The present invention relates to an assisted injection device (1) for injecting a composition contained in a medical container (40), comprising: - a body (10) adapted to receive a medical container (40) in a fixed position relative to the body (10), the body (10) being configured to be held in a user's hand, - a spring-loaded piston rod (30) translationally movable inside the body (10) between a proximal rest position and a distal operative position wherein the piston rod (30) engages a stopper (44) of the medical container (40) and pushes the stopper (44) in the medical container (40), - a blocking system comprising a locking member (50, 60, 70) configured to engage the piston rod, the locking member being movable between a locked position wherein the locking member retains the piston rod (30) in the proximal rest position, and a released position wherein the locking member allows the piston rod (30) to move from the proximal rest position to the distal operative position under the force of the spring (31), wherein the piston rod (30) includes a proximal end (32) extending out of the body (10) and configured to be pushed by the user in a distal direction to accelerate the movement of the piston rod (30) to the distal operative position when the locking member (50, 60, 70) is in the released position.