Drug delivery device for selecting and dispensing a number of user variable doses of a medicament, the device comprising - a housing, - a dose selector operable to set a dose by rotation relative to the housing, - a number sleeve arranged within the housing such that at least a portion of the number sleeve is visible through a first aperture in the housing, - a piston rod coupled to the housing and to a drive sleeve axially movable between a first, proximal dose setting position and a second, distal dose dispensing position such that rotation of the drive sleeve relative to the housing causes the piston rod to translate relative to the housing, - a clutch mechanism releasably coupling the number sleeve and the drive sleeve, the clutch mechanism comprising a first interface (42, 62) for rotationally constraining the drive sleeve and the number sleeve depending on the axial position of the drive sleeve, and - a drive spring, wherein relative rotation of the number sleeve and the drive sleeve during dose setting strains the drive spring and the drive spring is allowed to at least partially relax during dose dispensing thereby rotationally driving the number sleeve, the drive sleeve and the piston rod, characterized in that - the first interface comprises splines on the outer surface of the drive sleeve and corresponding splines on the inner surface of the number sleeve which are arranged such that in a first axial position of the drive sleeve relative to the housing and the number sleeve, the splines (42, 61) are engaged rotationally constraining the drive sleeve to the number sleeve and that in a second axial position of the drive sleeve relative to the housing and the number sleeve, the splines (42, 61) are disengaged allowing relative rotation of the drive sleeve and the number sleeve, and - the clutch mechanism comprises a second interface (43, 121) for rotationally constraining the drive sleeve and the number sleeve via a clutch element depending on the axial position of the drive sleeve and the bias of a clutch spring, wherein the clutch spring biases the drive sleeve towards the clutch element. - a housing, - a dose selector operable to set a dose by rotation relative to the housing, - a number sleeve arranged within the housing such that at least a portion of the number sleeve is visible through a first aperture in the housing, - a piston rod coupled to the housing and to a drive sleeve axially movable between a first, proximal dose setting position and a second, distal dose dispensing position such that rotation of the drive sleeve relative to the housing causes the piston rod to translate relative to the housing, - a clutch mechanism releasably coupling the number sleeve and the drive sleeve, the clutch mechanism comprising a first interface (42, 62) for rotationally constraining the drive sleeve and the number sleeve depending on the axial position of the drive sleeve, and - a drive spring, wherein relative rotation of the number sleeve and the drive sleeve during dose setting strains the drive spring and the drive spring is allowed to at least partially relax during dose dispensing thereby rotationally driving the number sleeve, the drive sleeve and the piston rod, - the first interface comprises splines on the outer surface of the drive sleeve and corresponding splines on the inner surface of the number sleeve which are arranged such that in a first axial position of the drive sleeve relative to the housing and the number sleeve, the splines (42, 61) are engaged rotationally constraining the drive sleeve to the number sleeve and that in a second axial position of the drive sleeve relative to the housing and the number sleeve, the splines (42, 61) are disengaged allowing relative rotation of the drive sleeve and the number sleeve, and - the clutch mechanism comprises a second interface (43, 121) for rotationally constraining the drive sleeve and the number sleeve via a clutch element depending on the axial position of the drive sleeve and the bias of a clutch spring, wherein the clutch spring biases the drive sleeve towards the clutch element.