Disclosed is a method for initializing an ambulatory infusion system. The method includes providing a liquid drug cartridge including a cartridge body, and a cartridge piston. The method further includes providing a dosing unit including a metering pump unit with a dosing cylinder and a dosing piston. The dosing unit further includes a valve unit configured for switching between a filling state and a draining state. The filling port being fluidically coupled with the dosing cylinder in the filling state and the draining port is fluidically coupled with the dosing cylinder in the draining state. The method further includes
a) exerting an initialization force displacing the cartridge piston, and
b) increasing a fluidic volume of the dosing cylinder by displacing the dosing piston, thereby sucking liquid out of the liquid drug cartridge into the dosing cylinder and further displacing the cartridge piston, without the initialization pushing force being exerted.