An injection device comprises housing (112, Figure 1) adapted to receive a syringe (122, Figure 3) having a discharge nozzle, so that the syringe is movable between a retracted position (nozzle contained within housing), and an extended position (nozzle extends from the housing). Trigger 114 is movable from a rest position, retaining drive 120, to an active position, in which it no longer retains drive 120. A releasable locking mechanism 116 is movable from a first position, prevented the trigger from moving into its active position, to a second position that allows trigger movement. The trigger 114 and locking mechanism 116 co-operate such that the trigger 114 moves to 'active' when the releasable locking mechanism is moved to its second position - for example, by pressing sliding sleeve 126 of mechanism 116 against a patient's skin so it moves into housing (112) - whilst pressure is applied to trigger surface 114c.
