The commission shall ensure that a prescription drug prescribed before implementation of a prior authorization requirement for that drug for a recipient or enrollee under Medicaid, the child health plan program, or another state program the commission or a health and human services agency administers, or for an individual who becomes eligible under Medicaid, the child health plan program, or another state program the commission or a health and human services agency administers, is not subject to any prior authorization requirement under this subchapter until the earlier of:
(1) the date the recipient or enrollee exhausts all the prescription, including any authorized refills; or
(2) the expiration of a period the commission prescribes.