A segmented piston rod for an injection device for delivering a medication from a reservoir having a stopper, the piston rod comprising: multiple segments that each are joined together via a hinge located on the lateral side of each segment such that the piston rod can be bent in one direction by articulating subsequent hinges while an axial force can be transmitted by the segmented piston rod to the stopper when the subsequent hinges are closed and the segments abut each other on the opposite side of the hinge, the segmented piston rod being configured to be guided in a housing of the injection device such that the segmented piston rod is secured against rotation around its longitudinal axis with respect to the housing, the segmented piston rod having a last element configured to abut the stopper in the reservoir and a first segment which is opposite to the last segment, the first segment of the segmented piston rod is equipped with an internal thread matching an external thread of a threaded rod wherein rotation of the threaded rod by a drive mechanism advances the piston rod towards the stopper of the reservoir, whereby the last segment of the segmented piston rod which is configured to abut the stopper of the reservoir has a guiding element which is attached to or attachable to the last segment, characterized by that the guiding element guides the last segment of the segmented piston rod in the reservoir, whereby the guiding element is designed for abutting the wall of the reservoir and guide the last segment such that the normal of the last segment is parallel to the longitudinal axis of the reservoir, whereby the guiding element protrudes from the last segment towards the second to last segment shaped as a fin, and which has an edge which is oriented parallel to the normal of the last segment. multiple segments that each are joined together via a hinge located on the lateral side of each segment such that the piston rod can be bent in one direction by articulating subsequent hinges while an axial force can be transmitted by the segmented piston rod to the stopper when the subsequent hinges are closed and the segments abut each other on the opposite side of the hinge, the segmented piston rod being configured to be guided in a housing of the injection device such that the segmented piston rod is secured against rotation around its longitudinal axis with respect to the housing, the segmented piston rod having a last element configured to abut the stopper in the reservoir and a first segment which is opposite to the last segment, the first segment of the segmented piston rod is equipped with an internal thread matching an external thread of a threaded rod wherein rotation of the threaded rod by a drive mechanism advances the piston rod towards the stopper of the reservoir, whereby the last segment of the segmented piston rod which is configured to abut the stopper of the reservoir has a guiding element which is attached to or attachable to the last segment, characterized by that the guiding element guides the last segment of the segmented piston rod in the reservoir, whereby the guiding element is designed for abutting the wall of the reservoir and guide the last segment such that the normal of the last segment is parallel to the longitudinal axis of the reservoir, whereby the guiding element protrudes from the last segment towards the second to last segment shaped as a fin, and which has an edge which is oriented parallel to the normal of the last segment.