The State
The fundamental legal relationship is between medical care practitioners and the state and federal governments. In the United States, most of the law governing medical practice and general medical care delivery, exclusive of payment issues, is state law. All medical care practitioner licenses are issued by the states, rather than by the federal government. State laws define the practice of medicine and nursing, who can write prescriptions, the supervision requirements for nonphysician practitioners, and the other parameters that establish the bounds of medical care delivery. These laws are subject to any applicable federal mandates, such as the special restrictions on controlled substance prescriptions, or requirements tied to reimbursement for care paid for with federal funds. While the state regulation and licensing of medical care practitioners is taken for granted, there is no legal principle that would prevent a state from allowing a layperson to practice medicine.
Medical care practitioners owe the state certain duties in return for the right to their professional licenses: the duty to protect the public health through various reporting and recordkeeping functions, the duty to practice competently and within the constraints established by the state laws, and the duty to help ensure that other medical care practitioners maintain proper standards of practice.