What if an over-the-counter allergy medicine could help halt and even reverse multiple sclerosis? And if it did, could patients return to their full capacity? Those were some of the questions posed by Jonah Chan, Ph.D., and Ari Green, MD. The two had discovered in the lab that the medication clemastine—originally approved as an antihistamine—could repair myelin, the protective insulation around nerve fibers that is damaged in MS. That early discovery in Chan's lab, in which clemastine was shown to stimulate the differentiation of myelin-making stem cells, led to clinical trials to test the therapy in people. Surprisingly, these were...