The term hallucinogen is often used to describe a drug that produces a change in sensory perception, usually either visual or auditory. Drugs commonly assigned to this class include lysergic acid diethylamide (LSD), mescaline (derived from the peyote cactus), and psilocybin (derived from a mushroom). However, this rather limited definition fails to include the other prominent property of this class of drugs, which is a change in thought and mood. For this reason the term is sometimes used interchangeably with psychedelic or psychotomimetic, the latter term representing the CNS effects beyond the hallucination itself. Most literal definitions of the term hallucinogen are inadequate, but it should be used to signify substances that consistently produce changes in sensory perception, thought, and mood. An hallucinogen is a drug that reliably produces alterations in perceptions as a primary effect. Drugs that should not be included are those that produce alterations in sensory perception only at toxic doses (e.g., antimuscarinic agents, anti-malarials, and opioids) and fail to produce these effects in all individuals. This does not preclude a drug's being classified as an hallucinogen if it has other properties as well. Several drugs that reliably alter mood at low doses and produce altered sensory perceptions at slightly higher doses are close chemical analogues to the CNS stimulant class of drugs. These drugs that also reliably produce differing degrees of CNS stimulation in a dose-
responsive fashion include phencyclidine (PCP), methyl-enedioxymethamphetamine (MDMA), and methylene-dioxyamphetamine (MDA).
The hallucinogens generally fall into two chemical classes. The indole alkylamines include LSD, psilocybin, psilocin, dimethyltryptamine (DMT), and diethyltrypta-mine (DET), all of which are structurally similar to serotonin. The other chemical subclass of hallucinogens contains phenylethylamine derivatives such as mescaline, MDMA, MDA, and DOM (dimethoxymethyl amphetamine). A related stimulatory hallucinogen, PCP, is a piperidine analogue that produces unique effects.
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