The present disclosure is directed towards a dosing unit (100) for an ambulatory infusion system, including:- a metering pump unit (110), the pump unit including a dosing cylinder (112) and a piston (111), the piston (111) being arranged in sealing sliding engagement inside the dosing cylinder (112)- a valve unit (120), the valve unit (120) having a filling port (127b), the filling port being designed for fluidic coupling with a liquid drug reservoir (300), a draining port (127c), the draining port (127c) being designed for fluidic coupling with an infusion site interface (890), and a shut-off body (126), the shut-off body (126) being movable between a filling position where it fluidic couples the filling port (127b) with the dosing cylinder (112) and an alternative draining position where it fluidic couples the dosing cylinder (112) with the draining port (127c)- a valve driver coupler (125), the valve driver coupler (125) being coupled to or integral with the shut-off body and being the output element of a step switching mechanism.The disclosure is further directed towards a drive unit that may be used in combination with a disclosed dosing unit, an ambulatory infusion system and a method for coupling a dosing unit and a drive unit.
