Hallucinogens are a class of drugs that affects the brain and alters perceptions and mood. Human beings have used hallucinogens for centuries, primarily in ancient cultural and religious ceremonies. As a part of their religion, for example, certain American Indian tribes used the hallucinogen peyote to experience "spiritual visions."

Unlike most other drug classifications, hallucinogens are grouped primarily by causing hallucinations (which comes from Latin meaning "to wander in the mind"). Hallucinogens may be synthetic (man made) or organic (occurring in nature, such as certain plants). Organic hallucinogens include mescaline, peyote, and various fungi (certain mushrooms). Synthetic hallucinogens first appeared in 1938 when German chemist Albert Hoffman created a derivative of lysergic acid (LSD) in the laboratory. Scientists largely ignored the discovery until medical researchers in the 1950s began investigating LSD's possible therapeutic use for treating some psychiatric disorders.

In the 1960s, with the youth counterculture movement, the "hippie...