Hallucinogenic, huh LOO suh nuh JEHN ihk, drug is any of several substances that distort perception of oneself and one's surroundings. These drugs temporarily change the chemistry of the brain. Also called psychedelic (mind-revealing) drugs, hallucinogenic drugs affect the senses, emotions, and reasoning and the brain's control of muscles and certain body functions. The changes may be extremely pleasant or highly unpleasant and frightening. In the United States, laws prohibit the manufacture, distribution, and possession of these drugs except for government-approved research and certain American Indian religious rituals.