Well obviously, but he was referring to a drug screen. And seeing as how marijuana can stay in your system for up to 30 days, there's very little way to prove whether you partook legally at home, or at the job.
I just asked the person in charge of drug screening/hiring for my company.
The way it works in our offices that are located in states with medical marijuana, if you have a prescription you are ok if you test positive. It is up to their manager as to whether it makes sense to put that employee in a safety sensitive position. It is also up to the employee to inform his/her manager about his/her prescription, because we can't ask on our own due to HIPPA regulations.
Our clients are very demanding when it comes to drug testing. We just got an edict from a client demanding even more sensitive testing levels. So, you could have a prescription for anything, but you might find yourself being excluded from working certain jobs due to client requirements or safety considerations. Don't think it would be a good idea for a bulldozer operator to be stoned, or a pipeline guy, or a truck driver, etc.