Multiple sclerosis attacks the brain and spinal cord, and can cause neurological issues which range from mild problems to serious disability, such as paralysis.
The drugs which were studied, known as beta interferons, have been used to treat the relapsing-remitting form of MS since 1995.
But a new study from the UBC Hospital MS Clinic and Brain Research Centre published in the Journal of the American Medical Association, suggests the drugs aren't preventing the progression of disability.
However, the researchers warn that the study's results weren't meant to suggest that MS patients stop taking the drugs as the medications are still effective at reducing relapse rates.
Dr. Helen Tremlett, one of the study's authors, says the finding is just coming to light because in the past, not enough time had elapsed from when the drugs were approved to treat the disease.
A total of 2,656 patients with MS were looked at. Of those, 868 were treated with the beta interferon drugs and were compared with both contemporary and historical cases of patients who went untreated.
Tremlett said researchers were trying to figure out whether taking the medications would reduce a patient's risk of needing a cane to walk.
"Our main finding is that, among patients with relapsing, remitting multiple sclerosis, which is the most common form of MS, we found that exposure to a group of drugs known as the beta interferons were not strongly associated with a reduction in progression of long-term disability," Tremlett said.
Relapsing-remitting MS is a form of the disease characterized by clearly defined attacks that worsen a patient's neurological function. These flare-ups or relapses are followed by a period of partial or complete recovery.
The study was supported by the University of British Columbia and Vancouver Coastal Health.
The researchers looked at the disease using an established disability scale.
They looked at the disease over the time period between the point at which a person qualified for interferon beta treatment until they reached the point on the scale where they would generally need a cane to walk.
While researchers found the medications did not impact whether disabilities became more severe, Tremlett pointed out that the study's results were not meant to suggest that people with MS should stop taking beta interferons.
"Nothing has actually changed," she said, pointing out that the drugs are still effective when it comes to reducing relapse rates.
She added there are many aspects of the disease that the researchers did not measure, including quality of life, cognition, and ability to work.
"This is just a more realistic expectation of what you could hope to gain from taking the treatment."