Swiss doctors conducted a trial among 468 patients diagnosed with early symptoms of MS, of whom 292 received the drug and 176 received a dummy, called a placebo.
Each volunteer was given an injection every other day for two years or until he or she was diagnosed with a clinically definite case of MS, a judgement based on worsening disability.
Using an internationally accepted scale of symptoms, the researchers found that among the "early treatment" group the drug reduced the risk of developing clinically definite MS by 41 percent.
In MS, the immune system attacks myelin, the fatty sheath that protects the cells of the central nervous system.
As a result, nerve signals get slowed or blocked, causing difficulties in movement and coordination, muscle weakness, cognitive impairment, slurred speech and vision problems.
The research was led by Ludwig Kappos of University Hospital Petersgraben in Basel.
Interferon beta-1b, marketed as Betaseron, is a member of the interferon family of drugs. It is already licensed for treating MS, and works by apparently reducing inflammation.