A needle stick safety device for a pre-filled syringe with an attached needle and a rigid needle shield over the needle, the needle stick safety device being intended to prevent needle stick injuries after medication has been injected, and comprising: - a main body for receiving the syringe, the main body comprising protrusions, - a RNS removal tool removably mounted at the distal end of the device, the RNS removal tool forming a removable covering over the rigid needle shield for facilitating removal of said rigid needle shield just before injecting medication to a patient, - a needle guard body forming a tube that is concentric and interior or exterior to the main body, the needle guard body being slidable with respect to the main body and being biased in the distal direction by a compression spring, the needle guard body including grooves with which the protrusions of the main body interfere in order to control the movement of the needle guard body with respect to the main body, wherein: - from a configuration in which the device is ready for medication dispensing, as the device is pushed against the injection site, the needle guard body moves proximally, against the spring force, allowing the needle to enter into the injection site, the groove-protrusion interface moving from a second position to a third position; - when the device is being withdrawn from the injection site, the spring pushes the needle guard body distally towards a fully forward extended position, the protrusions of the main body moving in the grooves of the needle guard body from the third position to a fourth position in which the needle guard body is locked from further motion; characterized in that, in order to place the device in the configuration in which it is ready for medication dispensing, the RNS removal tool is removed from the end of the device, thereby removing the rigid needle shield and causing the needle guard body to move distally in response to the spring force, so that the protrusions of the main body move in the grooves of the needle guard body from a first position to the second position. - a main body for receiving the syringe, the main body comprising protrusions, - a RNS removal tool removably mounted at the distal end of the device, the RNS removal tool forming a removable covering over the rigid needle shield for facilitating removal of said rigid needle shield just before injecting medication to a patient, - a needle guard body forming a tube that is concentric and interior or exterior to the main body, the needle guard body being slidable with respect to the main body and being biased in the distal direction by a compression spring, the needle guard body including grooves with which the protrusions of the main body interfere in order to control the movement of the needle guard body with respect to the main body, - from a configuration in which the device is ready for medication dispensing, as the device is pushed against the injection site, the needle guard body moves proximally, against the spring force, allowing the needle to enter into the injection site, the groove-protrusion interface moving from a second position to a third position; - when the device is being withdrawn from the injection site, the spring pushes the needle guard body distally towards a fully forward extended position, the protrusions of the main body moving in the grooves of the needle guard body from the third position to a fourth position in which the needle guard body is locked from further motion;