Mo. Rev. Stat. § 376.386

For any health carrier or health benefit plan, as defined in section 376.1350, that provides prescription drug coverage, if a prescription drug covered by a health carrier or health benefit plan is prescribed in a single dosage amount for which the particular prescription drug is not manufactured in such single dosage amount and requires dispensing the particular prescription drug in a combination of different manufactured dosage amounts, the health carrier or health benefit plan shall only impose one co-payment for the dispensing of the combination of manufactured dosages that equal the prescribed dosage for such prescription drug. Such co-payment requirement shall not apply to prescriptions in excess of a one-month supply. If technology does not permit such adjudication, the health carrier or health benefit plan shall provide reimbursement forms for the patient.