An injection device comprises a syringe (1) extendible against a spring bias (9) from a retracted position in a housing (2) to a projecting injecting position. A spring biased plunger has collapsible elbows (10) which, when the plunger is released, initially engage the end of the syringe to drive it to the projecting position, whereupon arresting of the syringe movement causes the elbows to collapse inside the syringe to allow the plunger to engage and drive the bung (3).