Mo. Rev. Stat. § 324.041
For the purpose of determining whether cause for discipline or denial exists under the statutes of any board, commission, or committee within the division of professional registration, any licensee, registrant, permittee, or applicant that tests* positive for a controlled substance, as defined in chapter 195, is presumed to have unlawfully possessed the controlled substance in violation of the drug laws or rules and regulations of this state, any other state, or the federal government unless he or she has a valid prescription for the controlled substance. The burden of proof that the controlled substance was not unlawfully possessed in violation of the drug laws or rules and regulations of this state, any other state, or the federal government is upon the licensee, registrant, permittee, or applicant.
§ 324.041, RSMo