A drive mechanism for an injection device in which a piston is successively moved in a first axial direction in relation to a medicament cartridge containing a medicament selectively to drive a bung closing a first end of the medicament cartridge into the medicament cartridge to expel medicament through a delivery member located at a second end of the medicament cartridge, the drive mechanism comprising: a base member, a first drive member fixed relative to the base member, a second drive member axially and rotatably moveable relative to the first drive member and a piston member axially moveable relative to the first and second drive member, wherein the second drive member is drivingly connected between the first drive member and the piston member such that rotation of the second drive member in a first direction of rotation relative to the first drive member and the piston member causes the second drive member to move in the first axial direction relative to the first drive member and the piston member to move in the first axial direction relative to both the first and second drive member, and an injection device comprising such a drive mechanism.
