Ayahuasca: A psychedelic tea made from a combination of plants native to the Amazon basin, typically Banisteriopsis caapi and Psychotria viridis (or chacruna), and used sacramentally by indigenous peoples of South America. The chacruna plant contains the psychedelic compound DMT (N,N-dimethyltryptamine), but it is deactivated by digestive enzymes unless it is ingested with a monoamine oxidase inhibitor such as Banisteriopsis. In 2006, the U.S. Supreme Court affirmed the right of the Brazil-based UDV Church to use ayahuasca as a sacrament.

DMT (or N,N-Dimethyltryptamine): A rapid-onset, intense, and short-acting psychedelic compound sometimes referred to as “the businessman’s trip.” This tryptamine molecule is found in many plants and animals for reasons not well understood.

5-HT2A receptor: One of several types of receptors in the brain that respond to the neurotransmitter serotonin. Psychedelic compounds also bind to this receptor, precipitating a cascade of (poorly understood) events that produce the psychedelic experience. Because of its distinctive molecular shape, LSD binds particularly well to the 5-HT2A receptor. In addition, a portion of the receptor folds over the LSD molecule and holds it inside the receptor, which might explain its intensity and long duration of action.

5-MeO-DMT (5-methoxy-N,N-dimethyltryptamine): A powerful, short-acting psychedelic compound found in certain South American plants and in the venom of the Sonoran desert toad (Incilius alvarius). The toad venom is typically vaporized and smoked; 5-MeO-DMT obtained from plants is usually made into a snuff. The compound has been used sacramentally in South America for many years; it was first synthesized in 1936 and was not made illegal until 2011.

Hallucinogen: The class of psychoactive drugs that induce hallucinations, including the psychedelics, the dissociatives, and the deliriants. The term is often used as a synonym for psychedelics, even though psychedelics don’t necessarily produce full-fledged hallucinations.

LSD (lysergic acid diethylamide): Also known as acid, this psychedelic compound was first synthesized in 1938 by Albert Hofmann, a Swiss chemist at Sandoz who was searching for a drug to stimulate circulation. LSD was the twenty-fifth molecule that Hofmann had derived from the alkaloids produced by ergot, a fungus that infects grain. Hofmann shelved the compound when it proved ineffective as a medicine, but five years later a premonition led him to resynthesize it. After accidentally ingesting a small quantity of LSD, he discovered its powerful psychoactive properties. In 1947, Sandoz began marketing LSD as a psychiatric drug under the name Delysid. It was withdrawn from circulation in 1966 after the drug appeared on the black market.

MDMA (3,4-methylenedioxymethamphetamine): A psychoactive compound first synthesized by Merck in 1912 but never marketed. After the compound was resynthesized by the Bay Area chemist Alexander “Sasha” Shulgin in the 1970s, it became a popular adjunct to psychotherapy, because its “empathogenic” qualities helped patients form a strong bond of trust with their therapists. In the 1980s, the drug showed up in the rave scene, where it was sold under the name of Ecstasy (or E or later Molly); in 1986, the U.S. government put MDMA on schedule 1, declaring it a drug of abuse with no accepted medical use. However, recent drug trials sponsored by MAPS have demonstrated MDMA’s value in treating PTSD. MDMA is not considered a “classical psychedelic,” because it appears to operate on different brain pathways from LSD or psilocybin.

Mescaline: A psychedelic compound derived from several cacti, including peyote and San Pedro. The compound was first identified and named by the German chemist Arthur Heffter in 1897. The Doors of Perception is a first-person account of Aldous Huxley’s first mescaline experience.

Microdosing: The practice of ingesting a small, “subperceptual” dose of a psychedelic, usually LSD or psilocybin, every few days as an aid to mental health or mental performance. A common protocol is to take ten micrograms of LSD (a tenth of a medium dose) every fourth day. The practice is fairly new, and as yet the evidence for its effectiveness is anecdotal. Several trials are under way.

Psilocybin: The main psychoactive compound found in psilocybin mushrooms and a shorthand for the class of mushrooms that contain it.

Psychedelic: From the Greek for “mind manifesting.” The term was coined in 1956 by Humphry Osmond to describe drugs like LSD and psilocybin that produce radical changes in consciousness.