(a) This section does not apply to a prescription for a controlled substance, as defined by Chapter 481, Health and Safety Code.
(b) In the rules and standards governing the vendor drug program, the executive commissioner, to the extent allowed by federal law and laws regulating the writing of prescriptions and dispensing of prescription medications, shall ensure that a prescription written by an authorized health care provider under Medicaid is valid for the lesser of:
(1) the period for which the prescription is written; or
(2) one year.