(a) Definitions for the purpose of this section:

(1) The term controlled substance means a drug or other substance, or immediate precursor, included in schedules I, II, III, IV, or V of part B of the Controlled Substance Act (21 U.S.C. 812) or any drug or substance added to these schedules pursuant to the terms of the Controlled Substance Act.

(2) The term practitioner means a physician, dentist, veterinarian, scientific investigator, pharmacist, or other person licensed, registered, or otherwise permitted by the United States or the jurisdiction in which he practices to distribute or possess a controlled substance in the course of professional practice.

(3) The term delivery means the actual, attempted or constructive transfer and/or distribution of a controlled substance, whether or not there exists an agency relationship.

(b) Offenses. (1) The delivery of any controlled substance on a national wildlife refuge is prohibited, except that distributed by a practitioner in accordance with applicable law.

(2) The possession of a controlled substance on a national wildlife refuge is prohibited unless such substance was obtained by the possessor directly, or pursuant to a valid prescription or order, from a practitioner acting in the course of his professional practice, or except as otherwise authorized by applicable law.

(3) Presence in a national wildlife refuge when under the influence of a controlled substance to a degree that may endanger oneself, or another person, or property, or may cause unreasonable interference with another person's enjoyment of a national wildlife refuge is prohibited.