WASHINGTON -- The Food and Drug Administration has approved the sale of a second pill to treat erectile dysfunction.

It's called Levitra and it's in the same family as Viagra. Both work by targeting an enzyme important for maintaining an erection.

"In clinical trials, Levitra was shown to work quickly. More importantly, Levitra was shown to improve the sexual response for the majority of men the first time they took it, and it worked consistently over time," said Dr. Myron Murdock, a Levitra researcher.

Its appearance in pharmacies in the coming weeks is expected to spark a fierce battle in the billion-dollar-plus impotence market.

Roughly 30 million American men have some degree of impotence, but most do not seek medical therapy.

Levitra, produced by Bayer, has recently been selling in Europe and has been approved by 50 countries, according to the company. A third impotence pill -- Cialis -- has also won European approval and is expected to be in U.S. pharmacies later this year.

Like Viagra, Levitra is available only by prescription. It can be taken up to once a day, and it should not be prescribed to men who take nitrate-containing drugs or alpha blockers, which are sometimes prescribed for high blood pressure or prostate symptoms.