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.

While LSD does not appear to be addictive, tolerance with use of increasing doses may occur. Adverse psychiatric reactions such as anxiety, paranoia, and delusions are possible. Distressing flashbacks, a condition called hallucinogen persisting perception disorder, may occur despite no further use. Death as a result of LSD, though occasionally occurring in accidents, is very rare. 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.

LSD was first made by Albert Hofmann in 1938 from lysergic acid, a chemical from the fungus ergot. Hofmann discovered its hallucinogenic properties in 1943. In the 1950s, the Central Intelligence Agency (CIA) believed the drug might be useful for mind control so tested it on people, some without their knowledge, in a program called MKUltra. LSD was sold as a medication for research purposes under the trade-name Delysid in the 1950s and 1960s. It was listed as a schedule 1 controlled substance by the United Nations in 1971. It currently has no approved medical use. In Europe, as of 2011, the typical cost of a dose was between €4.50 and €25.