A Doctor of Medicine, abbreviated MD, was a Federation title to an accredited medical physician. Leonard McCoy became an MD by the time he was assigned to the USS Enterprise in 2266. (TOS: "The Changeling"; Star Trek II: The Wrath of Khan)
Between 2270 and 2273, Christine Chapel became an MD, a fact noted by Leonard McCoy when he returned to the Enterprise during the V'Ger incident. (Star Trek: The Motion Picture)
It is unknown whether Starfleet physicians possessed this title in the 24th century or earned a different type of doctorate. McCoy's contemporaries, such as Doctor M'Benga, did not explicitly posses the appellation of MD either, and McCoy himself was still alive in the late 24th century.