the method includes providing a liquid drug cartridge (5) including a cartridge body (51) and a cartridge piston (52) disposable and releasably disposed within the cartridge body; providing a metering unit (3) including a metering pump unit (30) with a metering cylinder (300) and a metering piston (302) disposed within the metering cylinder (300) in a sealable and displaceable manner, the dosing unit (3) further including a valve unit (31) with a filler port (310) and a drainage port (311), the valve unit (31) configured to switch between the fill state and a alternative drainage state, wherein the filler port (310) is fluidly coupled with an internal volume of the metering cylinder (300) in the fill state and the drainage port is fluidly coupled with the internal volume of the cylinder dosage in the drainage state; provide a boot force member (61); (a) exerting with the initiating force member (61) an initializing thrusting force on the piston (52), thereby displacing the piston (52) within the cartridge body in a piston advance direction ( x) across an initialization distance; (b) increasing a fluid volume from the metering cylinder (300) by displacing the metering piston (302), thereby sucking the liquid drug out of the liquid drug cartridge (5) into the metering cylinder (300) and further moving the piston (52) into the cartridge body (51) in the forward direction of the piston (x) without exerting the initial thrust force.
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