Jerry Schatzberg turned down the script when he was offered it by the production company. He was then re-approached when Al Pacino took an interest in the project. He agreed then to direct on the condition that Pacino could star, however ironically it then became difficult to find a studio that would back the project with a newcomer like Pacino.
The studio notes reported that makeup man Herman Buchman studied the "track" marks on the arms of hospital patients and victims in morgues and achieved an authentic look for the actors by using a liquid called Flexible Collodian. In scenes in which actors appear to inject themselves, a registered nurse was on set, serving as a technical advisor.
The name Needle Park was the then-nickname of Sherman Square on Manhattan's Upper West Side near 72nd Street and Broadway. It received this nickname due to being a frequent hangout place for drug addicts.
Joan Didion and John Gregory Dunne visited Jim Morrison, lead singer of The Doors, during the recording of the album Waiting For The Sun as he was considered for the role of Bobby, but went with Pacino instead.
As noted in the onscreen credits, the film was shot entirely in New York City. According to the film's studio production notes, portions were shot at Needle Park and the Upper West Side area of New York City, at Riverside Park, in a New York City prison and hospital ward, on the Staten Island Ferry, and in the East Village.
According to a November 1967 Hollywood Reporter news item, film rights for the novel were purchased by Avco Embassy Pictures and, according to a March 1969 Variety news item, the film rights were later bought by producer Dominick Dunne, whose brother and sister-in-law, John Gregory Dunne and Joan Didion wrote the screenplay.