At this point, there is no evidence of widespread use of Viagra by elite athletes, Mr. Tygart said. Yet, because the drug is not prohibited and thus not screened for, there is no way to know precisely how popular it is.

There is some suspicion that Viagra may be used to circumvent doping controls in cycling, which has faced waves of scandal. Last May, the cyclist Andrea Moletta was removed from the Tour of Italy after a search of his father’s car turned up 82 Viagra pills, as well as syringes concealed in a tube of toothpaste, according to news accounts. An investigation ended without formal accusations of doping.

Image Kenneth W. Rundell, lead researcher of the Marywood study, said Viagra provides an unfair advantage, at least at altitude. Credit... Niko J. Kallianiotis for The New York Times

The former major league baseball player Rafael Palmeiro once served as a pitchman for Viagra and tested positive in 2005 for the steroid stanozolol, although the connection, if any, between the drugs in his case is not known. Some athletes are believed to take Viagra in an attempt to aid the delivery of steroids to the muscles and hasten recovery from workouts. Others take Viagra to counter the effects of impotence brought on by steroid use, said Dr. Gary I. Wadler, the chairman of the World Anti-Doping Agency’s committee on prohibited substances.

The agency, based in Montreal, is financing two studies related to Viagra and performance enhancement in sports. The University of Miami is studying whether Viagra benefits aerobic capacity at lower altitudes than the Stanford study  comparable to heights where elite competitions take place. This study is also examining whether there is a difference in the way Viagra affects male and female athletes.

The study at Marywood University is measuring the potential effects of Viagra as an antidote to air pollution, produced outdoors by the exhaust of factories and automobiles and indoors by ice-resurfacing machines. Studies involving animals, and children in Mexico City, have indicated that pollution causes pulmonary hypertension. If that could be alleviated for athletes by Viagra, “performance is going to be enhanced,” said Dr. Rundell, the lead researcher of the pollution study.

The Marywood study is expected to be completed by next month, and the Miami study is expected to conclude in February. The earliest that the World Anti-Doping Agency could place Viagra on its list of prohibited substances would be September 2009, five months before the 2010 Winter Olympics in Vancouver, British Columbia, a spokesman said.