I cannot remember the title but I remember the plot so here we go

The movie is shot in a mental institution. The main character thinks he’s a doctor of that place who is going around the place and trying to solve something. He had a patient friend who was always in his cell hiding in the shadow, by the end of the movie turns out that the friend and the main character are the same person talking to himself. The movie also has a nymphomaniac chick patient and the male nurse was banging her. The movie is full of tention and I would like to see it 100 more times.

Big fat thanks to whoever helps me!