What Is Blood Doping? Everything You Need To Know About EPO And Other Performance-Enhancing Drugs Lance Armstrong Says He Used

Armstrong, who before last night has never publicly confessed to doping, admitted to Winfrey that he used performance-enhancing drugs to win seven Tour de France titles.

The 41-year-old former Livestrong spokesman said that like many other cyclists of his generation, he used EPO, human-growth hormone, testosterone and other drugs to compete. "I'm a flawed character," Armstrong said during the hour-and-a-half program. "I viewed this situation as one big lie that I repeated a lot of times."

In October, the U.S. Anti-Doping Agency released more than 1,000 pages of evidence in doping allegations against Armstrong and his teammates. As a result, he was stripped of his seven Tour de France titles.

In similar fashion, cyclist Floyd Landis was stripped of his 2006 Tour de France title after failing a drug test. Eighty percent of the Tour de France medalists between 1996 and 2010 have been "similarly tainted by doping," according to the USADA report on Armstrong.

As the fallout from the interview continues, viewers are dissecting the terms that Armstrong used to describe how he went about winning.

What is “Blood Doping?”

EPO, or erythropoietin, is a hormone naturally produced by human kidneys to stimulate red blood cell production, according to the World Anti-Doping Agency. Cyclists and other athletes use EPO to raise their red blood cell counts, which increases the amount of oxygen that can be delivered to muscles, improving recovery and endurance.

Although EPO has been banned since the 1990s, the first screening test was used at the 2000 Summer Olympic Games in Sydney, CNN reports.

According to the WADA, blood transfusions have a similar effect on the body's red blood cell count. Usually an athlete will store some of his blood when his hemoglobin levels are high, then reinfuse it right before an event. This type of transfusion cannot be detected by current tests, according to the USADA.

Both methods can have dangerous side effects. Increased levels of hemoglobin, which literally thickens the blood, can lead to complications with circulation, putting athletes at risk for cardiovascular problems.

What are Corticosteroids?

Corticosteroids are man-made drugs that resemble the natural hormone cortisol, according to the Cleveland Clinic. These are different from anabolic steroids, which athletes take to increase strength. The most common types are cortisone, prednisone and methylprednisolone.

Cortisol is most commonly known as a stress hormone. Corticosteroids work to decrease inflammation that can cause swelling and pain, according to the Cleveland Clinic. They can be administered locally -- to the specific area that hurts -- or systemically through a pill or intravenously.

The list of possible side effects for corticosteroids is long, including weight gain, sudden mood swings, blurred vision, osteoporosis and high blood pressure.

"If steroid use involves high doses and is prolonged (for a few months to several years), an increase in the number of side effects might occur," the clinic's site states.

What is “Testosterone?”

Testosterone is a naturally occurring hormone that helps regulate bone density, fat distribution, muscle strength, red blood cell production and sex drive, according to the Mayo Clinic. It is found in both men and women; in men, it also helps to regulate sperm production.

Athletes generally abuse testosterone to "bulk up," according to the USADA. The side effects are similar to both blood doping and anabolic steroid use. Testosterone increases the body's red blood cell count, increasing the risk for cardiovascular disease. Mood swings, muscle weakness and liver dysfunction are also common for both sexes with overuse.

Using testosterone also shuts down the body's natural production of the hormone. This can cause the testicles to shrink in men, reducing sperm production.