This film was an American production that was made in Japan using a mostly Japanese crew and a number of Japanese actors. It was shot entirely in English and had two working titles, "Nightmare" and "The Two-Headed Monster", in that order.
Sam Raimi's film Uma Noite Alucinante 3 (1992) pays homage to this film. After Ash Williams swallows one of his small doppelgangers, he grows an eye on his right shoulder and then develops a second head, which eventually results in him splitting apart into two versions of himself named Good Ash and Evil Ash. In this film, Larry Stanford is injected with a serum after being rendered unconscious and later also grows an eye on his right shoulder, which then also develops into a second head and eventually results in him also splitting apart, but in his case, into himself and an ape-like monster.
For the film's English-language version, the producers needed to find two Japanese actors who could speak English fluently. They finally ended up casting Tetsu Nakamura (under the name of Satoshi Nakamura), an actor who was born in Canada and went to school there, but moved to Japan in 1940, and Jerry Itô, an actor who was born in New York, so English was his native language.
Several different sources state different release dates for this film. The American Film Institute suggests that it premiered in the YA in San Francisco, California on 3/28/62 with a run time of 72 minutes. Stuart Galbraith IV wrote in his book "The Japanese Filmography" that it was released in Japan in 1961 with a U.S. premiere on 3/28/62, but AllMovie states that it was released in the U.S. on 7/1/59. Lopert Pictures released it in the U.S. in 1962 as the bottom half of a double feature with Os Olhos Sem Rosto (1960) (which had been dubbed into English, edited and re-titled "The Horror Chamber of Dr. Faustus" specifically for said double feature). In the UK it was titled "The
Split". It was then shown several years later on "Elvira's Movie Macabre" and later released on DVD.
Originally released in the U.S. by Lopert Pictures in 1962 as the bottom half of a double feature. The top half of it was a dubbed and edited version of Georges Franju's film Os Olhos Sem Rosto (1960) (which had been re-titled "The Horror Chamber of Dr. Faustus" specifically for said double feature).