No other drugs are as misunderstood and controversial as the psychedelic drugs, in part because they do not form as cohesive a drug group as the stimulants or the depressants. Indeed, the group of psychedelic drugs includes several different drugs with dissimilar mechanisms of action, each with a range of psychological, emotional, and behavioral effects. Despite these differences, there are common features that define them all as psychedelic drugs.

Psychedelic drugs are used by different groups of people, for different purposes, with different expectations, in different settings, and with different results. They are used in the context of polydrug addiction, in infrequent social situations, for rare experimental purposes, in the supervised context of psychotherapy, and for spiritual and religious purposes. Addiction experts treat psychedelic drug abuse, whereas a few psychotherapists describe the usefulness of psychedelic drugs as an aid to psychotherapy.

Some confusion about psychedelic and hallucinogenic drugs relates to terminology. A hallucination is a sensory perception without basis in reality and without stimulation of the sensory organ(s) in question. It is the result of the stimulation of certain sensory activities such as vision, hearing, taste, and touch, but without stimulation of the sensory organs. People may "hear," "see," "smell," or "feel" things that do not exist.Sometimes people are aware that hallucinations are drug induced (pseudohallucinations); sometimes they believe that their hallucinations are, in fact, reality.

The literal definition of the term hallucinogen is a drug that causes hallucinations. However, some drugs that are casually called hallucinogens do not cause hallucinations. Conversely, some drugs cause hallucinations under special conditions, but the hallucinations are side effects rather than the primary effects or regular effects of the drugs. For example, stimulants can cause hallucinations with chronic, high-dose use, but hallucinations are not the primary effect of stimulants. Thus, the term "hallucinogen" is too restrictive to use as a general description of psychedelic drugs.

The term psychedelic means "mind-manifesting" or "mind-revealing" and was coined by psychiatrist Humphrey Osmond in 1956 because it is a neutral and less misleading term than "hallucinogen" to describe the variety and range of effects that these drugs can produce. Thus, the phrase psychedelic drugs is used in this book to describe those drugs that characteristically and consistently produce some combination of distortions of thinking, feeling, and perception, collectively known as the psychedelic experience. . .