A dosing device for an infusion system comprises a dosing unit having a variable volume and at least one opening in fluid connection with the variable volume, through which opening the variable volume can be filled with a substance or the substance can be dispensed from the variable volume. The dosing device is operable in a first state for filling the substance from a supply conduit, in a second state for preventing filling and dispensing, or in a third state for dispensing through a dispensing conduit. No direct fluid connection exists between the supply conduit and the dispensing conduit at any time. Methods for dosing a substance with the dosing device.