Requiem for a Dream may not be considered a horror movie to some, but it should be. I think it's one of the most psychologically messed up movies I've ever seen. It scared my friend so much that he couldn't sleep for a week thinking about it. That's the kind of movie this is, hard to forget and realistically horrifying.
Ellen Burstyn gives one of the best performances I've ever seen. She plays a lonely widow, who craves for something in her life to give her something to live for. She ends up finding that something when she thinks she's going to be on television. She's so desperate for the experience, to be able to be in the spotlight, talk about her husband and son, that she obsesses about it. She decides she needs to get on some diet pills to lose weight in time for her television appearance, so that she looks nice and is able to fit into the pretty red dress she wore to her son's graduation. The amount of emotions she's able to show in her face make this movie so realistic. It also makes the addiction and experience the character goes through that much more terrifying and heartbreaking to watch.
This film is dark and disturbing. Do not watch it thinking that it's not going to freak you out because it will. There's nothing light about this movie.