Protein Hormones
Erythropoietin (EPO) is a naturally occurring protein hormone that is secreted by the kidneys during low oxygen conditions. EPO stimulates the bone marrow stem cells to make red blood cells, which increase the delivery of oxygen to the kidney. Endurance athletes, such as those who compete in marathons, cycling or cross-country skiing, can use EPO to increase their oxygen supply by as much as seven to 10 percent. EPO is difficult to detect. The increased red cell density (secondary polycythemia) caused by EPO, however, can thicken the blood. The thickened blood, which is more like honey than water, does not flow through the blood vessels well. To pump the thickened blood, the heart must work harder, which increases the chances of heart attack and stroke.

Artificial Oxygen Carriers
Artificial oxygen carriers are man-made substances that can do the work of hemoglobin, the oxygen-carrying protein in your blood. Doctors use them to treat breathing difficulties in premature infants, in patients with severe lung injuries and in deep-sea divers. They include substances such as perfluorocarbons, synthetic- or modified-hemoglobins and liposome-encased hemoglobins (artificial red cells). It is not clear how they benefit athletes. Possible side effects include immune problems (fever, decreased platelets, increased phagocyte counts), cardiovascular problems (high blood pressure), iron overload and kidney damage.

Blood Doping
Blood doping is the practice of infusing whole blood into an athlete in order to increase oxygen delivery to the tissues. A similar effect can be achieved by training at high altitudes. An athlete who infuses his own blood may cause infection or cardiovascular problems because of the increased blood volume (high blood pressure, blood clots, heart failure and stroke). An athlete who uses someone else's blood runs the risk of acquiring viral infections (hepatitis, HIV/AIDS).