The present disclosure relates to a resettable drive mechanism for use in a drug delivery device. The drug delivery device is having a distal direction and a proximal direction. The drive mechanism comprises a drive rack engaged with a drive gear, a piston rack engaged with a piston gear and coupling means for coupling the drive gear with the piston gear. In a first state of the drive mechanism, the drive gear and the piston gear are coupled and the piston rack moves in distal direction when the drive rack is moved in distal direction. In a second state of the drive mechanism, the drive gear and the piston gear are decoupled and the piston rack is moveable in proximal direction for resetting the drive mechanism.