimen over this period of time.