Lysergic acid diethylamide, also known as LSD, LSD-25, lysergide or acid, is a semi-synthetic psychedelic drug well known for its psychological effects, which can include altered thinking processes, closed and open eye visuals, synesthesia, an altered sense of time and spiritual experiences, as well as for its key role in 1960s counterculture. It is used mainly as an entheogen, recreational drug, and as an agent in psychedelic therapy.

LSD is non-addictive, is not known to cause brain damage, and has extremely low toxicity relative to dose, although in rare cases adverse psychiatric reactions such as anxiety or delusions are possible.

It is produced in crystalline form and then mixed with excipients, or diluted as a liquid for production in ingestible forms. It is odorless, colorless and has a slightly bitter taste. LSD is sold in tablet form, on sugar cubes, in thin squares of gelatin, and most commonly, as blotter paper (sheets of absorbent paper soaked with LSD.

The effects of LSD are often unpredictable. Usually, the initial effects of the drug are felt within 30 to 90 minutes after ingesting it. The user may experience extreme changes in mood, feel several different emotions at once, or swing rapidly from one emotion to another. If taken in large enough doses, the drug may produce severe delusions and visual hallucinations. The physical effects can include dilated pupils; higher body temperature and sweating; nausea and loss of appetite; increased blood sugar, heart rate and blood pressure; sleeplessness; dry mouth and tremors.

The user may also suffer impaired depth and time perception, with distorted perception of the size and shape of objects, movements, color, sound, touch and own body image. Sensations may seem to “cross over,” giving the feeling of hearing colors and seeing sounds. These changes can be frightening and can cause panic. Some LSD users also experience severe, terrifying thoughts and feelings, fear of losing control, fear of insanity and death.

An experience with LSD is referred to as a “trip” and acute adverse reactions as a “bad trip”. These experiences are long, with the effects of higher doses lasting for 10 to 12 hours.

Under the effects of LSD, the ability to make sound judgments and see common dangers is diminished, making the user susceptible to personal injury, which can be fatal. After an LSD trip, the user may suffer acute anxiety or depression, and may also experience flashbacks, which are recurrences of the effects of LSD days or even months after taking the last dose.

The National Survey on Drug Use and Health (NHSDA) in 2010 reported that the percentage of the population aged 18 to 25 who had ever used LSD was 6.4%.