Abstract: A pen shaped syringe for repetitive injection of individually set doses of a medicine from a cylinder ampoule reservoir comprises a dose setting member which may be rotated to cause a rotative movement of a dosing member and a combined rotative and axial movement of an indicator member indicating the set dose, and a piston drive member which when rotated in one direction moves the piston into the cylinder ampoule. A unidirectional coupling is established between the dosing member and the piston drive member by each member carrying a disc having surfaces with sector shaped saw teeth riding over each other when the dosing member is rotated in the dose setting direction and engaging each other when the dosing member is rotated in the opposite direction corresponding to the direction of rotation by which the piston is moved into the cylinder ampoule.