It's a complicated, debilitating illness and drug companies have struggled for years to come up with viable treatments. Other big drug companies such as Roche Holdings and Eli Lilly are pursuing similar treatments, with results expected to come later this year and early next year, respectively. "I think people's confidence in the amyloid hypothesis would go down even further, but ultimately people would wait to see what those other data events show," he said. Negative results might give a boost to companies that are exploring alternate ways to treat Alzheimer's disease. And though Biogen's drug is furthest along in development, it's far from being the only treatment being tested for Alzheimer's.

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