The BD Ultra-Fine™ Nano Pen Needle is 32 gauge, 4 mm (5/32"). Is the shortest pen needle in the U. S. It can be safely used by both children and adults, providing reliable injections with no "pinch-up" of skin required. You should consult with a health care professional before switching to a shorter pen needle. ITEM CANNOT BE RETURNED IF PACKAGE HAS BEEN OPENED.