A drug delivery device comprising a housing, a piston rod, a transmission member, a helical torsion drive spring coupled to the transmission member and the housing, and a drive member in engagement with the piston rod and adapted to be rotated by the transmission member to thereby move the piston rod distally during expelling. The device further comprises dose setting means allowing a user to simultaneously set a dose and strain the drive spring correspondingly. The drive spring comprises a plurality of wire windings and is formed from a wire having a rectangular cross section wound on edge with the height larger than the thickness. The spring is operated such that the tilt angle for at least a portion of the rectangular wire will increase when the drive spring is strained from an initial strained state to a fully strained state, this resulting in a corresponding non-linear spring characteristic.
