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   Also known as hallucinogenic drug, hallucinogenic substance, magicum, pseudohallucinogen, illusinogen, mysticomimetic, phanerothyme, *psychedelic, psychedelic drug, psychedelic substance, psychotic, psychotomimetic, *phantasticum, and eideticum. The term hallucinogen comes from the Latin verb *alucinari (to hallucinate) and the Greek noun genesis (creation, origin). It translates loosely as 'creator of hallucinations'. The term hallucinogen was introduced into the biomedical literature in 1953 by the British physician Christopher Johnson, who had in turn borrowed it from the psychiatrists Abram Hoffer (b. 1917), Humphry Fortescue Osmond (1917-2004), and John Raymond Smythies (b. 1922), who did not use the term in print until 1954. The term hallucinogen is used more or less interchangeably with the other terms listed above to denote a group of chemical substances that have the potential to alter consciousness and to evoke phenomena such as hallucinations, * illusions, * sensory distortions, *delirium, loss of contact with reality, and sometimes coma and death. A classic definition of the term hallucinogen stems from the American psychiatrist Louis Jolyon West (1924-1999), who maintains that hallucinogenic drugs "may be defined as substances that create gross distortions in perception without causing loss of consciousness when administered in low doses (not toxic overdoses)." The latter designation is essential to a proper understanding of the notion of hallucinogens, since said substances are only considered 'hallucinogenic' above a certain dose, the level of which tends to vary somewhat across individuals. In addition, it should be noted that not all substances capable of evoking hallucinations are considered hallucinogens and that even psychopharmacologists do not agree on the proper delineation and classification of these substances. No doubt it is for this reason that there are so many different names, each used somewhat differently. A rather rigid classification of hallucinogens, used by the American pharmacologist and medicinal chemist David E. Nichols (b. 1944) includes only "substances with [a] psychopharmacology resembling that of the natural products mescaline and psilocybin and the semisynthetic substance known as lysergic acid diethylamide (LSD-25)." As Nichols explains, this includes only those substances which exert their CNS effects via an agonist or partial agonist action upon the serotonin receptor. The class of hallucinogens can be arranged in numerous ways. Using the criterion of psychoactive potential as a guiding principle, hallucinogens can be divided into three broad classes, referred to as psychedelics, * dissociatives, and * deliriants (or *true hallucinogens). Using their chemical structure as a guiding principle, they fall into two different classes, called the tryptamines and the phenethylamines. Among the substances generally classified as hallucinogens are bufotenine, datura, LSD, mescaline, psilocin, belladonna, mandrake, henbane, atropine, and scopolamine. In addition to their potential to mediate hallucinations during the state of intoxication, hallucinogens can induce long-term perceptual complications such as * flashbacks, * visual snow, and * hallucinogen-induced persistent perception disorder (HPPD). As to the mechanism ofaction of hallucinogens, the involvement of various neurotransmitter systems in the CNS (such as the serotonergic, glutaminergic, and dopamin-ergic systems) is known, but it is as yet unclear exactly how these neurotransmitter systems exert their influence upon the perceptual system, and even whether this influence constitutes the principal mechanism of action of hallucinogenic substances. Moreover, the CNS effects of hallucinogens are not entirely predictable, dependent as they are upon the expectations of the user (i.e. the 'set'), and the environment in which they are used (i.e. the 'setting'). When used in relatively high doses, hallucinogens can produce the experience of an 'alternate reality', also referred to as a transcendental or mystical experience. Adverse reactions to hallucinogens are commonly referred to as a 'bad trip'. In 1979 the term * entheogen was introduced as yet another alternative for the terms listed above, in an effort to reinstate the original spiritual connotations of these substances in * mysticism and shamanism. A person intentionally employing hallucinogens for the purpose of exploring the psyche may be called a * psychonaut. The term hallucinogen is used in opposition to the terms entactogen and empathogen, which both denote psychoactive substances capable of evoking distinctive emotional and social (as opposed to hallucinatory) effects, as in ecstasy use, for example.

   References

   Cohen, S. (1964). Drugs of hallucination. London: Secker & Warburg.

   Nichols, D.E. (2004). Hallucinogens. Pharmacology & Therapeutics, 101(2), 131-181.

   West, L.J. (1975). A clinical and theoretical overview ofhallucinatory phenomena.In: Hallucinations. Behavior, experience, and theory. Edited by Siegel, R.K., West, L.J. New York, NY: John Wiley & Sons.