The disclosure relates to a safety system for a medical injection device including a barrel, a needle, and a stopper. Said safety system includes a body, the medical injection device being movable relative to the body between an initial distal position and a final safety position. The body includes at least one first locking member configured to releasably lock the medical injection device in the initial position, and a plunger rod configured to drive the stopper in distal direction. Said plunger rod being axially collapsible between an initial extended configuration and a final retracted configuration. The plunger rod and the locking member being configured so that when the stopper abuts at the distal end of the barrel, a safety activation force exerted onto the plunger rod in distal direction collapses the plunger rod, causing the body to move to the final safety position.