Biogen Inc. took a colossal risk in late 2019. It revived a drug for Alzheimer's disease it had left for dead earlier that year after the medication appeared to fail two large clinical trials.

The company wasn’t ready to take no for an answer. Biogen, along with its Tokyo-based partner Eisai Co. Ltd., had poured years of effort into developing the therapy, then called aducanumab, and was counting on it as its next big therapeutic advance.

After months of controversy, Biogen's bet paid off in spectacular fashion Monday when the U.S. Food and Drug Administration granted the treatment its approval, making it the first new Alzheimer's drug in almost 20 years and first to attack an underlying process of the disease.