Biogen develops and sells drugs treating neurological diseases, with a historical focus on multiple sclerosis treatments alongside newer drugs targeting Alzheimer's disease and other neurodegenerative conditions. The company built its early growth around a portfolio of multiple sclerosis therapies that generated substantial revenue for many years, though that business has faced increasing competitive and pricing pressure as competing treatments, including generics, have entered the market. Biogen has pursued additional growth from Alzheimer's disease treatments targeting the underlying amyloid plaques associated with the disease, a therapeutic approach that has been scientifically contentious and commercially challenging to bring to market successfully. As a biopharmaceutical company, Biogen's results depend heavily on the drug development pipeline behind its marketed products, since patent expirations eventually open its established drugs to generic competition.