WASHINGTON (Reuters) - The U.S. Food and Drug Administration on Friday said it approved Eli Lilly & Co. and Icos Corp.'s anti-impotence drug, Cialis (tadalafil).

Cialis, which is already available in Europe, Australia, Brazil and other countries, is the second rival to Pfizer Inc.'s Viagra (sildenafil) to win U.S. approval this year. Levitra (vardenafil), made by Bayer AG and GlaxoSmithKline, was launched in the United States in August.

Cialis is a phosphodiesterase inhibitor, similar to Viagra and Levitra But Cialis has demonstrated it can be effective for up to 36 hours - some nine times longer than Viagra - and is fast gaining popularity in Europe, where it is touted as the "weekend drug."

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