I believe that as a professional athlete you have a duty to uphold a certain image in the media, not just performance in the sport. I believe that anything that is deemed illegal and can give you an unfair advantage over the competition such as any drug, that it is simply wrong. Drugs are meant to help elleviate pain and not to enhance certain athletic performance.
If we accept that it is okay for athletes to use drugs, then we are setting a bad example for young people. Young people look up to athletes, sometimes even when they abuse drugs. If we accept this as being okay, then our young people are more likely to use those drugs. It's already bad for a young person to see an athlete they look up to abusing drugs, but if society deems it acceptable, then those young people will be more susceptible to drug use.