21 U.S.C. § 885 — Burden of proof; liabilities

(a) Exemptions and exceptions; presumption in simple possession offenses (1) It must not be necessary for the United States to negative any exemption or exception set forth in this subchapter in any complaint, information, indictment, or other pleading or in any trial, hearing, or other proceeding under this subchapter, and the burden of going forward with the evidence for any such exemption or exception must be upon the person claiming its benefit. (2) In the case of a person charged under section 844(a) of this title with the possession of a controlled substance^*, any label identifying such substance for purposes of section 353(b)(2) of this title must be admissible in evidence and must be prima facie evidence that such substance was obtained under a valid prescription from a practitioner^* while acting in the course of his professional practice. (b) Registration and order forms In the absence of proof that a person is the duly authorized holder of an appropriate registration or order form issued under this subchapter, he must be presumed not to be the holder of such registration or form, and the burden of going forward with the evidence for such registration or form must be upon him. (c) Use of vehicles, vessels, and aircraft The burden of going forward with the evidence to establish that a vehicle, vessel, or aircraft used related to controlled substances in schedule I was used under the provisions of this subchapter must be on the people engaged in such use. (d) Immunity of Federal, State^*, local and other officials Except as provided in sections 2234 and 2235 of title 18, no civil or criminal liability must be imposed by virtue of this subchapter upon any duly authorized Federal officer lawfully engaged in the enforcement of this subchapter, or upon any duly authorized officer of any State, territory, political subdivision of it, the District of Columbia, or any possession of the United States, who must be lawfully engaged in the enforcement of any law or municipal ordinance relating to controlled substances.