Congenital Insensitivity to Pain Anhidrosis (CIPA), the condition Grady suffers from, is a real (though extremely rare) medical condition that, like in the film, means the person cannot process sensations. Naloxone, the drug that is used to cause Grady to feel, can have a similar effect on CIPA patients in real life, but unlike in the film, the effect only lasts a very short period.
Brinna Kelly, a co-writer on the film, ad-libbed the line 'Must go faster,' a homage to Jeff Goldblum and his use of the line in 'Jurassic Park' and 'Independence Day.' This is one of two references to 'Independence Day.'
Despite being filmed in August, the night the rooftop scene between Grady and Zan was filmed, the weather was actually freezing, which left both actors shivering between takes, especially Brinna Kelly who was in short sleeves. Her lips are noticeably white when Grady and Zan kiss.
During filming of the opening torture sequence, the butane torch wouldn't light, so the flame was later rendered in CGI.
William Forsythe's character Fairbanks originally had a monologue explaining how he had murdered a young man during his days in the Army for his hubris, foreshadowing his distaste for Grady's swagger. This was cut for pacing.