Cialis

Two new rivals are Cialis, made by Eli Lilly & Co. and its biotechnology partner, Icos Corp. and vardenafil, made by Bayer AG. Lilly and Icos have applied to the U.S. Food & Drug Administration to begin marketing Cialis. Vardenafil is in final clinical trials, and Bayer says it will seek FDA approval this fall. Both drugs could reach the market next year.

Cialis and vardenafil act in the same way as Viagra, by blocking an enzyme called phosphodiesterase-5, or PDE-5, and in the process relaxing smooth muscle cells in the penis and elsewhere. In causing these muscles to relax, the drugs effectively increase blood flow to the penis.

Though they work in similar ways, Cialis, vardenafil and Viagra are distinctly different chemical entities. Some urologists say vardenafil and Cialis are more effective at blocking the PDE-5 enzyme and hence potentially more potent than Viagra. This means they could act as well as, or better than, Viagra at smaller doses. They also interact with fewer other molecules and hence might have fewer side effects.

No one can make a definitive comparison between the drugs because there have been no public head-to-head clinical trials. But the newcomers are already zeroing in on what they believe are their drugs' advantages over Viagra. Cialis, for instance, gave men improved ability to have sex for up to 24 hours, according to results from a study presented at a medical conference recently. In contrast, Pfizer recommends that men have sex within four hours of taking Viagra.

Cialis and vardenafil may take effect faster than Viagra, urologists say. Pfizer advises patients to take Viagra an hour before attempting sexual intercourse. In a clinical trial, a majority of patients who took Cialis were able to engage in sexual intercourse within 30 minutes of taking the drug.