https://en.m.wikipedia.org/wiki/Erythropoietin http://news.bbc.co.uk/sport2/hi/front_page/4657010.stm Erythropoietin (EPO) is a hormone naturally produced by the kidneys. However, this hormone can be artificially produced to improve the performance of, for example, athletes or cyclists by injection. Its overall effect is to increase endurance and, in athletics, it is used mainly by long distance-runners. It is injected under the skin and stimulates red blood cell production. The more red cells there are in your body, the more oxygen that can be delivered to the muscles. This delays the onset of fatigue, meaning an athlete can run harder and for longer.