Disclosed is a method for initializing an ambulatory infusion system. The method includes providing a liquid drug cartridge (5), the liquid drug cartridge (5) including a cartridge body (51) and a cartridge piston (52), the cartridge piston (52) being arranged inside the cartridge body in a sealing and displaceable manner. The method further includes providing a dosing unit (3), the dosing unit (3) including a metering pump unit (30) with a dosing cylinder (300) and a dosing piston (302), the dosing piston (302) being arranged inside the dosing cylinder (300) in a sealing and displaceable manner, the dosing unit (3) further including a valve unit (31) with a filling port (310) and a draining port (311), the valve unit (31) being configured for switching between a filling state and an alternative draining state, wherein the filling port (310) is fluidic coupled with an inner volume of the dosing cylinder (300) in the filling state and the draining port is fluidic coupled with the inner volume of the dosing cylinder (300) in the draining state . The method further includes providing an initialization force member (61). The method further includes executing a sequence of a) exerting, with the initialization force member (61), an initialization pushing force onto the cartridge piston (52), thereby displacing the cartridge piston (52) inside the cartridge body in a cartridge piston advancement direction (X) by an initialization distance； b) increasing a fluidic volume of the dosing cylinder (300) by displacing the dosing piston (302), thereby sucking liquid drug out of the liquid drug cartridge (5) into the dosing cylinder (300) and further displacing the cartridge piston (52) inside the cartridge body (51) in the cartridge piston advancement direction (X), without the initialization pushing force being exerted.
