Erythropoietin (EPO) is found naturally in the human body and has been commercially available as recombinant human EPO (rEPO) for almost 20 years. Although the positive benefits of rEPO cannot be denied for those who suffer from anemia, athletes have also used the drug as an ergogenic aid. It has replaced conventional "blood doping" as the drug of choice to improve performance in contests requiring aerobic potential. Testing for the drug in the past has proven difficult due to several factors. The similarities in structure and metabolism of EPO and rEPO allow those who choose to abuse rEPO to avoid detection. Due to the inherent danger of abusing rEPO and its analogues, the various governing bodies of international sports continue to improve testing methods for this illegal substance.