The document provides an overview of the four main classes of hallucinogenic drugs: serotonergic hallucinogens like LSD and psilocybin; methylated amphetamines like MDMA; anticholinergic hallucinogens found in plants; and dissociative anesthetics like PCP. It discusses the history of hallucinogen use from the 1960s counterculture movement to current trends, physiological and psychological effects of different classes of drugs, risks and adverse effects, and mechanisms of action in the brain.