A needleless hypodermic injection system for injecting a liquid medication, which system comprises a disposable cartridge which contains a medication and which includes a propellant and an igniter, and a reusable application device which comprises a pressure chamber for receiving the medication cartridge, actuation means including an ignition system and means for ensuring reliability and safety of the system. The reusable application device comprises: (a) a housing including a fist housing section and a second housing section which are adapted to be assembled together by a screwing operation, the first housing section comprising a front part having an injection outlet and a chamber adapted to receive a the cartridge contains the medication to be injected and also contains a propellant and an igniter, and (b) means for selectively activating said igniter of said cartridge when predetermined conditions are fulfilled.