Viagra may be banned from international sports after indications that athletes are using the anti-impotence treatment to improve performance.

The World Anti-Doping Agency is considering adding the drug to its list of illegal substances. The agency tracks the use of drugs that enhance sporting prowess and is to conduct research into the medicine, which can boost blood flow to the lungs.

The results, however, are not expected until next year, which means competitors at the Beijing Olympics will be able to take Viagra, known as "Vitamin V" in sporting circles.

Viagra, and a similar drug, Cialis, are regularly found in the urine samples of male competitors, said Christiane Ayotte, a scientist at a WADA-accredited laboratory.

The drug is thought to increase the blood supply to the muscles and may also enhance endurance, especially in polluted or high-altitude areas.

A 2006 study in the Journal of Applied Physiology found using Viagra improved the times of some cyclists but it did not significantly enhance performance at sea level.