As the commercials continually remind us: Viagra is all about performance.
Now it turns out, that's not just referring to in the bedroom.
Researchers say the drug, approved for erectile dysfunction, could eventually help some athletes train at high altitudes and soldiers fight in the mountains of Afghanistan.
In a study at Stanford University, some volunteers riding stationary bicycles and breathing through masks to simulate the low oxygen conditions found at 12,700 feet, improved their times for six kilometers by an average of 39 percent after taking Viagra.
The drug, which became an instant blockbuster for Pfizer in 1998, works by causing blood vessels to relax -- not only in the penis but in the lungs.
Last year, the company won approval for the drug, also known as sildenafil, to treat a medical condition called pulmonary hypertension, or high fluid pressure in the lungs. Pulmonary hypertension is also one of the effects of exercising in oxygen-poor environments such as high altitudes.
"It provides a pretty clear advantage to some people," said Annie Friedlander, the senior author of the study, which appears in the Journal of Applied Physiology.
It does not help everyone. Only four of the 10 riders saw their times improve -- 10 minutes, 48 seconds with Viagra compared to 15 minutes when they took a placebo.
Researchers are not certain why only some volunteers responded to the drug, but they noticed that they were the ones whose times had suffered the most at high altitudes. Viagra, it seems, allowed them to make up the performance they had lost.
None of the riders saw any improvement from the drug at sea level, and none reported an erection during the trials.
The next step: The U.S. military plans to test Viagra, at high altitude, on about a dozen soldiers later this summer.