Mo. Rev. Stat. § 334.110: Retired licensees not required to register.

Any person licensed to practice as physician and surgeon in this state who retires from such practice shall file with the board an affidavit, on a form to be furnished by the board, which states the date on which he retired from such practice and such other facts as tend to verify the retirement as the board may deem necessary; but if he thereafter reengages in the practice, he shall renew his registration with the board as provided by section 334.090.