A drug delivery device, comprising: a housing, a cartridge, the cartridge containing a drug in a quantity sufficient for a plurality of doses of the drug, a bung, the bung being 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, and a drive mechanism, 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 the maximal driving force which is transferrable to the bung via the drive mechanism varies and is adjusted to the current position of the bung within the cartridge, wherein the drive mechanism comprises a piston rod configured to transfer the driving force to the bung, wherein the piston rod comprises a thread, the thread having a variable pitch and/or a variable lead, wherein the bung is displaceable with respect to the cartridge from an initial position via an intermediate position, when the cartridge is partly emptied, to an end position, characterised in that the maximal driving force is greater when the bung is in the initial position and/or between the initial position and the intermediate position than when the bung is in the intermediate position and/or between the intermediate position and the end position, wherein the thread has a distal section facing a distal end of the piston rod and a proximal section being arranged further away from the distal end of the piston rod than the distal section, wherein in the distal section the pitch of the thread and/or the lead of the thread is less than the pitch of the thread and/or the lead of the thread in the proximal section and wherein the maximal driving force varies between two subsequent doses, the maximal driving force being greater for the first dose which is dispensed from the cartridge than for any subsequent dose which is dispensed from the cartridge. a housing, a cartridge, the cartridge containing a drug in a quantity sufficient for a plurality of doses of the drug, a bung, the bung being 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, and a drive mechanism, 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 the maximal driving force which is transferrable to the bung via the drive mechanism varies and is adjusted to the current position of the bung within the cartridge, wherein the drive mechanism comprises a piston rod configured to transfer the driving force to the bung, wherein the piston rod comprises a thread, the thread having a variable pitch and/or a variable lead, wherein the bung is displaceable with respect to the cartridge from an initial position via an intermediate position, when the cartridge is partly emptied, to an end position, characterised in that the maximal driving force is greater when the bung is in the initial position and/or between the initial position and the intermediate position than when the bung is in the intermediate position and/or between the intermediate position and the end position, wherein the thread has a distal section facing a distal end of the piston rod and a proximal section being arranged further away from the distal end of the piston rod than the distal section, wherein in the distal section the pitch of the thread and/or the lead of the thread is less than the pitch of the thread and/or the lead of the thread in the proximal section and wherein the maximal driving force varies between two subsequent doses, the maximal driving force being greater for the first dose which is dispensed from the cartridge than for any subsequent dose which is dispensed from the cartridge.