Mo. Rev. Stat. § 105.1112

Any person who is not a public employee who, within three years prior to applying for public employment or appointment from an eligibility register, has been convicted under the laws of this state, the United States or any other state, of any criminal offense involving the use of a controlled substance, marijuana or other dangerous drug, as such substances are defined in chapter 195, shall be ineligible for any public employment unless such person has completed or shows evidence of enrollment and continuing progress in a state certified drug abuse treatment and education program within such three-year period or is currently undergoing treatment in such program.