Erythropoietin (pronounced, ah-rith-ro-poy-tin, and abbreviated, EPO) is a relatively recent entry into the arsenal of a endurance athletes. EPO is a protein hormone produced by the kidney. After being released into the blood stream it binds with receptors in the bone marrow, where it stimulates the production of red blood cells (erythrocytes). Medically, Erythropoietin is used to treat certain forms of anemia (e.g. due to chronic kidney failure). Logically, since Erythropoietin accelerates erythrocyte production it also increases oxygen carrying capacity. This fact did not long escape notice of the athletic community or for that matter Lance Armstrong.
Blood doping is the process of artificially increasing the amount of red blood cells in the body in an attempt to improve athletic performance. In the past this was accomplished by blood transfusion. The athlete would donate a unit of blood into storage and then 3 weeks later, after the body had completely replaced the blood loss, transfuse the unit back into the body. This would occur just before a big race, effectively giving the athlete an extra unit of blood. This enables performance improvements in endurance sports because of the extra oxygen carrying capacity. Administering EPO for 5 days prior to a race ?or endurance event achieves the same effect as the blood transfusion however, increase the dose and the period of time administered by a factor of 4 and you have effectively increased your blood oxygen capacity by 30% on baseline levels.