LSD builds extreme tolerance quickly. Use must be spaced out by at least three days for the tolerance to go down enough to trip again, any sooner and there will be little to no effect. There is a cross-tolerance to Psilocybin, the chemical found in hallucinogenic mushrooms.
LSD was originally synthesized in 1938 by Albert Hofmann. It was not discovered to be a hallucinogen until April 16, 1943 after Albert Hofmann accidentally ingested a small amount. LSD did not appear on the streets until 1963. Before then, the experiences of LSD were restricted to chemists who could synthesize it from Ergot alkaloids derived from the rye Ergot fungus.
It was federally banned in the US in 1967, simply because the government was afraid of the alternative thought methods and "enlightenment" it created, believing it would incite rebellion among the general public. It is one of the safest drugs, legal or illegal, known to man. It is safer than aspirin or advil. So-called flashbacks are simply the result of post-traumatic stress disorder, and can happen without LSD use, mostly in wartime veterans.
The chemical name is d-Lysergic Acid Diethylamide. The term LSD is an acronym for the German name, Lyserg-Saeure-Diaethylamid.
Many people in modern days have begun using the word Purple Haze in reference to certain strains of cannabis. This is a misnomer, in reference to the original definition as stated above.
Shit man, the walls are breathing!