[Udik]

You'd have to prove that the use of the medication brings them to a simply average level of performance in the specific domain, e.g. focus, or resistance to strain. (Which is in itself a contradiction, since by definition from an athlete you ask a performance which is well above average). Anything more than that would be an unfair advantage. Very hard to prove. On the other hand, being a professional athlete is not a basic right. Some people are simply not able to do it and I don't see the issue in that. Want a decent life? Take ritalin, but then don't ask to compete at the Olympic games.

Edit: I said average. That's wrong. It should bring them to the low end of the non-pathological performance spectrum.

[alexeldeib]

I don't disagree, but the flip side would be a high level athlete who suffers, for example, an injury. They can continue competing at the same level only with medication. For me, this is analogous to someone with ADHD taking medication to perform closer to their level without ADHD (impossible to prove, per your point), but I would prefer to give them the opportunity rather than broadly disqualify such people from competing.