The real Nic Sheff stated in a Q&A that the drawing book that David Steve Carell in the movie finds of his son's addiction struggles was drawn by Nic's real-life half-brother, Jasper Sheff.
The first scenes shot for the film were the most intense, according to Felix van Groeningen, and involved Timothée Chalamet's character hospitalization for a drug overdose. Chalamet was instructed to lose weight a couple of weeks before the filming of the "hospitalization scenes" and subsequently rested to recover to complete the rest of the shoot. Chalamet has stated that there were numerous "doctor visits" and "close calls" during the shoot, saying, "Your mind knows you're acting. But when you drop twenty pounds, and you are under a rain machine for eight takes in a T-shirt, your body doesn't know you're acting."
Timothée Chalamet lost 18 lbs for his role as Nic Sheff. He also had a consultant and doctor on set to ensure his acting as a drug addict was realistic and authentic.
Co-screenwriter Luke Davies is himself a recovering addict. During the 80s, Davies developed a dependency on heroin, which led him on a path of self-destruction. He managed to overcome his addiction in 1990 and later turned his experiences into the novel "Candy," published in mid-1998. The novel was later turned into a film in 2006, with Davies co-penning the screenplay.
The editing of the film took seven months in total, during which the movie was recut entirely multiple times. Initially Nico Leunen, Van Groeningen's long-time collaborator, was not involved in the project; however, Van Groeningen became dissatisfied with the editing process and brought Leunen to Los Angeles to reconstruct the film.