The present invention relates to an injection device (1) for injecting a composition contained in a medical container (60), comprising: a body (2) configured to receive the medical container (60) in a fixed position relative to the body, a spring-loaded piston rod (3) translationally movable inside the body (2) along a spring axis (A), between a proximal rest position and a distal operative position wherein the spring-loaded piston rod (3) engages a stopper (64) of the medical container (60), a selective blocking mechanism comprising a braking member (10) selectively tiltable relative to the piston rod (3) between: o a blocking position wherein the braking member impinges on the piston rod (3) so as to prevent any translational movement of the spring-loaded piston rod (3) in a distal direction by a bracing effect； and o a releasing position wherein the braking member disengages from the piston rod (3) so as to allow the piston rod (3) to move towards the distal operative position under the spring force.
