I've told my doc I smoke a lot and I also asked him about my few encounters with DXM and whether or not I should be worried about my future health. He said it wasn't the smartest thing but I shouldn't worry about it because I didn't do it enough. Just ask him something like "everything we discuss is completely confidential right? are there any situations where you'd be required by law to break this confidentiality?" Thats exactly what I did and my doc said "Yes this is confidential and only if I think you may be suicidal."

I know therapists and psychiatrists are required to get medical help for clients who admit to hurthing themselves physically (drug use is not hurting yourself according to them) but that won't affect you. And if you're uber paranoid just tape record him saying everything is confidential and if you get arrested, bring that shit up in court and you'll be taken care of.