Although a number of studies have provided supportive evidence that IFN beta favourably influences the short-term course of MS, the long-term effects on the development of disability are not well known. A definitive approach to this problem would be long-term randomised, placebo-controlled trials, but this is not practical, and, many would argue, not ethical. Attempts were made to follow up those patients who participated in the clinical trials mentioned above, but unfortunately all these extension studies suffered from methodological problems, most importantly a significant number of patients being lost to follow-up. These follow-up studies, despite their limitations, suggest that patients with RRMS can experience sustained benefit over many years.

Recently, a large Italian study which followed over 1500 patients for up to seven years, showed that interferon beta does slow progression in RRMS: both the time to sustained disability worsening and the time to the onset of the secondary progressive disease phase were significantly prolonged in interferon beta-treated patients compared to untreated patients.