A drive mechanism for a drug delivery device comprises a housing having a proximal end and a distal end and a piston rod, which is adapted to be displaced in a distal direction with respect to the housing when dispensing a drug dose. A drive member is adapted to be rotated when dispensing the drug dose, wherein the drive mechanism is configured to convert rotational movement of the drive member into a movement of the piston rod in the distal direction with respect to the housing. A stop member is adapted to prevent a rotational movement of the drive member with respect to the housing when setting a drug dose. The drive member comprises a drive portion being coupled to the piston rod and a first coupling portion which couples the drive portion and the stop member, wherein the drive mechanism is configured to be switchable between a set and dispense mode of operation and a reset mode of operation, and wherein, in the set and dispense mode, said first coupling portion is bi-directionally coupled to the drive portion for setting and dispensing the drug dose, and wherein, in the reset mode, said first coupling portion is decoupled from the drive portion to allow a reset operation to be performed for the drive mechanism.
