The researchers also identified a biomarker that can be used to measure the drug's effectiveness, opening the door to a potential new treatment for the debilitating disease.

In the autoimmune disease multiple sclerosis (MS), the body's immune system attacks the protective myelin sheath that covers nerves, causing irreversible damage and disrupting communication between the brain and body. The breakdown of myelin – called demyelination – can cause symptoms that range in severity from numbness and tingling to blindness and paralysis.

The goal of any MS treatment is to reverse demyelination, which would return proper nerve function and reduce or prevent long-term disability. Rather than reversing the demyelination process, current treatments focus on the autoimmune aspect of the disease, blocking immune cell access to myelin or otherwise lessening the body's inflammatory response.

Now, researchers from the University of California San Francisco have identified an over-the-counter antihistamine called clemastine that can reverse damage to the myelin sheath and, what's more, they've identified a biomarker that can measure the drug's effectiveness.

It all hinges on something called the "myelin water fraction" or MWF. Water that is trapped between the layers of myelin that wrap around nerves in the brain can't move as freely as water that floats between brain cells. The MWF measures the ratio of myelin water to the brain tissue's total water content and indicates myelin integrity.