A safety pen needle includes: a main body, a cannula fixed to the main body and at least partially projecting out of the main body, a lock element placed inside the main body, a thrust element placed inside the main body, and a shield movable relative to the main body while remaining at least partially inserted within the main body. The pen needle is configured to move from a first configuration, in which the shield entirely covers an upper end of the cannula, to a second configuration, in which the upper end of the cannula is exposed, due to the action of external thrust. The pen needle is configured to move from the second configuration to an irreversible third configuration, in which the shield entirely covers the upper end of the cannula, due to the action of thrust exerted by the thrust element.