Go to site For Pet Owners


Photo of VetPen


VetPen from Merck Animal Health is the first and only veterinary insulin pen for use in the treatment of diabetes mellitus in cats. To avoid dosing errors, VetPen must be used with 40 IU/mL, 2.7 mL Vetsulin cartridges.

When loading a new cartridge into VetPen, priming is necessary to remove air bubbles. And proper steps to prepare VetPen must be done before each injection. Click here for information on priming and preparing VetPen.

VetPen is available in 2 sizes: 8 IU VetPen with dosing increments of 0.5 IU and 16 IU VetPen with dosing increments of 1 IU. Vetsulin® (porcine insulin zinc suspension) (2.7 mL) cartridges are available in boxes of 10.

Advise pet owners not to reuse VetPen needles. After a single use, needles should be disposed of in approved sharps or biohazard containers.

Why U-40 syringes must be used with Vetsulin

Because Vetsulin has a concentration of 40 IU per mL of solution, a U-40 syringe or VetPen must be used to achieve correct dosing. USE OF A SYRINGE OTHER THAN A U-40 SYRINGE OR USE OF A 100 IU/mL CARTRIDGE OR HUMAN INSULIN PEN WILL RESULT IN INCORRECT DOSING.

Some pet owners may try to purchase syringe and insulin supplies from their local human pharmacies. Pharmacies carry U-100 syringes only. They do not stock either U-40 syringes, VetPen, or Vetsulin. And most pharmacists are not even aware of U-40 syringes, VetPen, or Vetsulin.

U-100 syringes and human insulin pens will deliver less than half the required Vetsulin dose

Human insulin is formulated at a concentration of 100 IU/mL. As a result, using a U-100 syringe or insulin pen designed for human use with Vetsulin will deliver two and a half times less insulin than required. Conversely, if a client uses a U-40 syringe or VetPen with a 100 IU/mL insulin preparation, he or she would inject about two and a half times the amount of insulin needed. This could cause fatal hypoglycemia.

Strongly encouraging clients to purchase Vetsulin, U-40 syringes, and VetPen from your veterinary clinic is the best way to protect your patients from incorrect insulin dosing. Remind your clients that using U-100 syringes and insulin pens from a human pharmacy to administer Vetsulin will result in incorrect insulin dosing, which puts their pet in danger of experiencing serious health consequences.


To help your clients prepare and administer Vetsulin to their cats, there are resources available that guide them step-by-step through the process: