The present invention relates to an assisted injection device (1) forinjecting a composition contained in a medical container (30), comprising: - abody(10) configured to receive the medical container (30) in a fixed positionrelativeto the body (10), - a spring-loaded piston rod (40) translationally movableinsidethe body (10) along a spring axis (A), between a proximal rest position and adistaloperative position wherein the piston rod (40) engages a stopper (34) of themedicalcontainer (30) and pushes the stopper (34) in the medical container (30), - alever(50) pivotably mounted on the body (10) about a first pivot axis (B)orthogonalto the spring axis (A) at a first distance from the spring axis (A),comprising anactuation zone (51) opposite the first pivot axis (B) relative to the springaxis (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), thelever (50) being pivotable between a rest position wherein the selectiveblockingsystem (60) engages the piston rod (40) to prevent any translational movementofthe piston rod (40) and a second position wherein the selective blockingsystem(60) releases the piston rod (40) to allow the piston rod (40) to move towardthedistal operative position under the spring force.
