First name. One introduced herself to me by first name only, the other signed an email setting up the first appointment with her first name only. So that was easy.

When they use first name-last name together, your choice. If it's a doctor (an MD), I'd use Doctor Last Name.

And may I air a pet peeve of mine, non-MDs with doctorates (PhD, JD) being addressed as Dr.? I realize they're entitled to the title, but it bugs me when people call me "Dr." Save it for the person who could actually answer the question "is there a doctor in the house?" in the affirmative.