Abstract: A self-use medical injector has a knob at the rear end of a barrel which is rotated from a zeroed position to set the dose and which rotates back again, to the zeroed position, when the injector is fired. A scale is provided on the knob (2) or barrel (1) and a datum mark or indicator on the barrel or knob co-operates with the scale. There is a raised stud (3, 11, 12, 16) providing the indicator or the zero mark of the scale. An annular member (4, 10, 14, 17) fitted over the knob if the stud is on the barrel or over the barrel if the stud is on the knob, has a projection (9, 12, 15, 18) which is registered with a mark on the scale corresponding to the required dose. This projection abuts the stud when the knob is rotated and thus sets the knob for the correct does.