There could be a very big problem if the medical requires a hair sample analysis for substance abuse. Information on this and other drug related problems, including alcohol, tobacco and women, is readily available on Dr Google and associated websites.
There is one medical examiner in particular at CAA/LGW who was very keen on the hair follicle test being imposed and so it might well have been by now.
Inso far as the FAA is concerned, I copied out the following for digestion from this site:
Removed commercial link. Better to use the FAA website.
(Substance Dependence/Substance Abuse - A diagnosis or medical history of substance dependence is disqualifying unless there is established clinical evidence, satisfactory to the Federal Air Surgeon, of recovery, including sustained total abstinence from the substance(s) for not less than the preceding 2 years. A history of substance abuse within the preceding 2 years is disqualifying. The list of substances includes alcohol and all illegal drugs (marijuana, cocaine, etc.) plus certain prescription drugs such as sedatives, anxiolytics, etc.)
So I suppose it could be said with a certain degree of exactitude that the FAA take this sort of thing quite seriously. Regular as opposed to irregular use of marijuana within the last two years disqualifies you from holding an FAA licence? I have always found the FAA office at KPNE to be extremely helpful. Perhaps you should contact them for clarification?
Risk taking poses its own problems in aviation and generally speaking such an attitude is not encouraged among pilots either amateur or professional. In the absence of gross neurological manifestation this characteristic is not likely to be determined at the initial medical stage. The unfortunate attitude which condones such a degree of infantility will, however, usually be determined at the psychology tests administered by an individual airline as part of its recruitment process.
I should state that I have no medical qualifications and that the above a personal thoughts and ruminations based on empirical research generally available on the internet to which I commend your further researches.