The invention concerns an injection device with a housing adapted to receive a syringe having a discharge nozzle and a plunger. There is a trigger and a drive arranged to act upon the plunger on actuation of the trigger to advance the syringe from a retracted position to an extended position through an opening in the housing. A locking mechanism is arranged in and moveable relative to the housing. The locking mechanism is engaged with the plunger when the syringe is not in its extended position and disengaged from the plunger when the syringe is in its extended position. This means that movement of the plunger relative to the syringe is prevented when, in use, the syringe is advanced from a retracted position to an extended position. Hence the contents of the syringe are not expelled from the syringe during advancement from the retracted position to the extended position. The contents are only expelled when the syringe reaches the extended position.