A person licensed as a physician assistant may perform, only under the direction and supervision of a physician, acts which constitute the practice of medicine and surgery to the extent and in the manner authorized by the physician responsible for the physician assistant and only to the extent such acts are consistent with rules and regulations adopted by the board which relate to acts performed by a physician assistant under the supervising physician’s direction and supervision. A physician assistant may prescribe drugs pursuant to a written agreement as authorized by the supervising physician.