Auto-injector having a body (1； 2), a reservoir (S) containing fluid product and having a piston (P) and a needle (A), such as a pre-filled syringe, said auto-injector having a piston rod (5) suitable for cooperating with the piston (P) of said reservoir (S), said piston rod (5) being movable between a first and inoperative position, a second position for pricking, in which said piston rod (5) has moved said reservoir (S) with respect to said body (1； 2) in order to perform the pricking, and a third position for injection, in which said piston rod (5) has moved the piston (P) of the reservoir (S) in order to inject the fluid product through the needle (A), an actuation spring (6) being provided to urge said piston rod (5) firstly towards its pricking position and then towards its injection position, the auto-injector comprising a force-regulating system (7, 8) that exerts a force (F1, F2) on said piston rod (5), said force (F1) being opposite to the force exerted by said actuation spring (6) on said piston rod (5) at the end of pricking in order to decrease the force exerted on said reservoir (S) by said piston rod (5) at the end of pricking, and said force (F2) adding to the force exerted by said actuation spring (6) on said piston rod (5) at the start of injection in order to amplify the force exerted on said piston (P) by said piston rod (5) at the start 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 dépl
