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The psychedelic drug (or entheogen) lysergic acid diethylamide (LSD) was first synthesized on November 16, 1938 by the Swiss chemist Albert Hofmann in the Sandoz (now Novartis) laboratories in Basel, Switzerland. It was not until five years later on April 16, 1943, that the psychedelic properties were found.

Albert Hofmann, born in Switzerland, joined the pharmaceutical-chemical department of Sandoz Laboratories, located in Basel as a co-worker with professor Arthur Stoll, founder and director of the pharmaceutical department. He began studying the medicinal plant squill and the fungus ergot as part of a program to purify and synthesize active constituents for use as pharmaceuticals. His main contribution was to elucidate the chemical structure of the common nucleus of Scilla glycosides (an active principle of Mediterranean Squill). While researching lysergic acid derivatives, Hofmann first synthesized LSD on November 16, 1938. The main intention of the synthesis was to obtain a respiratory and circulatory stimulant (an analeptic). It was set aside for five years, until April 16, 1943, when Hofmann decided to take a second look at it. While re-synthesizing LSD, he accidentally absorbed a small amount of the drug through his fingertips and discovered its powerful effects. He described what he felt as being:

... affected by a remarkable restlessness, combined with a slight dizziness. At home I lay down and sank into a not unpleasant intoxicated-like condition, characterized by an extremely stimulated imagination. In a dreamlike state, with eyes closed (I found the daylight to be unpleasantly glaring), I perceived an uninterrupted stream of fantastic pictures, extraordinary shapes with intense, kaleidoscopic play of colors. After about two hours this condition faded away.

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Wikipedia