What are the most historically significant intellectual or artistic achievements you can think of that were developed or achieved while the artist was under the effects of drugs or possibly a state of psychological, bi-polar type mania?
Ken Kesey’s “One Flew Over The Cuckoo’s Nest” would certainly qualify for a lasting work that was written while under the influence (LSD, Peyote)…
Vincent Van Gogh.
The Grateful Dead, Pink Floyd and The Doors (among others) were using assorted psychedelic drugs very heavily when they were writing thier earliest material----Of course much of it now sounds dated and trite, but there are some songs/albums from that period that have stood the test of time and are still fresh and introspective 40+ years later…
Hunter S. Thompson was intoxicated during much of the time he was writing “Fear And Loathing: On The Campaign Trail '72” and I would argue that that is one of his most important works, and still seems very relevant today.
Not quite drugs or mental illness, at least overtly, but Friedrich Kekulé claimed that the clue that the structure of the benzene molecule was a ring came to him in a vivid daydream about a snake biting its own tail. Now, whether something else (intoxication, sleep deprivation, etc) may have triggered that vivid daydream…
Samuel Taylor Coleridge’s poem “Xanadu” was inspired by images he saw while using opium.
I’ve always been dubious of the concept that artists who are associated with psychedelics actually “work” much while under the influence; booze, or certain other things, yeah, but as I’m sure others can tell ya, people aren’t very productive while hallucinating. As for Thompson, much of his stuff walks a fine line (at least to me) between truth and bullshit, but I assume he’d admit as much.
Oh, and you could argue that the GD and the Doors never produced anything truly “significant” 
(I kid! I kid!)
Did Pink Floyd use drugs after Syd Barret?
Also, I’m not sure about this but Hemingway might have been drunk while writing all the things he wrote.
Almost every piece of art or music, ever.
Related Cracked article: The Five Greatest Things Ever Accomplished While High
Paul Erdos (the most prolific mathematician ever) was a famous amphetamine user. His friends became concerned that he had become addicted to the drug, so one bet him that he couldn’t stop using for a month. Erdos won the bet, but complained that “mathematics had been set back a month”!
Louis Wain’s cats are said to chronicle the artist’s late-onset schizophrenia. (Video)
Oh for…
Not at all, and you should know better.
I’m a representationalist, and hence not a huge fan in any way, but I’ve read that Jackson Pollack used to paint after sucking down a fifth of whiskey.
Ernest Hemingway and Winston Churchill both suffered from depression and were heavy drinkers. They wrote some good stuff.
Moving thread from IMHO to Cafe Society.
It was a hyperbole, but meant as a light-hearted joke, not a serious assessment. 
From what I head, he didn’t actually write the book on LSD. He did, however, conceive of the Indian character while he was tripping.
The book is a large one, and Kesey was obviously not high during each and every keystroke it took to write it, but the amount of psychedelic drugs he was using (by his own admission) was staggering-----LSD, psilocybin, mescaline, peyote, IT-290, ditran, morning glory seeds; these being just the ones taken as part of research at Veterans Hospital in Menlo Park (paid 75 dollars a day).
Kesey was forthright about his use of psychedelic drugs and its profound effect on his writing-----“One Flew Over The Cuckoo’s Nest” was a direct result of Kesey’s experiences working nights in a mental hospital (as an orderly) and using (both on and off-shift) copious amounts of LSD, among many other substances.
It was a well-known secret that Carl Sagan enjoyed himself some Marry Jane. There are some essays written by an anonymous astrologist, unanimously believed to be written by Sagan, in which he describes being able to better understand complicated concepts while under the influence, and how he came up with a few specific ideas or equations.