Texas Health and Safety Code - HEALTH & SAFETY § 481.184. Burden of Proof; Liabilities

(a) The state is not required to negate an exemption or exception provided by this chapter in a complaint, information, indictment, or other pleading or in any trial, hearing, or other proceeding under this chapter. A person claiming the benefit of an exemption or exception has the burden of going forward with the evidence with respect to the exemption or exception.

(b) In the absence of proof that a person is the duly authorized holder of an appropriate registration or order form issued under this chapter, the person is presumed not to be the holder of the registration or form. The presumption is subject to rebuttal by a person charged with an offense under this chapter.

(c) This chapter does not impose a liability on an authorized state, county, or municipal officer engaged in the lawful performance of official duties.