See also injecting insulin.
Refillable vs. disposableEditInsulin pens come in 2 basic styles: prefilled which you discard after the insulin is used, or refillable. Refillable pens (Like the Novopen Junior ) are much like fountain pens; insulin cartridges are inserted and changed when empty. Disposable pens (like the Levemir FlexPen ) are cheaper but also less accurate -- the plunger mechanism on the disposable pens is a bit flimsy, and can give inaccurate doses by half a unit or so. You can use either type with a syringe, with some precautions as below.
As of January 1, 2010, Novo Nordisk discontinued the following cartridges for refillable pens: 
- Novolin® R PenFill® 3 mL cartridges (Regular, Human Insulin Isophane Suspension [recombinant DNA origin])
- Novolin® N PenFill® 3 mL cartridges (NPH, Human Insulin Isophane Suspension [recombinant DNA origin])
- Novolin® 70/30 PenFill® 3 mL cartridges (70% NPH, Human Insulin Isophane Suspension and 30% Regular, Human Insulin Injection [recombinant DNA origin])
MSD Animal Health has recently brought a refillable Caninsulin pen onto the market. Previously, no pens were designed for use with Lente-type insulins as the glass ball used to resuspend the insulin shattered the large Ultralente crystals present in both Ultralente and Lente insulins. The VetPen is designed to allow resuspension of the insulin without this occurrence. There are two sizes of refillable VetPens. One (blue cap) dispenses insulin doses from 1 to 16 units (IU) per dose in full unit increments. The other (tan cap) dispenses 0.5 (1/2) to 8 unit doses in full or half unit increments. Both pens use 2.7 ml insulin cartridges which hold a total of 108 units (IU) of U40 Caninsulin. The pen uses pen needles that are 29 gauge and 1/2" (12.7 mm) long. The pens are operated and maintained like those designed for human use. As with other insulin pens, the pen and its in-use cartridge do not need to be refrigerated.
Pens vs. vialsEdit
Before you decide to use a pen or not, you must first determine if the insulin you are using comes in one. Ultralente and PZI insulins do not. Lente-type insulins (Semilente, Ultralente, Lente) previously were unable to be dispensed in pen or cartridge form because the glass ball used to mix the insulin in these devices shattered the Lente crystals.  In Europe, things may be the other way round -- many insulins come only in pen form there, and if you wish to use syringes for their greater precision, you must use them with pen prefill cartridges.
The initial outlay for a refillable pen may cost more (around $40), and cartridges or disposable pens cost more per mL than vials. On the other hand, in the case where the insulin is expensive and expires quickly, as with Lantus, you may find yourself throwing away a lot of expired insulin -- it can then be cheaper to use pens, with their small 3ml cartridges. Cartridges may be used with syringes too.
Next you need to determine the pet's average insulin needs, and how precise the dose must be. Most pens inject full units of insulin only; some are designed to inject a minimum of two units with 1 unit increments. Smaller pets often require insulin doses that include half units or even smaller increments. If you decide on a refillable pen, keep in mind that Lilly insulin cartridges don't fit Novo's pens, and so on. The NovoPen 3 Demi  and Novopen Junior  (refillable) pens seem to be the only ones on the market for people allowing doses that include half-units, and they only work with Novo insulins.
This 1998 study,  aimed at discovering the accuracy of insulin pens to deliver small doses (less than 5 units) of U100 insulin to children, concluded that there can be problems with the accuracy of some pens regarding small insulin dosages. It notes the following problems regarding accuracy which are able to be controlled by the patient or caregiver:
- Removing the needle from the pen between injections.
- Doing an "air shot" each time before actually injecting insulin.
- Not leaving the needle in place for 5-10 seconds after injecting insulin may result in less insulin being delivered than the planned dose.
The study made the following observations:
- Insulin from any sort of cartridge device (refillable or disposable pens) is dispensed more slowly than when using syringes, because of the necessary compression action of all pens on cartridges.
- Low-dose insulin patients who switch from using syringes to pens may experience a significant change in insulin dosage.
- Children are more at risk of receiving accidental intramuscular injections, which results in faster absorption. (This would also hold true for pets, because of their smaller bodies.)
- The availability of U40 insulin would decrease the risks of hypoglycemia, as U40 insulins are able to be given in small doses with more accuracy than U100 insulins.
- Unlike syringes, pens can take a little while to squeeze out the last half-unit or so. If you remove the pen and see a drop of insulin at the needle tip, you have given slightly less insulin than you think. To avoid this, many pen makers specify leaving the pen under the skin for 5 to 10 seconds after injection.  This is usually not noticed in large humans, but is especially important for low-dose animals, since that extra half-unit may be a large proportion of the required dose.
- Most pens are now designed to hold 300 units (3ml) of insulin and do not need refrigeration after being started. Some insulins don't keep as well as they do in vial form; average time from first use to discarding/changing cartridges is as low as 14 days--even though there may be plenty of insulin left. Newer insulin analogs like Levemir and Lantus last about a month without refrigeration in their pens.
Pens with syringesEdit
Pen cartridge with a syringeEditIf you need more precise doses than a pen provides, you must use a syringe. Many penfill cartridges can be used with syringes, just like small vials,  but often require that you inject a similar volume of air into the cartridge every time you draw insulin. Once you've used a syringe on a cartridge, that cartridge should not be reinserted into a pen.
Disposable pen with a syringeEdit
- If you are stuck with a disposable pen (e.g. Novo Nordisk's Flexpen), and need the extra precision of a syringe, things get a little tricker. You must inject extra air BEFORE you draw out the insulin.
- Fill the syringe with air, empty it into the pen, then (with pen above syringe), suck the same amount of insulin back out. Do it this way whether you're transferring to a vial, extracting 1ml at a time, or just using the pen every day as a vial.
- Here's why -- as you remove the insulin from the Flexpen, the pressure inside the cartridge will drop, which will pull on the pen's screw-and-plunger mechanism, which doesn't expect to be pulled. (Just pushed).
- Eventually, the screw-and-plunger will break, possibly contaminating the insulin. So you must try to keep the pressure inside the pen about neutral or a bit positive (pushing back on the screw).
- Finally, don't ever try actually using the Flexpen again as a pen once you've begun this process! It is practically guaranteed to break eventually. Possibly spectacularly.
- Using a pen some days, and a syringe other days, has caused problems for some users. The dosages on the pen are not exactly the same as those shown on the syringe. If you must do it, keep one pen for syringe use, and another for pen use, and mark them clearly.
Along with the pens, you will need to purchase disposable screw-on pen needles. They come in lengths ranging from the standard 1/2' to the "ultra short" 3/16" length. Gauges are from 29-31.  Not all manufacturer's pen needles fit all pens. Novo's NovoFine pen needles fit only Novo's pens, etc. These needles can do double duty as lancets after being used, though, so consider their cost versus that of syringes AND lancets.
Ultra-Fine pen needles fitting all US-available pens are available in the following sizes/gauges in the US:
- 29g, 1/2" (The "standard"; this is the usual length and gauge seen on syringes),
- 31g 5/16" (short),
- 31g 3/16" (classed as mini--shorter than "short").
In Europe, Novo Nordisk has just launched the even thinner
- NovoFine® 32G Tip, 0.23/0.25 mm x 6 mm
Length of needle for a cat or dog is very much an individual preference. The injection technique, thickness of skin and fat layer, and injection site details of each pet may require a particular length, or may be more flexible. Once you find a length that works for your pet, it may be more difficult to change than you think.
- ↑ Flex Pen illustration. RxList.com.
- ↑ Optiset pen illustration page 2. Novo Nordisk.
- ↑ 3.0 3.1 Insulin injection Tips-Pens. BD Diabetes.
- ↑ 4.0 4.1 Novopen Junior. Novo Nordisk.
- ↑ Levemir FlexPen. Novo Nordisk.
- ↑ NovoPen 3 Demi. Novo Nordisk.
- ↑ Novo cartridge discontinuation notice. Novo Nordisk.
- ↑ larger view of VetPen technical details. Merck Animal Health.
- ↑ S. Burgaud; R. Guillot; G. Harnois-Milon (2012). Clinical Evaluation of a Veterinary Insulin Pen in Diabetic Dogs. BSAVA.
- ↑ S. Burgaud; S. Riant; N. Piau (2012). Comparative Laboratory Evaluation of Dose Delivery Using a Veterinary Insulin Pen. BSAVA.
- ↑ Hanas, Ragnar (1999). Insulin-Dependent Diabetes-Page 10. Children With Diabetes.
- ↑ 12.0 12.1 Frequently Asked Questions-VetPen. MSD Animal Health.
- ↑ BD Diabetes Pen Needle sizes.
- ↑ About VetPen. Merck Animal Health.
- ↑ Caninsulin Video use and care instructions. Merck Animal Health.
- ↑ VetPen FAQs. Merck Animal Health.
- ↑ Caninsulin product datasheet. MSD Animal Health.
- ↑ Hanas, Ragnar (1999). Insulin-Dependent Diabetes-Page 10. Children With Diabetes.
- ↑ Novopen 3 Demi. Novo Nordisk.
- ↑ Novopen Junior. Novo Nordisk.
- ↑ Accuracy & Reproducibility of Low-Dose Insulin Administration Using Pen-injectors & Syringes. British Medical Journal (sign up free to read).
- ↑ Tips for Injecting Insulin. Joslyn Diabetes Center.
- ↑ Drawing insulin from cartridge photo 1.
- ↑ Drawing insulin from cartridge photo 2.
- ↑ Diabetes Health chart of all available insulin pen needles. Diabetes Health.
- ↑ Insulin Pen Needles. BD Diabetes.
- ↑ Insulin Pen Needles. ReliOn.
- Treatment of Diabetes Mellitus in Dogs Using Isophane Insulin Penfills Thoresen SI, Lorenzen FH., 1997, Acta Veterinaria Scandanavica
- Insulin Pen Overview Diabetes123
- Compare insulin pens and other supplies here Children With Diabetes
- This Diabetes Health Guide compares all currently marketed (US) insulin pens--refillable and disposable Diabetes Health
- Diabetes UK's list of available insulin pens Diabetes UK
- BD Diabetes
- More Tips on Drawing Insulin From a Disposable Pen
- More Information on Using Syringes on Pen Cartridges
- Using Insulin Pen Cartridges/Penfills with Syringes-Page 2 BC Childrens Hospital