The hypodermic needle theory was a theory in the 1940s and 1950s which implied that mass media had a direct and immediate effect on its audience. Aspects of human history as well as specific events that transpired around that period contributed to the development of this theory, such as Hitler's propaganda during World War II and the rapid rise of radio and television popularity.
The hypodermic needle theory supports the implication that media pieces are able to trigger a specific, desirable response in its target audience by "injecting" or "shooting" an appropriate message into them. Another name for this theory is the magic bullet theory, which instead of using the metaphor of a direct injection delivered by a syringe, uses the metaphor of a bullet shot by a gun, in which the bullet is the message intended for the target audience and the gun is the media outlet.
As media became more interactive with programs such as radio call-in shows and mediums such as the Internet, the hypodermic needle theory began to dwindle in significance, replaced by more complex theories, such as the two step of flow theory and diffusion of innovations theory. This is due to the option in interactive media for the audience to "answer back" and send messages, rather than simply receiving them only.