Fresh allegations by Lance Armstrong's former teammate Tyler Hamilton are bad news for the man in yellow. Hamilton says Armstrong used EPO to train for the Tour de France. So what's EPO, and why is that a big deal?
EPO is a hormone...
EPO, or more properly erythropoietin, is a protein hormone produced by the liver and kidneys. However, it can also be produced via cell cultures in a lab. In an odd turn of events, one of the first companies to begin manufacturing EPO (under the trade name Epogen) was Amgen, the current sponsor of the Tour of California.
...that stimulates red blood cell production...
EPO regulates how many red blood cells your body produces. It binds with receptors in your bone marrow and cranks up your red blood cell count. It's commonly used to treat anemia, often due to kidney disease or cancer. Another twist: Armstrong was likely given EPO when he was recovering from cancer under a doctor's treatment.
...to increase blood oxygen capacity...
Red blood cells carry oxygen; each gram of hemoglobin—the oxygen-transport protein in red blood cells—carries about 1.34 ml of oxygen. So the more red blood cells your body has, the more oxygen it can transport.
...which gives you an unfair competitive advantage...
EPO effectively gives you more aerobic power. The more oxygen you have in your blood, the longer and harder your body can work before it becomes exhausted. So it's particularly effective at boosting performance in endurance events, like professional cycling. Studies have found that blood doping with EPO can make athletes faster and increase the amount of time they can work out before becoming exhausted. Studies on recreational athletes have found that EPO can increase the amount of time you work before becoming exhausted by 17 percent (login required), which is quite a lot on an otherwise level field.
...but it can also kill you.
But EPO poses real dangers as well. EPO raises your hematocrit level (that is, the percentage of red blood cells vs. plasma in the blood). In simple terms: EPO thickens the blood. That can become very dangerous, as it can cause a a clot, resulting in heart attack or stroke. In fact, it's not terribly uncommon for endurance athletes to drop dead due to EPO.