The problem is that many performance enhancing drugs are used to treat legitimate conditions. It will always be easier to get doctor prescription for some players than it would be for others. If you ban Adderal/Ritalin/Modafinil (although the last one is very questionable when it comes to performance benefit but still very useful for motivation/energy when putting long hours practicing) and allow medical exemptions you will end up with privileged group who have access to the "right doctors".

If you ban it and don't allow medical exemptions you will eliminate people who need those drugs to function normally.

I would prefer the latter out of the two: you need a drug to function - tough luck you can't compete but I prefer just allowing wider spectrum of drugs to begin with.