An autoinjector comprising a body (1, 2), and a reservoir (S) containing fluid and including a piston (P) and a needle (A), such as a pre-filled syringe, said autoinjector further comprising a piston rod (5) that is suitable for co-operating with the piston (P) of said reservoir (S), said piston rod (5) being movable between a rest position and an injection position in which said piston rod (5) has moved the piston (P) of the reservoir (S) so as to inject the fluid through the needle (A), an actuator spring (6) being provided so as to urge said piston rod (5) towards its injection position, the autoinjector further comprising a force-adjustment system (7, 8) that exerts a force (F2) on said piston rod (5), said force (F2) adding to the force exerted by said actuator spring (6) on said piston rod (5) at the beginning of injection, so as to amplify the force exerted on said piston (P) by said piston rod (5) at the beginning of injection.