An adapter for use with a drug delivery device having a reservoir for containing a product, the reservoir having a distally projecting end-piece defining a passageway for the transfer of the product from the reservoir, includes a collar engageable with the end-piece, an adaptor securing member limiting translational movement of the collar with respect to the end-piece once the collar is engaged with the end-piece, at least one adaptor anti-rotation member limiting the rotation of the collar with respect to the end-piece once the collar is engaged with the end-piece, at least one deactivating member neutralizing the adaptor anti-rotation member in response to a turning torque equal or greater than a predetermined value applied to the collar when the collar is engaged with the end-piece, thereby allowing the rotation of the collar with respect to the end-piece.