A drive mechanism for use in a drug delivery device comprising: a housing having a proximal and distal end; a drive sleeve non-rotatable with respect to said housing , wherein the drive sleeve comprises a helical thread ; a piston rod comprising a distal helical thread portion threadedly engaged with the said housing ; characterized in that , a) the piston rod comprises one or more flexible arms located at the proximal end of the said piston rod for releasable engagement to the helical thread on the said drive sleeve ; wherein the drive mechanism is configured such that when the said drive sleeve moves in a proximal direction away from a distal end of the said housing , the one or more flexible arms disengage from the helical thread and the said piston rod does not move with respect to the said housing ; and b) wherein at the end of the proximal movement of the drive sleeve the one or more flexible arms engage with the helical thread such that when the said drive sleeve moves in a distal direction towards a distal end of the said housing the said piston rod rotates with respect to the said housing due to its engagement with the housing and the drive sleeve so that a force is transferred in the longitudinal direction to the distal end of the said drug delivery device . a housing having a proximal and distal end; a drive sleeve non-rotatable with respect to said housing , wherein the drive sleeve comprises a helical thread ; a piston rod comprising a distal helical thread portion threadedly engaged with the said housing ; characterized in that , a) the piston rod comprises one or more flexible arms located at the proximal end of the said piston rod for releasable engagement to the helical thread on the said drive sleeve ; wherein the drive mechanism is configured such that when the said drive sleeve moves in a proximal direction away from a distal end of the said housing , the one or more flexible arms disengage from the helical thread and the said piston rod does not move with respect to the said housing ; and b) wherein at the end of the proximal movement of the drive sleeve the one or more flexible arms engage with the helical thread such that when the said drive sleeve moves in a distal direction towards a distal end of the said housing the said piston rod rotates with respect to the said housing due to its engagement with the housing and the drive sleeve so that a force is transferred in the longitudinal direction to the distal end of the said drug delivery device . a) the piston rod comprises one or more flexible arms located at the proximal end of the said piston rod for releasable engagement to the helical thread on the said drive sleeve ; wherein the drive mechanism is configured such that when the said drive sleeve moves in a proximal direction away from a distal end of the said housing , the one or more flexible arms disengage from the helical thread and the said piston rod does not move with respect to the said housing ; and b) wherein at the end of the proximal movement of the drive sleeve the one or more flexible arms engage with the helical thread such that when the said drive sleeve moves in a distal direction towards a distal end of the said housing the said piston rod rotates with respect to the said housing due to its engagement with the housing and the drive sleeve so that a force is transferred in the longitudinal direction to the distal end of the said drug delivery device . <EOS>