A mechanism for puncturing gas cartridges comprises a lever comprising a short lever arm and a long lever arm, both arms extending from a pivot. The mechanism also comprises a ramp for pushing laterally a tip of the long lever arm under application of an input force on the ramp. The lever is configured to rotate about the pivot under lateral movement of the tip of the long lever arm, causing a tip of the short lever arm to apply a puncturing force between the gas cartridge and a puncturing member. A main body may receive the gas cartridge, the puncturing member and the lever. An actuator may be slidably mounted on the main body for applying the input force on the ramp by sliding towards the main body. The mechanism may be incorporated in a gas injection device or in a needleless syringe.