The present invention relates to an assisted injection device (1) for injecting a composition contained in a medical container (30), comprising: - a main body (10) adapted to receive a medical container (30) in a fixed position relative to the main body (10), the main body (10) comprising an inner wall provided with a toothed surface (14), - a piston (40) comprising: a first part comprising an inner spring-loaded piston rod (50), and an outer piston body (60) provided with a locking member (65), the inner spring-loaded piston rod (50) and the outer piston body (60) being translationally movable inside the main body (10) between a proximal rest position and a distal operative position wherein the inner spring- loaded piston rod (50) engages a stopper (34) of the medical container (30) and pushes the stopper (34) in the medical container (30), a second part (70) arranged between the inner spring-loaded piston rod (50) and the outer piston body (60), and provided with an pressing member (75), the second part (70) being selectively slidable relative to the first part between a proximal first position wherein the pressing member (75) presses radially outwardly the locking member (65) in frictional engagement with the toothed surface (14), and a distal second position wherein the pressing member (75) is remote from the locking member (65) and allows radial inward movement of the locking member (65) and release of the frictional engagement with the toothed surface (14), - actuation means connected to the second part (70) of the piston (40), adapted to selectively move the second part (70) between the proximal first position and the distal second position.
