Ken Russell's 'Altered States'. Aye, he is a bit of a cheeseball, pretentious director*, but this one has enough quality in it to be recommended, and, y'know, it is still a bit pretentious, so it's a good laugh as well.
Basically it's about a scientist(played by William Hurt) who embarks on a series of experiments to explore altered states of conciousness, he meets up with tribespeople to get the lowdown on what psychedelics to take, and takes psychedelic drugs while in one of those sensory deprivation tanks, resulting in intense visions and some surprising physical results.
I have not seen it for many years, but I would say it is worth a watch, just, don't take it too seriously and you will enjoy it more.
(*his Uri Gellar biopic has to be seen to be believed, it is really terrible, but there is one scene that has to be seen, really funny, where Uri escapes from a military base, that has captured him and has been experimenting on him, it is probably quite a difficult film to track down so here is the scene...*spoilers*...he has this large metal helmet on that blocks his vision, but still he manages to use his powers of telekinesis to open doors and comandeer a jeep, he drives blindly through the base, his telekinesis bending the metal barrier blocking the enterance. He then drives the jeep at high speed back to his house, skids to a halt outside his front window, goes flying over the front of the jeer in a somersault, crashes through his front window, lands on his couch in his living room next to his friend who is sitting there, and high fives him.)
