A drive mechanism for a delivery device is presented. The drive mechanism comprises a housing (24) and a piston rod (6) having a longitudinal axis (x) and a piston rod feature (17). The drive mechanism further comprises a drive member (1) being movable with respect to the housing (24). The drive member (1) comprises a drive feature (15). The drive feature (15) is selectively engageable with the piston rod feature (17) by the radial displacement of the drive member (1) with respect to the piston rod (6). When the drive member (1) is in the first position, the drive member (1) is disengaged from the piston rod (6), and when the drive member (1) is in the second position, the drive feature (15) is engaged with the piston rod feature (17). When the drive member (1) moves distally with respect to the housing (24), said distal movement is transferred to the piston rod (6) such that the piston rod (6) moves distally with respect to the housing (24).