EPO is a natural protein that stimulates production of red blood cells, which carry oxygen. Taken as a supplement, it loads the blood with red cells and delivers more oxygen, permitting greater endurance. Some two dozen heart-attack deaths have reportedly been linked to EPO use. Like creatine, they permit muscles to recover faster from tough workouts. But the side effects include elevated cholesterol, uncontrollable outbursts of anger, possibly liver disease and cancer.
Men can develop low sperm counts.
Women may grow hair on their faces, lose hair from their scalps, get acne, stop menstruating and see their breasts shrink. This drug has become so popular with athletes for its muscle-building effects that the 1996 Atlanta Olympics were informally known as the Growth Hormone Games. Unfortunately, many of these performance boosters can't be picked out in a urine test, and even blood tests are tricky. Naturally occurring substances such as testosterone and EPO will always be present in some amounts. Moreover, new drugs come along all the time.