EPO, or erythropoietin (pronounced, ah-rith-ro-poy-tin), is a hormone produced by the liver and kidneys.
In the first part of a two-night interview broadcast Thursday, disgraced cyclist Lance Armstrong admitted to Oprah Winfrey that he took banned substances, including EPO, during all seven of his Tour de France victories.
Armstrong also told Winfrey that he repeatedly relied on blood transfusions and testosterone injections, among other things, to boost his performance.
According to Gizmodo, EPO — which can also be produced in a lab using cell cultures — regulates how many red blood cells your body produces.
Once released into the bloodstream, it binds with receptors in the bone marrow and stimulates red blood cell production — thus increasing the blood's oxygen carrying capacity (red blood cells carry oxygen).
EPO is commonly used to treat anemia resulting from chronic kidney failure or cancer.
Gizmodo notes the irony that Armstrong was "likely given EPO when he was recovering from cancer under a doctor's treatment."
Aside from the apparent endurance boost provided by EPO, Swiss researchers found that EPO also has a performance-enhancing effect in the brain, increasing motivation.
Science Daily cited experts as saying the substance might therefore be useful in treating depression.
After his damning admissions on Oprah, it remains to be seen whether Lance Armstrong will require its continued use for this purpose.