A needle assembly for an injection pen (22) has a sleeve (21) supporting internally a double-ended needle having an injection end and a non-injection end (25). The end of the sleeve surrounding the non-injection end (25) of the needle is divided into deformable segments (28) and is internally threaded for interengagement with a threaded boss (23) of the injection pen (22). A collar (29) is mounted on the sleeve (21) for sliding movement between a first position where the collar (29) overlies the segmented end and maintains interengagement of the threads at that end with those of the injection pen boss (23) and a second position where the segments may expand radially to come free of the injection pen boss (23). The needle may be mounted on a carrier (68) slidable with respect to the sleeve (21), the carrier (68) being coupled to the sleeve (21) for movement thereby.
