Do doctors have to keep colleagues drug use confidential?

Yes: All medical information is confidential. If the physician abuses drugs he will be removed from practice and will be sent for treatment. After full recovery several protective mechanisms exist in medical community to prevent reoccurrence. At the beginning physician will be supervised by other physicians after recovery.

That depends on: the context. If we notice red flags within the workplace that suggest that one of our colleagues suffers from a significant impairment - many medical providers would consider it an ethical obligation to address the situation or report it. My response is from the perspective of being a coworker (not being the medical provider to an impaired physician).