A hypodermic injection system for removing injectate from an injectate supply for rapid injection into a body, said injection system comprising: a housing, and characterised in that the housing has a forward end with a notch structure; a detachable and attachable head for operative attachment to said housing, said detachable head comprising: a ridge structure disposed at the rearward end of the detachable head for operatively connecting said detachable head to said housing, said detachable head being attachable to and detachable from said housing and being replaceable with at least one different detachable head adapted to hold a different amount of injectate; wherein said detachable head comprises a perforator shaft, the perforator shaft comprising an exit nozzle and extending from a hub to which the perforator shaft is attached; an injection chamber of a predetermined volume for receiving injectate from the injectate supply; and wherein the exit nozzle is for receiving injectate from said injection chamber for injection into a body; a filling device for transferring injectate from the injectate supply to said injection chamber in response to a pressure differential on injectate between the supply and said injection chamber, the pressure being lower in said injection chamber; and an electric motor drive assembly for establishing the pressure differential, said motor drive assembly including a reciprocal ram device for moving in one direction to apply positive pressure to injectate in said injection chamber to force injectate into said exit nozzle for injection into a body, and for moving in the opposite direction to relieve said positive pressure upon completion of an injection and to establish a lower pressure in said injection chamber to effect the transfer of injectate from the injectate supply to said injection chamber. a housing, and characterised in that the housing has a forward end with a notch structure; a detachable and attachable head for operative attachment to said housing, said detachable head comprising: a ridge structure disposed at the rearward end of the detachable head for operatively connecting said detachable head to said housing, said detachable head being attachable to and detachable from said housing and being replaceable with at least one different detachable head adapted to hold a different amount of injectate; wherein said detachable head comprises a perforator shaft, the perforator shaft comprising an exit nozzle and extending from a hub to which the perforator shaft is attached; an injection chamber of a predetermined volume for receiving injectate from the injectate supply; and wherein the exit nozzle is for receiving injectate from said injection chamber for injection into a body; a ridge structure disposed at the rearward end of the detachable head for operatively connecting said detachable head to said housing, said detachable head being attachable to and detachable from said housing and being replaceable with at least one different detachable head adapted to hold a different amount of injectate; wherein said detachable head comprises a perforator shaft, the perforator shaft comprising an exit nozzle and extending from a hub to which the perforator shaft is attached; an injection chamber of a predetermined volume for receiving injectate from the injectate supply; and wherein the exit nozzle is for receiving injectate from said injection chamber for injection into a body; a filling device for transferring injectate from the injectate supply to said injection chamber in response to a pressure differential on injectate between the supply and said injection chamber, the pressure being lower in said injection chamber; and an electric motor drive assembly for establishing the pressure differential, said motor drive assembly including a reciprocal ram device for moving in one direction to apply positive pressure to injectate in said injection chamber to force injectate into said exit nozzle for injection into a body, and for moving in the opposite direction to relieve said positive pressure upon completion of an injection and to establish a lower pressure in said injection chamber to effect the transfer of injectate from the injectate supply to said injection chamber.