In
1935, Sartre experimented briefly with
mescaline, the only
psychedelic drug available at the time. A couple of days later, he started having the persistent impression that he was being pursued by a giant
lobster. The hallucinations lasted for some time - some sources say almost a year - and were a cause of concern to the budding existentialist.
The pivotal scene in his (1938) La Nausee, in which Roquentin, the hero, experiences a loss of the boundaries that his conceptual structures impose on the world, 'merging' with his immediate surroundings, (starting with a tree, as I recall) is very reminiscent of a description of a psychedelic experience.
Given that this passage is often taken as his presentation in literary form of the idea of the en-soi - 'being in itself' - it may not be going too far to attribute a significant role to his mescaline experience in the development of this idea, presented more explicitly in the weighty Being and Nothingness (L'Etre et le Neant) - though of course it wasn't the only factor: Sartre had already spent a year in Berlin (around 1933) in order to hear Edmund Husserl's lectures, and so on.