In pre-production the website featured in the film was called Fear.com, despite the producers not owning that website in real-life. They had hoped to buy the domain name from its owners at the time, but were told that it was not for sale at any price, leading to the website's name in the film being changed to Feardotcom.com.
The 'mutilated' bodies in Pratt's apartment are real bodies, transformed into art by the German sculptor Gunther von Hagens.
The name "Dr. Gogol" is written on the subway wall in the first scene. This is a reference to Mad Love (1935), another film about a mad surgeon.
One of the three films that received an "F" CinemaScore from audiences upon their release in 2002, along with Solaris (2002) and Darkness (2002).
Originally given an NC-17 rating for extreme violence, but was re-rated R for violence, including grisly images of torture, nudity and language.