A drug delivery device comprising or adapted to receive a drug-filled cartridge, comprising: - a housing, - an expelling assembly comprising: - a piston rod adapted to engage and axially displace a piston in a loaded cartridge in a distal direction to thereby expel a dose of drug from the cartridge, the piston rod having a proximal-most position, - a drive structure at least partially accommodating the piston rod, - a drive member arranged in engagement with the piston rod and adapted to rotate the piston rod, - a helical drive spring arranged in engagement with the housing and the drive structure, - setting means (280, 380) allowing a user to simultaneously set a dose amount to be expelled and strain the drive spring correspondingly by rotation of the drive structure, - a scale drum helically coupled to the housing as well as axially moveable but non-rotatably coupled to the drive structure, the scale drum thereby being moved helically as the drive structure is rotated, - coupling means (610, 663) actuatable between a dose setting state in which the drive structure can be rotated to a set position, and an expelling state in which the drive structure driven by the drive spring can rotate the drive member, and - release means (290, 390) actuatable between a dose setting state and an expelling state to thereby actuate the coupling means, - wherein the drive spring can be released to rotate the drive structure and thereby the drive member to thereby rotate and move the piston rod in the distal direction, characterized in that the drive structure is in the form of a drive tube, - wherein the scale drum is coupled to the drive tube via a spline connection (265, 261, 662, 672), the spline connection during dose setting having a proximal-most position corresponding to a proximal-most position of the scale drum, - wherein the drive spring is arranged proximally of the spline connection when the spline connection is in its proximal-most position, and - wherein the drive spring with the scale drum in its proximal-most position being arranged at least partially between the scale drum and the drive tube. - a housing, - an expelling assembly comprising: - a piston rod adapted to engage and axially displace a piston in a loaded cartridge in a distal direction to thereby expel a dose of drug from the cartridge, the piston rod having a proximal-most position, - a drive structure at least partially accommodating the piston rod, - a drive member arranged in engagement with the piston rod and adapted to rotate the piston rod, - a helical drive spring arranged in engagement with the housing and the drive structure, - setting means (280, 380) allowing a user to simultaneously set a dose amount to be expelled and strain the drive spring correspondingly by rotation of the drive structure, - a scale drum helically coupled to the housing as well as axially moveable but non-rotatably coupled to the drive structure, the scale drum thereby being moved helically as the drive structure is rotated, - coupling means (610, 663) actuatable between a dose setting state in which the drive structure can be rotated to a set position, and an expelling state in which the drive structure driven by the drive spring can rotate the drive member, and - release means (290, 390) actuatable between a dose setting state and an expelling state to thereby actuate the coupling means, - a piston rod adapted to engage and axially displace a piston in a loaded cartridge in a distal direction to thereby expel a dose of drug from the cartridge, the piston rod having a proximal-most position, - a drive structure at least partially accommodating the piston rod, - a drive member arranged in engagement with the piston rod and adapted to rotate the piston rod, - a helical drive spring arranged in engagement with the housing and the drive structure, - setting means (280, 380) allowing a user to simultaneously set a dose amount to be expelled and strain the drive spring correspondingly by rotation of the drive structure, - a scale drum helically coupled to the housing as well as axially moveable but non-rotatably coupled to the drive structure, the scale drum thereby being moved helically as the drive structure is rotated, - coupling means (610, 663) actuatable between a dose setting state in which the drive structure can be rotated to a set position, and an expelling state in which the drive structure driven by the drive spring can rotate the drive member, and - release means (290, 390) actuatable between a dose setting state and an expelling state to thereby actuate the coupling means, - wherein the drive spring can be released to rotate the drive structure and thereby the drive member to thereby rotate and move the piston rod in the distal direction, characterized in that the drive structure is in the form of a drive tube, - wherein the scale drum is coupled to the drive tube via a spline connection (265, 261, 662, 672), the spline connection during dose setting having a proximal-most position corresponding to a proximal-most position of the scale drum, - wherein the drive spring is arranged proximally of the spline connection when the spline connection is in its proximal-most position, and - wherein the drive spring with the scale drum in its proximal-most position being arranged at least partially between the scale drum and the drive tube.