FIELD : medicine.SUBSTANCE : invention relates to medical equipment, specifically to the dose setting operation member for a drug delivery mechanism. Operation member has a central opening with an edge and at least one protruding element located on the edge and extending into the opening which is surrounded by a protruding rim. Protruding rim is interrupted along the edge by at least one gap. If more than one gap is provided, the gaps are arranged such that the pattern of the gaps does not comprise a symmetry that would allow a rotation different from an integer multiple of 360??around a central axis of the opening to map the complete pattern of gaps onto itself. Operation member may be combined with a drive member comprising abutments in positions corresponding to the positions of the gaps.EFFECT : operation member and mechanism for a drug delivery device and a drug delivery device are disclosed.20 cl, 5 dwg