Every two years as the Olympic Games begin, we hear about athletes using or at least being tested for performance-enhancing drugs. 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 edition of HowStuffWorks, 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.