A drug delivery device (100, 200) for delivering a fixed dose, wherein the drug delivery device comprises: - a housing assembly, - a drug reservoir with a piston - a drive mechanism comprising a piston rod (109, 209) operationally arranged in the housing assembly and adapted for advancing the piston to expel a drug during dosing, and a drive tube (180, 280) operationally arranged with the piston rod (109, 209), wherein the drive tube (180, 280) is adapted to be axially movable relative to the housing assembly and rotationally blocked in a stationary state, and adapted to be axially movable and rotatable relative to the housing in a dosing state, wherein the drive mechanism is adapted for axial movement and rotation of the drive tube (180, 280) in the dosing state and thereby advance the piston rod (109, 209) to deliver the fixed dose in response to activation; - an activation mechanism for activating the drive mechanism, in response to moving the drive tube in an axial direction, wherein the housing assembly comprises a guide structure for guiding the drive tube during activation and delivery of a dose, wherein the guide structure comprises an axial portion (156, 256) providing a sliding surface for guiding the drive tube during activation, and for providing a rotational stop defining an end of dose position, and a circumferential portion (157, 257) providing a sliding surface adapted for guiding the drive tube (180, 280) during dosing; wherein the drive tube (180, 280) comprises a guide structure comprising an axial portion (182, 282) adapted for slidably engaging the corresponding axial portion of the housing assembly for activation, and a circumferential portion (189, 289) for slidably engaging the corresponding circumferential portion of the housing assembly during dosing, whereby the drive tube (180, 280) can be guided along the axial portion (156, 256) without rotation and thereby changed from the stationary state to the dosing state during activation, whereby the drive tube (180, 280) can be moved axially and rotated by the drive mechanism in the dosing state, and whereby the drive tube (180, 280) can be guided along the circumferential portion (157, 257) until the end of dose position and thereby changed from the dosing state to the stationary state, in response to activation of the drive mechanism. - a housing assembly, - a drug reservoir with a piston - a drive mechanism comprising a piston rod (109, 209) operationally arranged in the housing assembly and adapted for advancing the piston to expel a drug during dosing, and a drive tube (180, 280) operationally arranged with the piston rod (109, 209), wherein the drive tube (180, 280) is adapted to be axially movable relative to the housing assembly and rotationally blocked in a stationary state, and adapted to be axially movable and rotatable relative to the housing in a dosing state, wherein the drive mechanism is adapted for axial movement and rotation of the drive tube (180, 280) in the dosing state and thereby advance the piston rod (109, 209) to deliver the fixed dose in response to activation; - an activation mechanism for activating the drive mechanism, in response to moving the drive tube in an axial direction, wherein the housing assembly comprises a guide structure for guiding the drive tube during activation and delivery of a dose, wherein the guide structure comprises an axial portion (156, 256) providing a sliding surface for guiding the drive tube during activation, and for providing a rotational stop defining an end of dose position, and a circumferential portion (157, 257) providing a sliding surface adapted for guiding the drive tube (180, 280) during dosing; wherein the drive tube (180, 280) comprises a guide structure comprising an axial portion (182, 282) adapted for slidably engaging the corresponding axial portion of the housing assembly for activation, and a circumferential portion (189, 289) for slidably engaging the corresponding circumferential portion of the housing assembly during dosing, whereby the drive tube (180, 280) can be guided along the axial portion (156, 256) without rotation and thereby changed from the stationary state to the dosing state during activation, whereby the drive tube (180, 280) can be moved axially and rotated by the drive mechanism in the dosing state, and whereby the drive tube (180, 280) can be guided along the circumferential portion (157, 257) until the end of dose position and thereby changed from the dosing state to the stationary state, in response to activation of the drive mechanism.