Patent Denied

In China, where supplies of the traditional cure for erectile dysfunction—tiger penis—have shriveled, men in search of an extra boost have been snapping up doses of knockoff Viagra. Although they're illegal, the fakes are so widespread that American pharmaceutical giant Pfizer says it's having a difficult time selling the real thing. Last week Pfizer's job grew even tougher. China's State Intellectual Property Office (SIPO) overturned the company's patent rights to Viagra, potentially opening the Chinese market to a flood of cheap, locally made generic rivals.

Details of the decision have not been released, but SIPO says Pfizer's...