U40 syringes and U100 syringes are NOT interchangeable You can only accurately select a U100 insulin dose with a U100 syringe It is very important that you select the correct dosing system syringe in order to not overdose or underdose insulin to your dog or cat How do you use UltiCare U100 Insulin Syringes? PetTest U40 Syringes, 31 Gauge x 5/16in are some of the thinnest, featuring 31gauge needles This helps make injection more comfortable for your furry family member compared to thicker 29gauge U40 needles The length of the 5/16" needle also helps reduce the risk of a painful injection to your pet's muscleThese items are shipped from and sold by different sellers Show details Hide details Choose items to buy together This item Brandzig U100 Pet Insulin Syringes 29G 3/10cc, 1/2" 100Pack $1999 ($0/Count) In Stock Sold by Brandzig and ships from Amazon Fulfillment FREE Shipping on orders over $2500