WEDNESDAY,Sept. 05, 2001 (HealthDayNews) -- The latest Viagra TV commercials are linking the erectile dysfunction drug with the NASCAR car racing circuiut. But soon, there may be some turbocharged competition on Viagra's track.

The FDA has accepted for review a new drug application for the erectile dysfunction drug Cialis. If approved, Cialis will be the first direct prescription competition in the United States to the enormously successful Viagra. There are many herbal and "natural" formulas on the commercial market, but only Viagra has been approved by the FDA after a series of clinical trials.

Erectile dysfunction (ED) is the consistent inability to attain and keep an erection sufficient for sexual intercourse. ED affects an estimated 152 million men worldwide. For a thorough discussion of ED, visit the Urology Channel.

Cialis' active ingredient inhibits the effects of phosphodiesterase (PDE), and it's the same basic substance found in Viagra, although the scientific PDE designation for Cialis is different from Viagra.

Phosphodiesterases are cellular enzymes that help control blood flow. Cialis and Viagra reduce PDE effects, and this reinforces blood flow to the penis, causing an erection.

Clinical studies have concluded that the effects of Cialis last for 24 hours or longer. According to Lily Icos, the maker of Cialis, this sets its product apart from Viagra which, it claims, loses its effect much sooner. No published clinical studies comparing the two drugs have been published.

The August 31 follows Lilly Icos July filing for European Union approval of Cialis. Viagra and another ED drug, Uprima, are already approved throughout the European Union. Uprima works by stimulating the brain chemical dopamine, which is associated with sexual performance in men. Links to the latest ED drug studies are available at the Doctor's Guide to the Internet