Every two years as the Olympic Games begin, we hear about athletes using or at least being tested for performance-enhancing drugs. Every time the Tour de France rolls around, there's talk of possible doping. Sometimes, competitors raise the question when one athlete does particularly well. Other times, tests catch athletes with drugs in their systems.

The practice of using artificial substances or methods to enhance athletic performance is called doping. Doping has become such a great concern that the United States formed an Anti-Doping Agency.

In this article, we will discuss why some athletes take drugs, what the major classes of drugs and their side effects are and how drug use is tested for.

How strong are you?
You'd probably be amazed to learn how much force your body's joints and muscles actually support on a daily basis. This interactive segment from Discovery takes you inside the body and explains how much strength your bones and muscles really possess. Other activities let you explore even more of your body's systems to see exactly how they move you through your daily life