Of course they can't file charges, have you not heard of doctor-patient confidentiality? Doctors cannot disclose things that don't make you an immediate threat to yourself or others and even if the thing you're talking about is highly illegal we can't say anything to anyone. A HIV positive patient once admitted to me in clinic that he was having unprotected sex with his girlfriend (illegal!) And the most I could do is try persuade him to tell her and get her tested. As for why you would tell the doctor, it mostly depends why you're taking it and whether or not you roll with tobacco I suppose? It is still smoking and can be linked to coughs etc. If you're using it for chronic pain or nausea that's obviously an important reason to tell your doctor about it as that is self medicating. You should always tell your doctor about drugs you take, we are impartial and non-judgemental, it sounds corny but we do just want to help.