Lysergic acid diethylamide (LSD), also known as acid, is a hallucinogenic drug. Effects typically include altered thoughts, feelings, and awareness of one’s surroundings. Many users see or hear things that do not exist.Dilated pupils, increased blood pressure, and increased body temperature are typical. Effects typically begin within half an hour and can last for up to 12 hours. It is used mainly as a recreational drug and for spiritual reasons. LSD does not appear to be addictive, although tolerance may occur with use of increasing doses. Adverse psychiatric reactions are possible, such as anxiety, paranoia, and delusions. Distressing flashbacks might occur in spite of no further use, a condition called hallucinogen persisting perception disorder. Death is very rare as a result of LSD, though it occasionally occurs in accidents. The effects of LSD are believed to occur as a result of alterations in the serotonin system. As little as 20 micrograms can produce an effect. In pure form, LSD is clear or white in color, has no smell, and is crystalline. It breaks down with exposure to ultraviolet light. About 10 percent of people in the United States have used LSD at some point in their lives as of 2017, while 0.7 percent have used it in the last year. It was most popular in the 1960s to 1980s. LSD is typically either swallowed or held under the tongue. It is most often sold on blotter paper and less commonly as tablets or in gelatin squares. There are no known treatments for addiction, if it occurs.