The present invention provides a drug delivery device comprising a housing (2) extending along a longitudinal axis, a dose dial member (3) being rotatable relative to the housing (2) to set a dose of drug to be expelled from an attached reservoir and comprising a first serrated transversal surface (7), a second serrated transversal surface (16) being axially and rotationally fixed with respect to the housing (2), and being configured for slipping engagement with the first serrated transversal surface (7) in a bi-directional ratchet interface in response to the dose dial member (3) being rotated relative to the housing (2) to increase or decrease the dose, and a dose delivery mechanism for expelling a set dose, the dose delivery mechanism comprising a piston rod (50), a torsion spring (30) for providing energy to drive the piston rod (50), an activation button (8) being axially movable relative to the housing (2) between an inactivated position in which the torsion spring (30) is retained and an activated position in which the torsion spring (30) is released to cause an expelling of the set dose, the activation button (8) being biased towards the inactivated position by a bias structure (40) arranged to act between the dose dial member (3) and the activation button (8), the bias structure (40) thereby further biasing the first serrated transversal surface (7) towards the second serrated transversal surface (16), and a drive member (20) being rotationally biased by the torsion spring (30), wherein the drive member (20) is axially fixed to the activation button (8) and axially movable relative to the housing (2) between a dose setting position in which the drive member (20) is rotationally fixed to the dose dial member (3) and a dose expelling position in which the drive member (20) is rotationally freed from the dose dial member (3) and rotationally interlocked with the piston rod (50).
