The main ingredient in Viagra is reborn as Revatio, a drug used to treat a fatal lung disease. Viagra, the erectile-dysfunction drug used by more than 26 million men worldwide, was reborn Monday as Revatio, a drug to treat pulmonary hypertension, a rare, fatal lung disease caused by constrictions in the blood vessels that supply the lungs. The Food and Drug Administration approved the drug's main ingredient, sildenafil citrate, for the new indication in a different dose, 25 milligrams vs. 20, 50 and 100 milligrams for Viagra, and with a new look. Revatio is white and round; Viagra is a...