Drug delivery device with variable piston force

A drug delivery device, comprising:

• a housing,
• a cartridge containing a drug in a quantity sufficient for a plurality of doses of the drug,
• a bung movably retained within the cartridge to dispense a dose of the drug from the cartridge upon movement of the bung with respect to the cartridge,
• a drive mechanism comprising a piston rod, the drive mechanism being operable to transfer a driving force to the bung to dispense the dose of the drug from the cartridge, wherein the drug delivery device is configured such that a maximal driving force which is transferrable to the bung via the drive mechanism varies and is adjusted to a current position of the bung within the cartridge, and
• an energy storage member adapted to store energy which, when released, provides at least a fraction of the driving force or the whole driving force, wherein the bung is displaceable with respect to the cartridge from an initial position via an intermediate position, in which the cartridge is partly emptied, to an end position, and wherein the maximal driving force is greater when the bung is in the initial position or between the initial position and the intermediate position than when the bung is in the intermediate position or between the intermediate position and the end position,
• wherein the piston rod is configured to transfer the driving force to the bung and wherein the piston rod comprises a thread, the thread having a variable pitch or a variable lead, wherein the thread has a distal section facing a distal end of the piston rod and a proximal section being arranged farther away from the distal end of the piston rod than the distal section, wherein in the distal section the pitch of the thread or the lead of the thread is less than the pitch of the thread or the lead of the thread in the proximal section.