This movie will likely always be found in the Drama section, but I'm adding Requiem For A Dream to the horror list because the entire story is a tragic tale about addiction and love (which combined is rather horrific).
Also, this movie haunts me. Haunts me! The characters are so engaging; you want to love and hold them. You want to help them and show them different ways, betters ways - but no. Instead you have to sit back and watch in horror as their lives unfold one unfortunate moment at a time.
I love this stunning movie, even though it haunts me, I love it so much. A must see for drug counselors, therapists, and students of the above.