An injection device 1 has a plunger 60 to act on a syringe 20. An actuation spring 40 provides a biasing force to urge the plunger from a first rearward position to a second forward position to express medicament from the syringe 20. A latch member 50 has an aperture 54 through which the rearward end of the plunger extends in its rearward position so as to be retained against the force of the actuation spring. A trigger 70, preferably an axially moveable button, can release the plunger from the latch member. The rearward end of the plunger is provided with an enlarged head 66 and the latch member is arranged to at least partially radially displace to expand the aperture and so release the enlarged head. The trigger is moveable between a primed position in which it prevents displacement of the latch member and a release position in which it allows displacement of the latch. It may engage directly with the plunger to reinforce the force of the spring.
