I read with interest Tony Cotton's piece on drugs in motorsport. We as part of MSA are already under the auspices of WADA which I think means world anti doping authority. The illegal drugs are specified in the very small print of the application form accompanying documents.
In practice if you are on any prescription drugs just send a photocopy of your current prescription sheet with your annual licence application. The MSA medical people will check what you are on against their rules. I have done this on many occasions for drivers with their medicals.
The rules vary with each sport, for example beta blockers are highly performance enhancing in sports that need a steady hand like shooting or archery (and heart surgery) but would cause negative performance in say cycling. Drugs like erythropoetin (the drug much loved by Lance Armstrong) increase one's blood count which is going to be very little help in say darts...
The most likely drug which we are likely to run into trouble with is alcohol. Those evenings in the motorhome or caravan chatting, telling lies about taking Eau Rouge 'flat' and sipping on a pleasant Merlot could well result in being over the limit in the morning. Although I have been known to join in such activities I have often wondered why some folk crash on the out lap of an early morning qualifying session.
In general if you are up front about what you are on and follow the 8 hour 'bottle to throttle' rule you should be okay.