A third pill to treat impotence was approved for sale in the United States on Friday, intensifying competition in a billion-dollar slice of the drug market. The new pill will be sold under the name Cialis. It joins Viagra, the oral drug that went on the market in 1998, and Levitra, which was approved earlier this year by the Food and Drug Administration.

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All three drugs act on an enzyme that helps prompt and maintain erections by relaxing muscles in the penis and blood vessels. The duration and onset of the drug action is different, however, with each pill.

Cialis, manufactured by Eli Lilly & Co., was found in studies to stay longer in the body than Viagra. Studies suggest that a 20 mg dose of Cialis is active for 24 to 36 hours. In France, where the drug has been on the market for some time, Cialis is called “Le weekend” pill. Levitra and Viagra reach their full effectiveness in an hour, on average.

Some market studies suggest that about 30 million men over the age of 40 have symptoms of impotence. The sales of Viagra last year were reported at $1.2 billion.

Cialis, whose chemical name is tadalafil, was tested in trials with 4,000 men. The studies found that it helped promote erection within a half hour and enhanced that ability for up to 36 hours.

The drug is not recommended for patients on some heart medications, such as nitroglycerin tablets or some alpha blockers, because the combination can cause a sharp drop in blood pressure. This can cause fainting or even death in some men.

Recommendations call for dosage limitations of Cialis for patients with kidney or liver disorders. The drug should not be taken by men for whom sexual activity is inadvisable because of heart conditions.

The most common reported side effects from clinical trials of Cialis were headache, indigestion, back pain, muscle aches and flushing.

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