According to the invention, a back-end device of an auto-injector adapted to be attached to a packaged syringe comprisesa plunger arranged to abut against a stopper of the packaged syringe to translate an injection needle of the packaged syringe in a proximal direction and to expel a dose of a medicament contained in the packaged syringe through the injection needle,a drive means capable of, upon release, driving the plunger in the proximal direction towards a proximal position,a rotary sleeve with a cam surface adapted to be engaged by a plunger follower formed to the plunger,a torsion spring arranged to exert a torque upon the rotary sleeve anda release element capable of, upon manual actuation, releasing the engagement of the plunger follower with the cam surface. The plunger follower is arranged to engage the cam surface to prevent the rotary sleeve from rotating with respect to the chassis in an initial state. Furthermore, in the initial state the plunger follower is arranged to engage the cam surface to prevent an axial translation of the plunger in the proximal direction. Upon release of the engagement of the plunger follower with the cam surface, the rotary sleeve is arranged to rotate within the chassis. The cam surface re-engages the plunger follower after a time delay determined by a rotation speed of the rotary sleeve. The time delay is set so as to allow the cam surface to re-engage the plunger follower after the plunger reaches the proximal position. The cam surface is arranged to withdraw the plunger from the proximal position in a distal direction by continued rotation of the rotary sleeve.
