What is a performance enhancing drug? Where do we draw the line between nutritional supplement and illegal substance?  There are many substances banned by the Olympics and other sporting organization, and while they all artificially enhance performance they don't all do the same thing.  

There are a number of different types and they work by different mechanisms.  Lets go through the main players.  The scientific name for all of these drugs is Ergogenic Aids (meaning "energy giving" medications).