Overview
Hallucinogens are defined in the American Psychological Association’s Dictionary of Psychology as:
“a substance capable of producing a sensory effect (visual, auditory, olfactory, gustatory, or tactile) in the absence of an actual stimulus. Because they produce alterations in perception, cognition, and mood, hallucinogens are also called psychedelic drugs or psychedelics (from the Greek, meaning “mind-manifesting”). They are a group of heterogeneous compounds, many of which are naturally occurring; others are produced synthetically. Many hallucinogens are structurally similar to one of several neurotransmitters, which may be used as a mechanism of categorization.” 1
The Diagnostic and Statistical Manual of Mental Disorders (DSM-5-TR) describes many disorders and symptoms related to hallucinogens, though emphasizes that the dynamics and mechanisms of their chemistry are substantially different from each other:
Hallucinogens comprise a diverse group of substances that despite having different chemical structures and possibly involving different molecular mechanisms, produce similar alterations of perception, mood, and cognition in users. Hallucinogens included are phenylalkylamines (e.g., mescaline, DOM [2 5-dimethoxy-4-methylamphetamine], and MDMA [3,4-methylenedioxymethamphetamine; also called “ecstasy” or “molly”]); the indoleamines, including psilocybin (and its metabolite psilocin, the compound primarily responsible for the psychedelic effects of hallucinogenic mushrooms) and dimethyltryptamine (DMT); and the ergolines, such as LSD (lysergic acid diethylamide) and morning glory seeds. In addition, miscellaneous other ethnobotanical compounds are classified as hallucinogens, of which Salvia divinorum and jimsonweed are two examples.
To emphasize this distinction between hallucinogens and substances with hallucinogenic effects, the DSM-5-TR explicitly indicates that cannabis is not in and of itself a hallucinogen:
Excluded from the hallucinogen group are cannabis and its active compound, delta-9-tetrahydrocannabinol (THC) (see the section “Cannabis-Related Disorders”). These substances can have hallucinogenic effects but are diagnosed separately because of significant differences in their psychological and behavioral effects.