Autoinjector comprising a body (1； 2), a reservoir (S) containing fluid and comprising a piston (P) and a needle (A), such as a pre-filled syringe, said autoinjector comprising a piston rod ( 5) adapted to cooperate with the piston (P) of said tank (S), said piston rod (5) being movable between a first rest position, a second stitching position, in which said piston rod (5) has moved said tank (S) with respect to said body (1； 2) for performing stitching, and a third injection position in which said piston rod (5) has moved the piston (P) of the tank (S) to inject the fluid product through the needle (A), an actuating spring (6) being provided for urging said piston rod (5) first to its stitching position and then to its injection position, the autoinjector comprising a force control system (7, 8) adapted to exert a force (F1, F2) on said rod piston (5), said force (F1) being opposed to the force exerted by said actuating spring (6) on said piston rod (5) at the end of stitching to reduce the force exerted on said reservoir (S) by said piston rod (5) at the end of stitching, and said force (F2) adding to the force exerted by said actuation spring (6) on said piston rod (5) at the beginning of injection to amplify the force exerted on said piston (P) by said piston rod (5) at the beginning of injection. Autoinjecteur comportant un corps (1 ； 2), un réservoir (S) contenant du produit fluide et comportant un piston (P) et une aiguille (A), tel qu'une seringue pré-remplie, ledit autoinjecteur comportant une tige de piston (5) adaptée à coopérer avec le piston (P) dudit réservoir (S), ladite tige de piston (5) étant déplaçable entre une première position de repos, une seconde position de piquage, dans laquelle ladite tige de piston (5) a déplacé ledit réservoir (S) par rapport audit corps (1 ； 2) pour réaliser le piquage, et une troisième position d'injection dans laquelle ladite tige de piston (5) a déplacé le piston (P) du réservoir (S) pour injecter le produi
