A dispensing device is disclosed having a cylinder, a piston disposed in the cylinder and moveable in priming and dispensing directions, including a spring device biasing the piston to move in the priming direction at a predetermined rate, and an operating plunger connectable to the piston and moveable along a path to move the piston in the dispensing direction. The operating plunger has an engagement device and is moveable between an engaged position in which the engagement device is connected to the piston and movement of the plunger along the path is transmitted to the piston, and a disengaged position in which the engagement device is disconnected from the piston and movement of the plunger along the path is independent of the piston. The engagement device is arranged such that movement of the plunger between the engaged and disengaged positions is of a different kind to movement along the path.