Mo. Rev. Stat. § 404.825
Unless the patient expressly authorizes otherwise in the power of attorney, the powers and duties of the attorney in fact to make health care decisions shall commence upon a certification by two licensed physicians based upon an examination of the patient that the patient is incapacitated and will continue to be incapacitated for the period of time during which treatment decisions will be required and the powers and duties shall cease upon certification that the patient is no longer incapacitated. One of the certifying physicians may be the patient's attending physician. The certification shall be made according to accepted medical standards. The determination of incapacity shall be periodically reviewed by the attending physician. The certification shall be incorporated into the medical records and shall set forth the facts upon which the determination of incapacity is based and the expected duration of the incapacity. Other provisions of this section to the contrary notwithstanding, certification of incapacity by at least one physician is required.
§ 404.825, RSMo