Assisted injection device for injecting a composition contained in a medical container, comprising: - a main body adapted to receive a medical container in a fixed position relative to the main body, the main body comprising an inner wall provided with a toothed surface, - a piston comprising: • a first part comprising an inner spring-loaded piston rod, and an outer piston body extending coaxially around the inner spring-loaded piston rod, the outer piston body being connected to the inner spring-loaded piston rod and provided with a locking member, the inner spring-loaded piston rod and the outer piston body being translationally movable inside the main body between a proximal rest position and a distal operative position wherein the inner spring-loaded piston rod engages a stopper of the medical container and pushes the stopper in the medical container, • a second part arranged between the inner spring-loaded piston rod and the outer piston body, and provided with a pressing member, the second part being selectively slidable, relative to the first part, between a proximal first position wherein the pressing member presses radially outwardly the locking member in frictional engagement with the toothed surface, and a distal second position wherein the pressing member is remote from the locking member and allows radial inward movement of the locking member and release of the frictional engagement with the toothed surface, - actuation means connected to the second part of the piston, adapted to selectively move the second part between the proximal first position and the distal second position, wherein the actuation means together with the second part of the piston is biased by a spring in the proximal direction and the movement of the second part relative to the first part is determined by an amount of pressure exerted by a user onto the actuation means in a distal direction, such that by selectively actuating or releasing the actuation means, the user can start or stop injection of the composition contained in the medical container, and/or adjust an injection rate of said composition. - a main body adapted to receive a medical container in a fixed position relative to the main body, the main body comprising an inner wall provided with a toothed surface, - a piston comprising: • a first part comprising an inner spring-loaded piston rod, and an outer piston body extending coaxially around the inner spring-loaded piston rod, the outer piston body being connected to the inner spring-loaded piston rod and provided with a locking member, the inner spring-loaded piston rod and the outer piston body being translationally movable inside the main body between a proximal rest position and a distal operative position wherein the inner spring-loaded piston rod engages a stopper of the medical container and pushes the stopper in the medical container, • a second part arranged between the inner spring-loaded piston rod and the outer piston body, and provided with a pressing member, the second part being selectively slidable, relative to the first part, between a proximal first position wherein the pressing member presses radially outwardly the locking member in frictional engagement with the toothed surface, and a distal second position wherein the pressing member is remote from the locking member and allows radial inward movement of the locking member and release of the frictional engagement with the toothed surface, • a first part comprising an inner spring-loaded piston rod, and an outer piston body extending coaxially around the inner spring-loaded piston rod, the outer piston body being connected to the inner spring-loaded piston rod and provided with a locking member, the inner spring-loaded piston rod and the outer piston body being translationally movable inside the main body between a proximal rest position and a distal operative position wherein the inner spring-loaded piston rod engages a stopper of the medical container and pushes the stopper in the medical container, • a second part arranged between the inner spring-loaded piston rod and the outer piston body, and provided with a pressing member, the second part being selectively slidable, relative to the first part, between a proximal first position wherein the pressing member presses radially outwardly the locking member in frictional engagement with the toothed surface, and a distal second position wherein the pressing member is remote from the locking member and allows radial inward movement of the locking member and release of the frictional engagement with the toothed surface, - actuation means connected to the second part of the piston, adapted to selectively move the second part between the proximal first position and the distal second position,