Cycling – Factbox: Performance enhancing drugs in sports

Armstrong's revelation that he cheated in international cycling races acknowledges that he used a cocktail of performance-enhancing drugs to try to boost red blood cells, improve strength and stamina, and mask his cheating from anti-doping authorities.

Here are details of some of the drugs commonly used in sports doping.

ERYTHROPOIETIN (EPO)

* EPO is a peptide hormone that is produced naturally by the human body. It is released from the kidneys and acts on the bone marrow to stimulate red blood cell production. By injecting EPO, athletes aim to increase the concentration of red blood cells to boost their aerobic capacity.

* EPO abuse has serious health risks. By thickening the blood, EPO abuse can lead to an increased risk of heart disease, stroke and cerebral or pulmonary embolism. The drug has been implicated in the deaths of several athletes.

BLOOD DOPING

* There are two forms of blood doping. Autologous blood doping is the transfusion of an athlete's own blood, which has been stored, refrigerated or frozen, until needed. Homologous blood doping is the transfusion of blood taken from another person with the same blood type.

* Although the use of transfusions for blood doping dates back several decades, experts say it has seen a recent resurgence, probably due to the introduction of more efficient EPO detection methods.

ANABOLIC STEROIDS

* Anabolic steroids are drugs that resemble testosterone, a hormone produced in men's testes. Because these drugs affect muscle growth, raising their levels in the blood can help athletes to increase muscle size and strength. Athletes who use anabolic steroids also claim they reduce body fat and recovery time after injury.

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Cycling - Factbox: Performance enhancing drugs in sports