An autoinjector, comprising: - a reservoir module comprising a reservoir containing fluid and a piston, and including a needle, - a driver module comprising: - a body, - a piston rod that co-operates with the piston of said reservoir, said piston rod being moved between a rest position and an injection position in which said piston rod has moved the piston of the reservoir so as to inject the fluid through said needle, - an injection spring for urging said piston rod towards its injection position, - force control means (280, 290), for causing the force exerted by said injection spring on said piston rod to vary in predetermined manner throughout the injection stage, said force control means comprising a lever and a cam, said lever being pivotally mounted at one end on said piston rod, being connected to said injection spring, and co-operating at its other end with said cam, characterized in that it comprises an interface module that is fastened to the driver module and that receives a reservoir module before each actuation, said interface module comprising a selector ring that is mounted to turn on the driver module, and pivot elements for fastening the reservoir module in the interface module, said selector ring is movable in turning between a locked position in which insertion of a reservoir module is possible and actuation of the driver module is not possible, an injection position in which actuation of the driver module is possible, a safety position in which an actuator sleeve of the reservoir module returns into its deployed position around said needle, and an ejection position in which ejection of the used reservoir module is possible. - a reservoir module comprising a reservoir containing fluid and a piston, and including a needle, - a driver module comprising: - a body, - a piston rod that co-operates with the piston of said reservoir, said piston rod being moved between a rest position and an injection position in which said piston rod has moved the piston of the reservoir so as to inject the fluid through said needle, - an injection spring for urging said piston rod towards its injection position, - force control means (280, 290), for causing the force exerted by said injection spring on said piston rod to vary in predetermined manner throughout the injection stage, said force control means comprising a lever and a cam, said lever being pivotally mounted at one end on said piston rod, being connected to said injection spring, and co-operating at its other end with said cam, characterized in that it comprises an interface module that is fastened to the driver module and that receives a reservoir module before each actuation, said interface module comprising a selector ring that is mounted to turn on the driver module, and pivot elements for fastening the reservoir module in the interface module, said selector ring is movable in turning between a locked position in which insertion of a reservoir module is possible and actuation of the driver module is not possible, an injection position in which actuation of the driver module is possible, a safety position in which an actuator sleeve of the reservoir module returns into its deployed position around said needle, and an ejection position in which ejection of the used reservoir module is possible. - a body, - a piston rod that co-operates with the piston of said reservoir, said piston rod being moved between a rest position and an injection position in which said piston rod has moved the piston of the reservoir so as to inject the fluid through said needle, - an injection spring for urging said piston rod towards its injection position, - force control means (280, 290), for causing the force exerted by said injection spring on said piston rod to vary in predetermined manner throughout the injection stage, said force control means comprising a lever and a cam, said lever being pivotally mounted at one end on said piston rod, being connected to said injection spring, and co-operating at its other end with said cam,