An infusion pump for administering controlled doses of a fluid comprises: a receptacle (2) for storing the fluid (6), the receptacle having an outlet (8) for dispensing the fluid; a plunger (12); a propulsion mechanism (14) which urges the plunger (12) in a direction to pressurise the fluid (6) and dispense it through the outlet (8); and a retaining member (16) which acts on the plunger (12) to control or prevent travel of the plunger (12); characterised in that the retaining member (16) is operably connected to a stepper motor (24) via a worm (22) and worm-wheel (18) drive.