that's a difficult one. Strictly speaking ANYONE has an obligation to report illegal acts, but doctors and other medical staff also have a duty of confidentiality.
I guess it would depend if the use endangered or put others at risk, eg using when in charge of children or vulnerable people, stealing to fund the habit, selling to others etc
It's not illegal to take any form of drug. It's only illegal to possess certain drugs, or to supply them. So doctors are not obliged to report that a patient is taking drugs (and can ignore the method by which they came to take them).
A drug user who goes to a doctor might well be seeking a legal alternative to the drug which they've been taking. (e.g. a prescription for methadone instead of heroin). If doctors were obliged to report every heroin addict who came into their surgery, nobody would ever approach a doctor for help in coming off the illegal drugs.