This is one of the few movies out there that should come with a literal "shock" rating, since there are at least three characters who have to experience intense electroshock treatments.
But on top of that, this movie definitely has an "R" rating. Whether it's the stories of McMurphy raping a fifteen year-old girl or Billy Bibbit slitting his own throat, there's more than enough in this film to upset anyone with the slightest bit of sensitivity. Heck, even some of the toughest viewers can find some of the scenes in this movie hard to watch. Which is kind of the point.