Jason Patric actually did shoot up for authenticity's sake for the film, though the substances he used in his syringes were either saline or vitamins.
As composer of the music score, Eric Clapton was initially reluctant to include "Tears in Heaven" in the film. Mourning the death of his son Conor, about the loss of whom the song had been written, Clapton wasn't sure if it should be made public. Director Lili Fini Zanuck managed to change his mind by convincing him that the song might help someone dealing with grief in their own life.
When the cast gathered together for the first time, they were either too shy or reluctant to speak to each other. Sam Elliott, hand outstretched, walked over to Greg Allman, a member of the Allman Brothers Band, and said, 'I have all your albums.' This broke the ice, and the cast followed suit.
Jason Patric appeared as an undercover Narcotics officer in the movie Narc (2002), which appeared to be a familiar resemblance to his character in this movie, with the difference of a "post drug addiction officer". Tom cruise financed the project that also starred Ray Liotta.