Unusually for an Italian film of the time, all the actors spoke their dialogue in English regardless of nationality, instead of being overdubbed in the English soundtrack. The Italian track however is dubbed by other actors voicing the characters, even the film's Italian actors.
In the early 1990's, Quentin Tarantino announced that he would be directing a remake with Bridget Fonda starring. It failed to materialize, however Tarantino later used the titular seven-note theme song in Kill Bill - Vol. 1 (2003). More specifically the dramatic polyphonic version of the music which plays near the climax of this film, is also played in Kill Bill as the Bride waits on the floor of her hospital room for Buck to re-enter.
The film was initially going to be directed by Alberto Pugliese based on a 1973 novel by Vieri Razzini titled "Terapia Mortale" (Deadly Therapy). However producer Luigi De Laurentiis wasn't satisfied with the script by Ernesto Gastaldi and after several years brought on Lucio Fulci and his co-writers, with hardly anything left of the novel in the finished product.
The Tamil-language Indian film Nooravathu Naal (1984) is believed to be an unofficial remake of this film. It was remade itself twice, as Aayiram Kannukal (1986) and 100 Days (1991).