Biogen's (NASDAQ: BIIB) Tecfidera launched a year ago and has already become a blockbuster likely to notch more than $2 billion in sales this year. Much of Tecfidera's success is coming at the expense of recently launched oral multiple sclerosis drugs from Novartis (NYSE: NVS) and Sanofi (NYSE: SNY), which market Gilenya and Aubagio, respectively. As Tecfidera rolls out globally, Novartis and Sanofi are likely to lose additional market share over the coming year, but neither company is sitting still. Both are among a slate of generic drug makers that have filed opposition to a key EU patent on Tecfidera that gives Tecfidera exclusivity until 2028. In the following slideshow, you'll learn more about the patent threat to Biogen's fast growing drug and what the impact on Biogen may be if the EU's patent office sides with Novartis and Sanofi.