Method for initializing an ambulatory infusion system, including the steps of: providing a liquid drug cartridge, the liquid drug cartridge including a cartridge body and a cartridge piston, the cartridge piston being arranged inside the cartridge body in a sealing and displaceable manner; providing a dosing unit, the dosing unit including a metering pump unit with a dosing cylinder and a dosing piston, the dosing piston being arranged inside the dosing cylinder in a sealing and displaceable manner, the dosing unit further including a valve unit with a filling port and a draining port,the valve unit being configured for switching between a filling state and an alternative draining state, wherein the filling port is fluidic coupled with an inner volume of the dosing cylinder in the filling state and the draining port is fluidic coupled with the inner volume of the dosing cylinder in the draining state; providing an initialization force member; the method further including a) exerting, with the initialization force member, an initialization pushing force onto the cartridge piston, thereby displacing the cartridge piston inside the cartridge body in a cartridge piston advancement direction by an initialization distance; b) increasing a fluidic volume of the dosing cylinder by displacing the dosing piston, thereby sucking liquid drug out of the liquid drug cartridge into the dosing cylinder and further displacing the cartridge piston inside the cartridge body in the cartridge piston advancement direction, without the initialization pushing force being exerted. providing a liquid drug cartridge, the liquid drug cartridge including a cartridge body and a cartridge piston, the cartridge piston being arranged inside the cartridge body in a sealing and displaceable manner; providing a dosing unit, the dosing unit including a metering pump unit with a dosing cylinder and a dosing piston, the dosing piston being arranged inside the dosing cylinder in a sealing and displaceable manner, the dosing unit further including a valve unit with a filling port and a draining port,the valve unit being configured for switching between a filling state and an alternative draining state, wherein the filling port is fluidic coupled with an inner volume of the dosing cylinder in the filling state and the draining port is fluidic coupled with the inner volume of the dosing cylinder in the draining state; providing an initialization force member; the method further including a) exerting, with the initialization force member, an initialization pushing force onto the cartridge piston, thereby displacing the cartridge piston inside the cartridge body in a cartridge piston advancement direction by an initialization distance; b) increasing a fluidic volume of the dosing cylinder by displacing the dosing piston, thereby sucking liquid drug out of the liquid drug cartridge into the dosing cylinder and further displacing the cartridge piston inside the cartridge body in the cartridge piston advancement direction, without the initialization pushing force being exerted. a) exerting, with the initialization force member, an initialization pushing force onto the cartridge piston, thereby displacing the cartridge piston inside the cartridge body in a cartridge piston advancement direction by an initialization distance; b) increasing a fluidic volume of the dosing cylinder by displacing the dosing piston, thereby sucking liquid drug out of the liquid drug cartridge into the dosing cylinder and further displacing the cartridge piston inside the cartridge body in the cartridge piston advancement direction, without the initialization pushing force being exerted.