Pfizer ( PFE) chemists were developing a pill to treat cardiovascular ailments. In 1991 something came up (literally) that sidetracked those efforts: Test subjects exhibited a rather virile side effect to the treatment. The pill, later named Viagra and approved by the FDA in 1998, proved a bust in terms of treating heart disease. But the unexpected benefit pumped up its maker's bottom line and set the stage for a brand-new industry. Today, drugs to treat erectile dysfunction command more than $5 billion a year in sales globally. Of that take, Viagra accounts for about $1.9 billion, but its chief rival in the space, Eli Lilly's ( LLY) Cialis, closing in on parity.