A drug delivery device, comprising: - a body having a receptacle for a drug; - a piston rod in the body, the piston rod being movable relatively to the body and being provided for expelling the drug out of the receptacle; - a mechanism (11, 12, 13, 14) provided to drive the piston rod while delivering the drug, the piston rod having a start position with respect to the mechanism; - a device (14, 17, 25) for guiding the movement of the piston rod relatively to the body while delivering the drug, the device for guiding the movement of the piston rod comprising a piston rod nut; - a device for releasing the piston rod from the device for guiding the movement, the device for releasing being provided to enable a reset of the piston rod to the start position, - a stop element provided for inhibiting a movement of the piston rod away from the start position, when the piston rod has been released, - a device (13, 25) for fixing the piston rod nut relatively to the body, the device for fixing the piston rod nut relatively to the body comprising a lock nut the lock nut being the stop element, - wherein the device for releasing the piston rod comprises a spring provided for releasing the piston rod nut from being fixed relatively to the body, wherein the receptacle is provided for inserting a cartridge, wherein the cartridge bars the stop element, and wherein the piston rod has a weight, which resets the piston rod by a gravitational force when the piston rod is released and the drug delivery device is directed in such a manner as to make the gravitational force drive the piston rod towards the start position. - a body having a receptacle for a drug; - a piston rod in the body, the piston rod being movable relatively to the body and being provided for expelling the drug out of the receptacle; - a mechanism (11, 12, 13, 14) provided to drive the piston rod while delivering the drug, the piston rod having a start position with respect to the mechanism; - a device (14, 17, 25) for guiding the movement of the piston rod relatively to the body while delivering the drug, the device for guiding the movement of the piston rod comprising a piston rod nut; - a device for releasing the piston rod from the device for guiding the movement, the device for releasing being provided to enable a reset of the piston rod to the start position, - a stop element provided for inhibiting a movement of the piston rod away from the start position, when the piston rod has been released, - a device (13, 25) for fixing the piston rod nut relatively to the body, the device for fixing the piston rod nut relatively to the body comprising a lock nut the lock nut being the stop element, - wherein the device for releasing the piston rod comprises a spring provided for releasing the piston rod nut from being fixed relatively to the body, wherein the receptacle is provided for inserting a cartridge, wherein the cartridge bars the stop element, and wherein the piston rod has a weight, which resets the piston rod by a gravitational force when the piston rod is released and the drug delivery device is directed in such a manner as to make the gravitational force drive the piston rod towards the start position.