According to the invention, a safety device for a pre-filled syringe comprises a hollow support body having a releasable mounting means adapted to mount a pre-filled syringe within the support body, a hollow outer body sildably arranged with respect to the support body and adapted to receive the support body and a release collar non-rotatably arranged within the support body. The release collar is movable relative to the support body and adapted to release the mounting means. A guide track is formed into at least one substantially planar side wall of the support body. The release collar comprises at least one flexible arm with a guide pin that extends from the flexible arm and protrudes through the guide track. The guide pin is movable within and along the guide track in a plane defined by the substantially planar side wall. A guide rail is formed into at least one substantially planar inner surface of the outer body. The guide rail is adapted to abut against the guide pin to guide the movement of the guide pin along the guide track when the outer body is slid relative to the support body.
