A new warning has been added to the label of the impotence drug Viagra due to its association with fainting when used within four hours of taking an alpha-blocker, a group of medications used to treat high blood pressure and enlarged prostates. Because Viagra already lowers a user's blood pressure, researchers say it may react adversely when used in combination with alpha-blockers, leading to dangerously low drops in blood pressure and eventual fainting.

The drug's maker, Pfizer, began a study of Viagra's interaction with the group of medications after the U.S. Food and Drug Administration received reports of fainting among Viagra users. Over 10 million men have used Viagra since its introduction in 1998.