In this video, Motley Fool health-care analyst David Williamson discusses Spectrum Pharmaceuticals (SPPI), and why more than 50% of the stock's float is sold short. The company took off in 2011, when production problems with the generic drug Leucovorin gave it a chance to get doctors to switch to its more expensive branded drug, Fusilev, but sales of Fusilev have flagged lately. David tells investors whether Fusilev's days of sales growth are over, and what else Spectrum has up its sleeve that may prove the shorts wrong.