Tens of millions of Americans have tried LSD, mushrooms, mescaline, and other hallucinogens, and millions more have experienced MDMA (3,4-methylene-dioxymethamphetamine) and other empathogenic drugs that share features in common with psychedelic drugs. The illicit use of psychedelic drugs reached a modest peak about 1980 and then declined steadily before a slight rise beginning in the late 1980's. According to the National Household Survey, 9.5 percent of the population over 12 years of age in 1995 (20 million people) had ever used a drug described as a hallucinogen, 1.6 percent had used a hallucinogen in the past month, and 9 percent had used a hallucinogen in the past year. Psychedelic drugs are used almost exclusively by persons under 30 years of age. A slight increase in use has occurred in the past 5 years, partly because LSD is being taken at lower doses that cause proportionately fewer bad trips and other adverse reactions. The authors review the sources and effects of major psychedelic drugs, the nature of the psychedelic experience, adverse effects of psychedelic drugs and treatment, therapeutic uses of psychedelic drugs, and the future of psychedelic drug use and research. An appendix contains supplemental information on the legal status of psychedelic drugs. References