In these days of quick-fire rapid communication such as texting and instant messaging, writing letters is a lost art. The idea of writing letters actually scares some people, because they're not sure how to properly address the recipient, particularly if they have a professional or business relationship, rather than a casual, friendly one. In particular, writing letters to a doctor seems to make people nervous. How, exactly, do you address an envelope when you send a letter to a doctor?
Use the term "Dr." whether the recipient is an M.D. or a Ph.D. It's not necessary to write out the word "Doctor." If you're writing to your physician, John Smith, you would address the envelope "Dr. John Smith" or "John Smith, M.D." If John Smith is not your physician, but a person who holds a Ph.D., you should write "Dr. John Smith," or "John Smith, Ph.D." If Dr. Smith is a professor or lecturer, you can also address the envelope to him by that title, "Professor John Smith" or "John Smith, Professor of Humanities."
If you're addressing an invitation or letter to the doctor and his wife, you would write, "Dr. and Mrs. Smith". It is also acceptable to write "Dr. Smith and Mrs. Smith" or "Dr. John and Mary Smith." If the wife has a different name, you can write "Dr. Smith and Mrs. Jones" or "Dr. John Smith and Mary Jones."
If the wife is the doctor and uses a different last name from her husband, address the letter this way: "Dr. Jones and Mr. Smith." If their last names are the same, you can use "Mr. John and Dr. Mary Jones".
If you are acquainted with someone casually rather than professionally, it's not necessary to use their formal title. For example, if Dr. Smith is your next door neighbour, but not your physician, you can send him a holiday card that is addressed to "John and Mary Smith" or just "John and Mary." It's not necessary to refer to him as "Dr." if your relationship is a friendly, casual one and you have no professional connection to him.
If you are called on to introduce two doctors in a social setting, introduce them by saying, "Dr. Smith, this is Dr. Mary Jones. Dr. Jones, this is Dr. Bob Smith." Women should always be introduced first.
If you recieve a reply from Dr. Smith, and he signs his name "John," it doesn't mean you should respond with "Dear John". Continue to use his title in correspondence unless he tells you to do otherwise.