From the Times:
Viagra, or sildenafil citrate, was devised to treat pulmonary hypertension, or high blood pressure in arteries of the lungs. The drug works by suppressing an enzyme that controls blood flow, allowing the vessels to relax and widen. The same mechanism facilitates blood flow into the penis of impotent men. In the case of athletes, increased cardiac output and more efficient transport of oxygenated fuel to the muscles can enhance endurance.A number of easy jokes with this one; I'm too lazy to write them though.
“Basically, it allows you to compete with a sea level, or near-sea level, aerobic capacity at altitude,” Kenneth W. Rundell, the director of the Human Performance Laboratory at Marywood, said of Viagra.