The use of naturally occurring and synthetically derived compounds for their "psychedelic" effects has been a part of human culture for thousands of years. The basic pharmacology of the major synthetic psychedelic compounds (primarily lysergic acid diethylamide [LSD]-25) is described and reference is made to their potentially beneficial psychological effects. Adverse reactions, defined as dysphoric and/or maladaptive/dysfunctional responses to the use of these drugs, sometimes require careful clinical judgment in order to diagnose. These reactions can be effectively classified along a temporal continuum. Acute, short-lived reactions are often fairly benign, whereas chronic, unremitting courses carry a poor prognosis. Delayed, intermittent phenomena ("flashbacks") and LSD-precipitated functional disorders that usually respond to treatment appropriate for the non-psychedelic-precipitated illnesses they resemble, round out this temporal means of classification. The question of organic brain damage as well as permanent changes in personality, attitudes, and creativity in patients and normals who have repeatedly ingested psychedelic drugs is controversial, but tends to point to subtle or nonsignificant changes. Future areas for study of the psychedelics' pharmacological, psychological, and therapeutic effects are suggested.