Question regarding legalitys of drug test

I've been wondering for a while how drug tests by employers and medicinal mj affect one another. What I mean is say ya get a new job that requires passing a drug test, who do you talk to about having a perscription, and how can you be sure that because you tested positive for mj that that is not the reason you were denied the job? I haven't been able to find out the legal answers to this, but I would sure like to since I might be lookin for a new job in the near future.