Abstract: In use a knob (5) is rotated from its zero position to set a dose. This rotates an indexer (16), which through its peg (20) turns a ring (7) by pushing on one of the teeth (8). A position is reached wherein the free end portion of the peg (20) meets a cam surface (13). On continued rotation of the knob (5), the peg (20) is forced radially inwards to clear the tooth (8) that it has just been pushing against. The ring (7), having been shifted through one-seventh of a complete revolution is then left stationary while the knob (5) is turned further to whatever dose is required. When a syringe actuation trigger (4) is pressed, the knob (5) winds back again to its zero position, taking with it the indexer (16). The peg (20) is still held clear of the ring (7) until it hits the sloping side of the tooth following the one which it had previously pushed. As the knob finally reverts to zero, the peg (20) slides along that slope and then snaps outwardly after passing the tip.