An auto-injector comprising an outer casing (30) in which a syringe comprising a cylinder (9) for containing a volume of medicament, a needle (10) at one end of the cylinder (9) and a plunger (8) is mounted axially movable in the cylinder (9), the autoinjector further comprises an inner casing (7) between the outer casing (30) and the syringe and a power source in communication with said inner casing (7), wherein the inner casing ( 7) is mobile by the power source between three positions, specifically a first position in which the inner casing (7) is in communication with the cylinder (9), the plunger (8) and the cylinder (9) that can move axially to move at least part of said needle (10) out of the outer shell (30)； a second position in which the inner casing (7) is in communication with the plunger (8) but not with the cylinder (9), said plunger (8) can move axially within said cylinder (9) to expel the medication to through the needle (10)； and a third position in which the inner casing (7) is not in communication with the plunger (8) or the cylinder (9), the plunger (8) and the cylinder (9) are retractable to retract the needle (10 ) towards the outer housing (30), characterized in that the auto-injector further comprises means associated with a rear flange (8F) of said piston (8) that provides a plurality of axial positions in which it is possible that said inner housing (7) is couple said plunger (8) in said second position.Un autoinyector que comprende una carcasa exterior (30) en la cual se monta una jeringa que comprende un cilindro (9) para contener un volumen de medicamento, una aguja (10) en un extremo del cilindro (9) y un émbolo (8) móvil axialmente en el cilindro (9), el autoinyector comprende además una carcasa interior (7) entre la carcasa exterior (30) y la jeringa y una fuente de energía en comunicación con dicha carcasa interior (7), en donde la carcasa interior (7) es móvil por la fuente de energía entre tres posiciones, específicamente una prim
