Drive mechanism for a drug delivery device

According to one aspect, a resettable drive assembly for a drug delivery device is provided for. The resettable drive assembly may comprise a housing having a proximal end and a distal end, a piston rod being rotatable with respect to the housing and being axially displaceable with respect to the housing between a proximal start position and a distal end position, a drive member for distally displacing the piston rod towards the end position when dispensing a dose, and a stop member. The drive assembly may be configured such that the drive member is operable to interact with the piston rod for forming an unlockable first interlock, the first interlock being operable to block proximal movement of the piston rod with respect to the drive member. The drive assembly may be configured such that the stop member is operable to interact with the piston rod for forming an unlockable second interlock, the second interlock being operable to block proximal movement of the piston rod with respect to the housing. The drive assembly may be configured such that, when the drive assembly is in a drive mode, the first and second interlocks are locked such that proximal movement of the piston rod from the end position to the start position is prevented by the first interlock and the second interlock. The drive assembly may be configured such that, for switching the drive assembly from the drive mode to a reset mode, the piston rod is rotatable with respect to the drive member for unlocking the first interlock and the stop member and the piston rod are rotatable with respect to each other for unlocking the second interlock. The drive assembly may be configured such that, when the drive assembly is in the reset mode, the first interlock and the second interlock are unlocked such that proximal movement of the piston rod to the start position is allowed. Further, a drug delivery device and a use of a spring are provided for.