A safety device for a pre-filled syringe, comprising: - a release collar (1.4) configured to receive the pre-filled syringe and comprising a flexible arm (1.4.5) with a guide pin (1.4.6) extending from the flexible arm (1.4.5), - a support body (1.2), and - a hollow outer body (1.3) receiving the support body (1.2) having two side walls opposing each other, wherein the outer body (1.3) slides relative to the support body (1.2) to inject a medication contained in the pre-filled syringe through a hypodermic needle and the release collar (1.4) is slidable relative to the support body (1.2) when the outer body (1.3) is slid relative to the support body (1.2) to release and retract the pre-filled syringe retained within the safety device; wherein - a guide track (1.2.4) is formed into at least one side wall (1.2.1) of the support body (1.2), - a guide rail (1.3.5) is formed into at least one inner surface of the outer body (1.3) facing the side wall of the support body with the guide track, - the guide pin (1.4.6) is movable within and along the guide track (1.2.4) in a plane defined by the side wall (1.2.1) when the outer body (1.3) is slid relative to the support body (1.2), and - the guide rail (1.3.5) is adapted to abut against the guide pin (1.4.6) to guide the movement of the guide pin (1.4.6) along the guide track (1.2.4) when the outer body (1.3) is slid relative to the support body (1.2), and - the guide pin protrudes through the guide track and is retained within the guide track in a start position prior to use of the safety device and in an end position after the injection has been carried out, so that a re-usage of the safety device is prevented. - a release collar (1.4) configured to receive the pre-filled syringe and comprising a flexible arm (1.4.5) with a guide pin (1.4.6) extending from the flexible arm (1.4.5), - a support body (1.2), and - a hollow outer body (1.3) receiving the support body (1.2) having two side walls opposing each other, - a guide track (1.2.4) is formed into at least one side wall (1.2.1) of the support body (1.2), - a guide rail (1.3.5) is formed into at least one inner surface of the outer body (1.3) facing the side wall of the support body with the guide track, - the guide pin (1.4.6) is movable within and along the guide track (1.2.4) in a plane defined by the side wall (1.2.1) when the outer body (1.3) is slid relative to the support body (1.2), and - the guide rail (1.3.5) is adapted to abut against the guide pin (1.4.6) to guide the movement of the guide pin (1.4.6) along the guide track (1.2.4) when the outer body (1.3) is slid relative to the support body (1.2), and - the guide pin protrudes through the guide track and is retained within the guide track in a start position prior to use of the safety device and in an end position after the injection has been carried out, so that a re-usage of the safety device is prevented. - the guide rail (1.3.5) is adapted to abut against the guide pin (1.4.6) to guide the movement of the guide pin (1.4.6) along the guide track (1.2.4) when the outer body (1.3) is slid relative to the support body (1.2), and - the guide pin protrudes through the guide track and is retained within the guide track in a start position prior to use of the safety device and in an end position after the injection has been carried out, so that a re-usage of the safety device is prevented.