Auto-injector having a body ( 1 2), a rservoir (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 rservoir (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 rservoir (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 rservoir (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 (Fl, F2) on said piston rod (5), said force (Fl) 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 rservoir (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.
