An add-on device adapted to be releasably mounted on a drug delivery device, the add-on device comprising an outer assembly with an add-on dose setting member adapted to be coupled to the outer housing of a pen housing, as well as an inner assembly engaging the pen dose setting member and being in rotational engagement with the add-on dose setting member during dose setting. The invention addresses the issue of preventing transfer of movement from the add-on dose setting member to the inner assembly during dose expelling, which in accordance with the invention is realized by rotationally de-coupling the inner and outer assembly during dose expelling.
