A drive mechanism for a medication delivery device (1) is proposed, comprising a housing (13,17,40), a rotation member (21) which is adapted to be rotated in a first direction with respect to the housing during setting of a dose of a medication and to be rotated in a second direction during delivery of the dose, the second direction being opposite to the first direction, a piston rod (12) which is adapted to be displaced in a distal direction with respect to the housing for delivering the dose, a drive member (20) which follows rotational movement of the rotation member in the second direction during delivery of the dose, and a stop member (26) which prevents rotational movement of the drive member with respect to the housing in the first direction during setting of the dose, wherein the rotational movement of the drive member in the second direction is converted into movement of the piston rod in the distal direction with respect to the housing.