An autoinjector having a body (1; 2), a reservoir (S), a piston (P), a needle (A), and a piston rod (5) co-operating with the piston. The piston rod is movable between a first rest position, a second pricking position, and a third injection position. An actuator spring (6) is provided to urge the piston rod towards its pricking position and then towards its injection position. A force-adjustment system (7, 8) exerts a force (F1, F2) on the piston rod (5). The force (F1) opposes the force exerted by the actuator spring at the end of pricking to decrease the force exerted on the reservoir by the piston rod at the end of pricking, and the force (F2) adding to the force exerted by the actuator spring at the beginning of injection, to amplify the force exerted on the piston by the piston rod at the beginning of injection.