Athletes are turning to a new performance enhancing drug: Viagra. Traces of the drug, which is intended to alleviate sexual dysfunction, are increasingly appearing in the testing of samples from sports competitors.

It has become so widespread that the World Anti-Doping Agency (Wada) is considering whether to include Viagra in its list of substances banned in international sports.

Christiane Ayotte, head of a Wada-accredited laboratory in Montreal, said she regularly identifies Viagra and Cialis, a similar product, in the urine of male sports competitors. No action is taken because it is not illegal.

“Each time there is a seizure of banned drugs you can be sure that there’s Viagra and Cialis found as well,” she said.

Experts believe that Viagra, which dilates blood vessels, could help in events requiring explosive power, such as sprinting. Others suggest it could help endurance – not so much marathon sex sessions as marathon running – particularly at high altitude or in polluted conditions, such as those expected at the Beijing Olympics. The drug is believed to aid the delivery of oxygen and nutrients to muscles.

Don Catlin, an American expert on doping, said he had suggested several years ago that Viagra could be used to enhance athletic performance.

“I thought there was some reason to be concerned about it and to put it on [Wada’s banned] list,” he said.

A study of cyclists who took sildenafil citrate (the chemical name for Viagra) while racing at altitude found that it reduced their time over 6km by 15%.

Victor Conte, who ran the Bay Area Laboratory Cooperative in San Francisco and supplied banned substances to top competitors, such as Marion Jones and Dwain Chambers, the runners, has claimed of Viagra: “All my athletes took it. It’s bigger than creatine [a legal muscle-building supplement]”.

There may be another reason athletes are taking Viagra – to compensate for the impotence brought on by using banned hormones.

A Wada spokesman said yesterday that it was “monitoring this substance . . . and is funding research on [its] performance enhancing potential”. Results of the study are expected next year.

Wada updates its list of proscribed drugs annually and could add Viagra from 2010, but only if it was satisfied that it was safe to do so, both scientifically and legally.

Meanwhile, British officials are taking action to prevent athletes fooling dope testers by using false penises when giving urine samples. The penises, and untainted urine, are widely available on the internet.

Doping control officers have been given strict guidelines that athletes must be observed and their bodies visible from the stomach to the knees when they produce urine specimens.

Russell Langley, of UK Sport, said: “This is necessary because there have been cases in other countries where sportsmen have tried to manipulate the sample provision.”

One early case occurred in the 1978 Tour de France when the race leader, Michel Pollentier of Belgium, was found to have a rubber tube containing untainted urine hidden in his armpit.

The problem has grown because of the sophisticated equipment available online. One site advertises a kit that includes a “very realistic prosthetic penis” and dried urine.