An arrangement for coupling a plunger to either a syringe or a stopper arranged in the syringe. The arrangement includes a syringe holder which is slidably arranged in a housing. The syringe holder is provided with at least one resilient syringe holder arm arranged distally, the syringe holder arm having an inclined surface for bearing against a shoulder, which is arranged at the plunger. The syringe holder arm is supportable by an inner surface of the housing in order to prevent it from being flexed outward in a first position. A widened portion is provided in the housing for allowing the syringe holder arm to flex outwards and disconnect from the plunger when in a more proximal position so as to switch load of the plunger from the syringe to the stopper.