A drive mechanism for a drug delivery device, comprising: - a piston rod, which is displaced in a distal direction for a delivery of a dose and in a proximal direction for a reset, and - a resilient member which acts on the piston rod during the reset and tends to restrict the movement of the piston rod in the proximal direction, wherein - the resilient member is a biasing member provided for an automatic reset, wherein the drive mechanism further comprises - a rotatable drive member, which is threadedly engaged with the piston rod, - a housing - a lock nut releasably rotationally locked to the housing and threadedly engaged with the piston rod, and - a cartridge holder, which can be attached or removed, the lock nut being rotationally locked with the housing when the cartridge holder is attached and not being rotationally locked with the housing when the cartridge holder is removed, characterised in that - the drive member rotates during the delivery of the dose, wherein the biasing member is loaded by the rotation of the drive member during the delivery of the dose and tends to rotate the piston rod with respect to the drive member in such a way that the piston rod is screwed into the drive member and thereby advanced in the proximal direction, until the biasing member is relaxed and the movement of the piston rod in the proximal direction is stopped, - the biasing member is latched to the lock nut and to the drive member so that the biasing member is loaded when the drive member rotates relative to the housing during the delivery of the dose, and - the biasing member relaxes when the rotational locking of the lock nut is removed, resulting in a rotation of the lock nut and a corresponding rotation of the piston rod with respect to the drive member. - a piston rod, which is displaced in a distal direction for a delivery of a dose and in a proximal direction for a reset, and - a resilient member which acts on the piston rod during the reset and tends to restrict the movement of the piston rod in the proximal direction, wherein - the resilient member is a biasing member provided for an automatic reset, wherein the drive mechanism further comprises - a rotatable drive member, which is threadedly engaged with the piston rod, - a housing - a lock nut releasably rotationally locked to the housing and threadedly engaged with the piston rod, and - a cartridge holder, which can be attached or removed, the lock nut being rotationally locked with the housing when the cartridge holder is attached and not being rotationally locked with the housing when the cartridge holder is removed, characterised in that - the drive member rotates during the delivery of the dose, wherein the biasing member is loaded by the rotation of the drive member during the delivery of the dose and tends to rotate the piston rod with respect to the drive member in such a way that the piston rod is screwed into the drive member and thereby advanced in the proximal direction, until the biasing member is relaxed and the movement of the piston rod in the proximal direction is stopped, - the biasing member is latched to the lock nut and to the drive member so that the biasing member is loaded when the drive member rotates relative to the housing during the delivery of the dose, and - the biasing member relaxes when the rotational locking of the lock nut is removed, resulting in a rotation of the lock nut and a corresponding rotation of the piston rod with respect to the drive member.