Difference between "he said something" and "he mentioned something"

(a) What is the difference between he said something and he mentioned something? Can they be used interchangeably?

(b) Is it proper to respond to Did I tell you X? with Yes, you mentioned it?

Solution 1:

Any place where you would use "mention" you could probably also use "said" (which is more general), but the reverse doesn't apply. Mention is less substantial -- a passing reference, a brief comment, a name on a list, that sort of thing.

My doctor said I need to lose 20 pounds and reduce my salt intake because of my blood pressure. He also mentioned that I'm overdue for a tetanus shot.

This example is primarily about lifestyle changes and reports a passing reference to something else the speaker might want to attend to.

Per your second question, yes, saying "you mentioned it" is generally appropriate.