Abstract: A syringe for injection of doses of a medicine from an exchangeable cartridge has a housing comprising a cartridge holder and a dosing mechanism by which a dose is set and injected by successively advancing a piston rod to press a piston into the cartridge. The dosing mechanism comprises a threaded spindle and a nut member cooperating with the spindle. A dose is set by relative rotation of the spindle and nut member whereby the nut member is moved along the spindle. The position of the nut member on the spindle defines how far the piston rod is advanced. The spindle and the nut are locked against relative rotation when the dosing mechanism is not operated to set a dose. Access to the cartridge holder is obtained by opening a lid connected to the housing and coupled to the dosing mechanism in such a way that the locking of the nut element in its position on the spindle is released when the lid is opened, and the opening movement of the lid is transmitted to a piston rod retraction mechanism.