Mo. Rev. Stat. § 408.080: Interest may be paid on interest — compounding limited to once a month — prohibited for certain loans.

Parties may contract, in writing, for the payment of interest upon interest; but the interest shall not be compounded more often than once a month. Where a different rate is not expressed, interest upon interest shall be at the same rate as interest on the principal debt. Loans governed by section 408.035 are not subject to the provisions of this section.