A drug delivery device comprising a pumping system (4) and a liquid reservoir (7) fluidly connected to a delivery system outlet (12), the liquid reservoir comprising an elastic plunger (14) sealingly slidable within a container wall (13) of the liquid reservoir for expelling liquid out of the reservoir. The pumping system comprises a piston pump (5) comprising a plunger actuator arranged to displace the plunger (14), and a dosing unit (6) arranged downstream of the liquid reservoir (7) and fluidly connected to the liquid reservoir. The dosing unit (6) comprises a chamber portion (22) arranged between an inlet valve (24) and an outlet valve (26), the chamber portion (22) arranged to receive from the liquid reservoir a pump cycle volume of liquid under an operational pressure greater than ambient pressure generated by the piston pump, and to the deliver said pump cycle volume of liquid to the delivery system outlet, said pump cycle volume being dependent on the operational pressure.
