A friend just after receiving his Ph.D. in Economics boarded a plane for an interview on the East Coast. He signed in as Dr. ..... During the flight there was a medical emergency. He was paged and asked to assist. He had to swallow hard and admit his doctorate was not in medicine. Needless to say, he never did that again. Humility, as they say, is a humbling thing.
Funny, I have a PhD in economics but have NEVER used Dr. anywhere.