To be a "doctor" one had only to be a barber.
The majority of medieval physicians believed in the doctrine of health as taught by the Greek physician Galen which was that the humors of the body could change and affect the moods and health of the patient. The four vital humors are: yellow bile which made you choleric, black bile which caused melancholy, the blood associated with sanguine, and phlegm which caused you to be phlegmatic.
The Medieval Dentist, illustrated here, with a necklace of teeth as he extracts a tooth, possibly to add to his necklace, from a patient.
To balance the humors, physicians would "bleed" their patients, either by cutting and letting blood drain...
Or they would use leeches to drain blood from the patient.