A drive mechanism (10) for an injection device in which a piston (26) is successively moved in a first axial direction in relation to a medicament cartridge (12) containing a medicament selectively to drive a bung (18) closing a first end (15) of the medicament cartridge (12) into the medicament cartridge (12) to expel medicament through a delivery member (16) located at a second end of the medicament cartridge (12), the drive mechanism (10) comprising: a base member (20), a first drive member (22) fixed relative to the base member (20), a second drive member (24) axially and rotatably moveable relative to the first drive member (22) and a piston member (26) axially moveable relative to the first (22) and second (24) drive member, wherein the second drive member (24) is drivingly connected between the first drive member (22) and the piston member (26) such that rotation of the second drive member (24) in a first direction of rotation relative to the first drive member (22) and the piston member (26) causes the second drive member (24) to move in the first axial direction relative to the first drive member (22 ) and the piston member (26) to move in the first axial direction relative to both the first (22) and second (24) drive member, and an injection device comprising such a drive mechanism (10).Linvention porte sur un mécanisme dentraînement (10) pour un dispositif dinjection dans lequel un piston (26) est déplacé de façon successive dans une première direction axiale par rapport à une cartouche de médicament (12) contenant un médicament, pour entraîner dune manière sélective un bouchon (18), fermant une première extrémité (15) de la cartouche de médicament (12), dans la cartouche de médicament (12) pour expulser un médicament à travers un élément dadministration (16) disposé à une seconde extrémité de la cartouche de médicament (12). Ledit mécanisme dentraînement (10) comporte : un élément de base (20), un premier élément dentraînement (22) fixé par rapport à lél
