A drive mechanism for use in a drug delivery device is provided comprising: a housing (103, 203) having a proximal and a distal end; a drive member (114, 214) located within the said housing (103, 203) such that the said drive member (114, 214) is movable longitudinally and is non-rotatable with respect to the said housing (103, 203); a piston rod (110, 210) that is non-rotatable with respect to the said housing (103, 203) and having at least one set of teeth (109, 209), and is adapted to operate through the housing (103, 203) and transfer a force in the longitudinal direction to the distal end of the drug delivery device; a rotating means releasably engaged with the said piston rod (110, 210) and engaged to the said drive member (114, 214) and engaged to the said housing (103, 203) characterized in that the rotating means is selected from the group of (i) to (ii), (i) a gear the axle of which is engaged with the set of teeth of the piston rod, (ii) a pulley comprising a belt and a wheel the axle of which is engaged with the set of teeth of the piston rod; wherein, a) when the said drive member (114, 214) moves proximally with respect to the said housing (103, 203) the said rotating means moves proximally with respect to the said piston rod (110, 210); b) when the said drive member (114, 214) moves distally the said rotating means moves distally displacing the said piston rod (110, 210) towards the distal end of the device. a housing (103, 203) having a proximal and a distal end; a drive member (114, 214) located within the said housing (103, 203) such that the said drive member (114, 214) is movable longitudinally and is non-rotatable with respect to the said housing (103, 203); a piston rod (110, 210) that is non-rotatable with respect to the said housing (103, 203) and having at least one set of teeth (109, 209), and is adapted to operate through the housing (103, 203) and transfer a force in the longitudinal direction to the distal end of the drug delivery device; a rotating means releasably engaged with the said piston rod (110, 210) and engaged to the said drive member (114, 214) and engaged to the said housing (103, 203) characterized in that the rotating means is selected from the group of (i) to (ii), (i) a gear the axle of which is engaged with the set of teeth of the piston rod, (ii) a pulley comprising a belt and a wheel the axle of which is engaged with the set of teeth of the piston rod; (i) a gear the axle of which is engaged with the set of teeth of the piston rod, (ii) a pulley comprising a belt and a wheel the axle of which is engaged with the set of teeth of the piston rod; wherein, a) when the said drive member (114, 214) moves proximally with respect to the said housing (103, 203) the said rotating means moves proximally with respect to the said piston rod (110, 210); b) when the said drive member (114, 214) moves distally the said rotating means moves distally displacing the said piston rod (110, 210) towards the distal end of the device. a) when the said drive member (114, 214) moves proximally with respect to the said housing (103, 203) the said rotating means moves proximally with respect to the said piston rod (110, 210); b) when the said drive member (114, 214) moves distally the said rotating means moves distally displacing the said piston rod (110, 210) towards the distal end of the device.