A drive mechanism for use in a drug delivery device is provided comprising: a housing (3, 7) having a helical thread, preferably an internal helical thread; an activation means that is non-rotatable with respect to the housing (3, 7); a drive sleeve wherein the activation means is designed to transmit force from a user to the drive sleeve and wherein the drive sleeve is coupled with the activation means for longitudinal travel and relative rotation between the drive sleeve and the activation means being allowed; a piston rod comprising a distal threaded portion and a proximal threaded portion characterized in that, a) the distal threaded portion of the said piston rod is threadedly engaged with the housing (3, 7); b) the proximal threaded portion of the piston rod is threadedly engaged with the drive sleeve; wherein the housing (3, 7) forms a track comprising a helical thread portion and a linear portion extending longitudinally, wherein the drive sleeve is engaged with the track; such that, c) when the activation means moves in a proximal direction away from a distal end of the drug delivery device the drive sleeve is engaged with the helical portion of the track such that the drive sleeve rotates with respect to the activation means and moves in the proximal direction; d) when the activation means moves in a distal direction towards a distal end of the drug delivery device the drive sleeve is engaged with the linear portion of the track such that the drive sleeve moves in the distal direction and is prevented from rotating with respect to the housing (3, 7) and the piston rod is caused to rotate with respect to the housing (3, 7) due to its engagement with the housing (3, 7) and the drive sleeve so that a force is transferred in the longitudinal direction to the distal end of the drug delivery device. a housing (3, 7) having a helical thread, preferably an internal helical thread; an activation means that is non-rotatable with respect to the housing (3, 7); a drive sleeve wherein the activation means is designed to transmit force from a user to the drive sleeve and wherein the drive sleeve is coupled with the activation means for longitudinal travel and relative rotation between the drive sleeve and the activation means being allowed; a piston rod comprising a distal threaded portion and a proximal threaded portion characterized in that, a) the distal threaded portion of the said piston rod is threadedly engaged with the housing (3, 7); b) the proximal threaded portion of the piston rod is threadedly engaged with the drive sleeve; wherein the housing (3, 7) forms a track comprising a helical thread portion and a linear portion extending longitudinally, wherein the drive sleeve is engaged with the track; such that, c) when the activation means moves in a proximal direction away from a distal end of the drug delivery device the drive sleeve is engaged with the helical portion of the track such that the drive sleeve rotates with respect to the activation means and moves in the proximal direction; d) when the activation means moves in a distal direction towards a distal end of the drug delivery device the drive sleeve is engaged with the linear portion of the track such that the drive sleeve moves in the distal direction and is prevented from rotating with respect to the housing (3, 7) and the piston rod is caused to rotate with respect to the housing (3, 7) due to its engagement with the housing (3, 7) and the drive sleeve so that a force is transferred in the longitudinal direction to the distal end of the drug delivery device. a) the distal threaded portion of the said piston rod is threadedly engaged with the housing (3, 7); b) the proximal threaded portion of the piston rod is threadedly engaged with the drive sleeve; wherein the housing (3, 7) forms a track comprising a helical thread portion and a linear portion extending longitudinally, wherein the drive sleeve is engaged with the track; such that, c) when the activation means moves in a proximal direction away from a distal end of the drug delivery device the drive sleeve is engaged with the helical portion of the track such that the drive sleeve rotates with respect to the activation means and moves in the proximal direction; d) when the activation means moves in a distal direction towards a distal end of the drug delivery device the drive sleeve is engaged with the linear portion of the track such that the drive sleeve moves in the distal direction and is prevented from rotating with respect to the housing (3, 7) and the piston rod is caused to rotate with respect to the housing (3, 7) due to its engagement with the housing (3, 7) and the drive sleeve so that a force is transferred in the longitudinal direction to the distal end of the drug delivery device.