Mo. Rev. Stat. § 376.392

For any health carrier or health benefit plan, as defined in section 376.1350, that provides prescription drug coverage or contracts with a third-party for prescription drug services, the health carrier or health benefit plan shall notify enrollees presently taking a prescription drug electronically, or in writing, upon request of the enrollee, at least thirty days prior to any deletions, other than generic substitutions, in the health carrier's or health benefit plan's prescription drug formulary that affect such enrollees.