What does a doctor do if they find out you use illegal drugs?

A doctor has a legal responsibility to make sure that the drugs s/he prescribes are used for valid medical purposes. Failing to do so can cause the doctor to lose his license to practice, cost him financial penalties, and may even lead to jail terms. Doctors may refuse to prescribe certain drugs if they think that the patient is likely to abuse those drugs, though they have a responsibility to make sure that the patient has sufficient care through the use of equivalent medicines. Doctors may also refuse to prescribe a drug if they are concerned about possible interactions with illegal drugs - street drugs combined with prescribed drugs can produce unfortunate side effects, up to and including death.

Doctors are (within limits) obligated not to report you to the police or authorities if they discover that you've used illegal drugs (patient confidentiality), so it is safe to tell your doctor if you've been indulging. Plus it's a good idea: it will help the doctor diagnose your current condition, and make sure that s/he doesn't prescribe something that (when mixed with your recreational drugs) will destroy your liver, kidneys, or other organs.