When does a medical doctor becomes Mr?

A medical doctor becomes "Mr." (or "Dr.") when they are addressed formally, regardless of their gender. The title "Mr." is used for men, while "Dr." is a title indicating a doctoral degree, which is what a medical doctor (MD or DO) holds.

So, a male medical doctor can be addressed as either "Mr." or "Dr.," but "Dr." is generally preferred as it reflects their professional achievement. Using "Mr." for a male doctor is not incorrect but is less common and might seem to downplay their professional status.