The present invention refers to a drug delivery device comprising a housing (5, 10) with an opening (12), a piston rod (11) having a proximal end and a distal end, at least a portion with an interface (14) adapted to engage the opening (12) for guiding the piston rod (11) during a distal or proximal movement of the piston rod (11) relative to the housing (5, 10) and at least one first attachment element (15) located at the distal end, and a bearing (16) with at least one second attachment element (17, 18) for rotatable attachment of the bearing (16) at the distal end of the piston rod (11). The at least one first attachment element (15) and/or the at least one second attachment element (17, 18) is elastically deformable in a radial direction for snap engagement of the piston rod (11) and the bearing (5, 10). The opening (12) comprises at least one portion having a contour adapted to receive the at least one first attachment element (15) and the at least one second attachment element (17, 18) if attached to each other but preventing disengagement of the at least one first attachment element (15) and the at least one second attachment element (17, 18) if received in the opening (12). Further, the present invention refers to an assembly method for a drug delivery device.
