A substance can be included on the "World Anti-Doping Code Prohibited List" if it meets two of the following three criteria: 1) it is performance-enhancing, 2) it is harmful to the athlete's health, and 3) it violates the spirit of sport.
Of the 21,849 drug tests conducted at the Summer Olympics from 1968-2008, there were 105 cases (0.49%) of doping violations. There have been 22 positive results (0.28%) from 7,783 tests conducted at the Winter Olympics between 1968 and 2010.
Although not a performance-enhancing drug, it seems relevant in the context of this debate to note that professional golfer Tiger Woods used LASIK eye surgery to improve his vision to 20/15, meaning that he can see at 20 feet what the average person can see at 15 feet. Professional baseball player Mark McGwire wore specially-designed contact lenses to improve his vision to 20/10.