The invention relates to an automatic injector device comprising : a single-use, disposable, drug delivery assembly comprising a housing and a syringe assembly located at least partially within the housing, said syringe assembly including a plunger, a pre-filled unit-dose drug containing chamber, and needle, said plunger, drug containing chamber and needle being configured and dimensioned to function as an injection syringe； a reusable motorised transmission assembly comprising a housing, a motor and transmission assembly located within the housing, said transmission assembly being configured and dimensioned to engage the plunger of said syringe in the drug delivery assembly and expel said unit dose dmg from the drug containing chamber, into the needle and out of the drug delivery assembly； said single-use disposable drug delivery assembly and said reusable motorised transmission assembly are in substantial axial alignment along a longitudinal axis defined by the syringe, plunger, pre-filled unit-dose drug containing chamber, and needle； and the housing of the single-use, disposable, drug delivery assembly is removably coupled to the housing of the reusable motorised transmission assembly via a coupling system configured and dimensioned to provide substantial axial alignment between said single-use disposable drug delivery assembly and said reusable motorised transmission assembly.L'invention concerne un dispositif d'injection automatique comprenant : un ensemble d'administration de médicament jetable à usage unique comprenant un boîtier et un ensemble seringue situé au moins partiellement à l'intérieur du boîtier, ledit ensemble seringue comprenant un piston, une chambre contenant un médicament à dose unitaire pré-remplie, et une aiguille, ledit piston, ladite chambre contenant un médicament et ladite aiguille étant configurés et dimensionnés pour fonctionner comme une seringue d'injection； un ensemble de transmission motorisé réutilisable comprenant un boîtier, un
