From a legal/contractual standpoint maybe.
From a logical standpoint though this is madness! The list of things that go in and out of legal-state over time (slavery, voting rights, opium use, homosexuality, race-based-laws, rape in marriage, etc, ad nauseum) goes to show that 'legality' is not only dynamic but deeply rooted in human opinion rather than objective science or rationality.
Are you capable to do your job right now? Safely and professionally? Then you're good to go. End of argument.
So you in the past ingested a variety of interesting chemicals? Irrelevant.
We're all in the medical profession, so it boggles my mind that people with the same, if not likely more medical knowledge than me, can seriously defend "Oh alcohol and tobacco is legal so okay, whilst marijuana is illegal so therefore evil" Really? Can you not see the hypocrisy in this?
Add to that, that North Americans are practically rattling from the number of prescribed medications in their systems, that it shocks me more about the fantastic puritan nature of its residents in regards to chemical ingestion. I know (and do know personally) that not all Americans believe this hype, but I would have thought medically trained people would be more rational.