The most widely prescribed drug for treating multiple sclerosis has little or no effect on a patient’s progression to disability, a new study has found.

The medicine, interferon beta, does help reduce the development of brain lesions and limit the frequency of relapses, but until now there have been few well-controlled long-term studies demonstrating its effectiveness at preventing the onset of irreversible disability.

Researchers at the University of British Columbia prospectively collected data on 868 M.S. patients treated with interferon beta, comparing them with 1,788 patients who never took the drug. Using a well-validated scale, they found that those who took interferon beta were no less likely to suffer long-term disability than those who took none.

Interferon beta drugs are commonly used to treat relapsing-remitting M.S., the most common form of the disease. M.S. is an autoimmune disease that damages the myelin sheath surrounding the nerve cells. Its course varies widely, but it is usually a relapsing illness that produces a variety of symptoms, including muscle spasms and difficulty walking, bladder and bowel problems, vision and hearing disturbances, speech problems, difficulties with reasoning and attention span and more.