The present invention relates to an assisted injection device (1) for injecting a composition contained in a medical container (30), comprising: - a body (10) configured to receive the medical container (30) in a fixed position relative to the body (10), - a spring-loaded piston rod (40) translationally movable inside the body (10) along a spring axis (A), between a proximal rest position and a distal operative position wherein the piston rod (40) engages a stopper (34) of the medical container (30) and pushes the stopper (34) in the medical container (30), - a lever (50) pivotably mounted on the body (10) about a first pivot axis (B) orthogonal to the spring axis (A) at a first distance from the spring axis (A), comprising an actuation zone (51) opposite the first pivot axis (B) relative to the spring axis (A), at a second distance from the spring axis (A), - a selective blocking system (60) coupled to the lever by a second pivot axis (C) orthogonal to the spring axis (A), the lever (50) being pivotable between a rest position wherein the selective blocking system (60) engages the piston rod (40) to prevent any translational movement of the piston rod (40) and a second position wherein the selective blocking system (60) releases the piston rod (40) to allow the piston rod (40) to move toward the distal operative position under the spring force.
