Doctor of Medicine

From Memory Alpha, the free Star Trek reference.

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")

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 if this title continued into the 24th century, or if a different doctorate was awarded.