An unethical process that has been used in the past to give athletes an advantage over their competitors.

The ability to efficiently bring in, absorb and transport oxygen to active muscle cells (see myoglobin and sarcomere for details) is essential to athletes. There are several ways to accomplish this. Naturally, when we exercise, our respiration and heartbeat increase. Trained athletes develop larger hearts because they need to pump more blood past their lungs to effectively absorb oxygen and expire carbon dioxide.

Another method to increase the absorbtion of oxygen is to increase the number of red blood cells in the body blood. One way of doing this is called "blood doping" and requires the athlete to withdraw blood a couple or more days prior to the athletic event. The body recognizes the blood loss and slowly replaces the lost blood cells with new ones until the person is back to normal. Right before the race, however, the person then re-injects their own blood causing them to have more blood cells than normal.

Just like steroid use, this was once not uncommon practice for athletes competing in the olympics. It does, however, have several problems. The most significant is that the infusion of additional blood followed by vigorous activity results in extremely high blood pressure and can lead to stroke or other problems (see why people die on the toilet). The second problem, is that the blood must be stored with chemicals that prevent it from clotting and these chemicals can sometimes be detected so the athlete could get caught.

More recently, use of genetic techniques has lead to similar and until recently undetectable phenomena. Drugs that promote creation of EPO (a hormone causes bone marrow to make more red blood cells) or are themselves synthetic forms EPO are increasingly common and hard to detect.

See also Marco Pantani (accused of EPO use).