The present disclosure is directed towards dosing unit (1) for use in an ambulatory infusion system, the dosing unit (1) including: a dosing cylinder (11) and a piston (12), the piston (12) being arranged inside the dosing cylinder and in a sliding displaceable manner along a displacement axis (A). The piston (12) is convertible form a storing configuration into an operational configuration, wherein a circumferential sealing member (121) of the piston (12) is mechanically relieved in the storing configuration and is in sealing and sliding engagement with a circumferential inner surface of the dosing cylinder (11) in the operational configuration. The dosing unit (1) includes a configuration switch member (3) in operative mechanical coupling with the piston (12), the configuration switch member (3) being movable relative to the piston (12) from a storing position into an operational position, thereby switching the piston configuration from the storing configuration to the operational configuration.
