[loramin]

I do all the time (and not just to the doc who gives me my weed prescription).

Let me put it like this:

A) the doc is bound by law (patient confidentiality) not to tell anyone (unless maybe it's a felony or something ... not sure about the details, but don't tell your doc you deal heroin and you'll be fine)
B) would you want to die because you didn't tell your doctor about a drug that interferes with the one he just prescribed to you? (not sure such a drug even exists, but I'd rather be safe than sorry)

Usually when I tell the doc it's because he/she asked "Do you smoke", and when I reply "Just weed" they laugh and move on to the next question.