The BD Ultra-Fine III Short Pen Needle is 31 gauge, 8 mm (5/16"). No pen needle is thinner. It is BD's best-selling pen needle. You should consult with a health care professional before switching to a shorter pen needle. Today's insulin needle is thinner and more delicate for greater comfort, so needle reuse can damage the tip and cause injury. May be used as replacement needles for all Type A insulin pen delivery devices including: