"All my athletes took it," Victor Conte said recently. "It is bigger than creatine."

Conte was the owner of the BALCO laboratory in San Francisco, the focus of the largest doping scandal in recent years, which put an end to the career of former athletics stars such as Marion Jones.

Viagra dilates blood vessels to take more blood to the penis and thus improve a man's sexual performance. However, experts think the same process takes more oxygen and nutrients to muscles and can be a non-negligible aide in explosive disciplines like sprinting.

"Each time there is a seizure of banned drugs you can be sure that there's Viagra and Cialis found as well," said Christiane Ayotte, head of a laboratory in Montreal, the city where the World Anti- Doping Agency (WADA) has its headquarters.

Like Viagra, Cialis is used to treat erectile dysfunction. According to some experts in sport, its use helps performance at high altitude and with high pollution.

Beijing, the city where the 2008 Olympics are set to be held in August, features significant pollution levels, to the point that Ethiopian Haile Gebrselassie declined to run the marathon there.

"The pollution in China is a threat to my health," said the athlete who suffers from asthma.

Viagra has a great advantage for sports professionals: it is legal, it is not a banned substance. Ayotte noted that Viagra and Cialis are regularly found in sportsmen's urine.

"WADA monitors this substance, as it does with many other substances, and is currently funding a research project on the performance-enhancing potential of Sildenafil (Viagra) at various altitudes," WADA communcations manager Frederic Donze told Deutsche Presse-Agentur dpa.

"It is a WADA decision with which the IOC has nothing to do," the German Thomas Bach, vice president of the International Olympic Committee (IOC) told dpa.

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