Autoinjector comprising a body (1, 2), a rservoir (S) containing fluid product and comprising a piston (P) and a needle (A), such as a pre-filled syringe, said autoinjector comprising a plunger (5) able to collaborate with the piston (P) of said rservoir (S), said plunger (5) being movable between a rest position and an injection position in which said plunger (5) has moved the piston (P) of the rservoir (S) in order to inject the fluid product through the needle (A), an actuating spring (6) being provided to urge said plunger (5) toward the injection position thereof, the autoinjector comprising a force-regulating System (7, 8) applying a force (F2) to said plunger (5), said force (F2) being added to the force applied by said actuating spring (6) to said plunger (5) at the start of injection in order to amplify the force applied to said piston (P) by said plunger (5) at the start of injection.
