Abstract: Add-on device for a drug delivery pen device, comprising a drive member adapted for coupling to a dose dial member of the pen device, a dial member adapted to rotate the drive member in first and second directions to set/adjust a pen dose, a release member axially moveable to actuate a pen device release button, a linear-to-rotational converter mechanism adapted to convert axial movement of the release member to rotational movement of the drive member in a first direction, whereby, when the add-on device is mounted on the drug delivery pen device: the dose dial member of the pen device is biased in the first direction by the drive member thereby taking up rotational slack in the pen device.