An injection device (100) including a tubular body (3) within which a cartridge is inserted (10) equipped with a bottom (14) and a back chamber (8) containing one first substance (SI). The device (100) also has a front chamber (9) containing one second substance (S2) intended to be mixed inside the tubular body (3) with the first substance (SI) to reconstitute the injectable substance (S). The device (100) is characterized in that the cartridge (10) has a flexible body (13) suitable for tearing as a result of the increase of pressure inside the cartridge (10) in order to put in communication the back chamber (8) with the front chamber (9) and to allow the reconstitution of the injectable solution (S).
