Hallucinogens is a broad pharmacological and common-language term for psychoactive substances that produce hallucinations — perceptions in the absence of external stimuli — and other profound alterations of consciousness, including distorted sensory experiences, ego dissolution, mystical states, and altered time perception. The term encompasses several pharmacologically distinct subclasses that produce similar phenomenological outcomes through very different mechanisms.
The major subclasses are: (1) classical psychedelics (serotonergic hallucinogens), including LSD, psilocybin, DMT, and mescaline, which act primarily as 5-HT2A receptor agonists; (2)dissociatives, including ketamine, PCP, DXM, and nitrous oxide, which produce altered consciousness through NMDA receptor antagonism; (3)deliriants, including tropane alkaloid-containing plants (datura, belladonna), diphenhydramine, and benzydamine, which produce true hallucinations through anticholinergic or other mechanisms; and (4)miscellaneous hallucinogens that do not fit neatly into these categories, including Salvia divinorum (kappa-opioid agonist) and Amanita muscaria (GABA-A modulator/NMDA agonist).
"Hallucinogen" is increasingly disfavored as a primary technical term in pharmacological literature in favor of more mechanistically precise classifications (serotonergic psychedelic, dissociative anesthetic) or culturally contextual terminology (entheogen). The broader term remains useful as a category that captures the common phenomenological thread across these mechanistically diverse compounds — that of profoundly altered states of consciousness involving non-ordinary perceptions — while acknowledging that the experiences differ substantially in character, utility, and risk across subclasses.