MD vs Dr.

MD and Dr. both are nouns.

Word	Noun	Adjective	Verb	Adverb
MD	Yes	No	No	No
Dr.	Yes	No	No	No

As nouns, MD and Dr. are synonyms defined as:

Other synonyms of MD include doc, doctor, medico, physician.

MD (noun)	Dr. (noun)
a doctor's degree in medicine	a licensed medical practitioner
a Mid-Atlantic state; one of the original 13 colonies, with capital Annapolis	a person who holds Ph.D. degree (or the equivalent) from an academic institution
a licensed medical practitioner