Not the typical “look a famous person who did drugs” post, but Sagan loved the stuff. Boing Boing has an essay linked that he wrote for the book Marihuana Reconsidered. There have been quite a few scientists who have experimented with drugs. Steven Jay Gould smoked to ease his cancer pain, Richard Feynman did it to expand his consciousness and Francis Crick was high on LSD when he came up with the idea of the double helix for DNA.
Sagan used cannabis to inspire him to do much of the work that garnered him great fame. If the comments on BB are to be believed then he was probably high much of the time (he would supposedly get high before his talks). In his essay he emphasizes that the great insights he was able to get while high would be lost if he did not write them down or tell them to someone else. Maybe if I would have followed that suggestion I would be a better physicist.

