Using insulin pens with their cartridges can be very convenient. As an alternative to insulin in vial form and syringes, they can give you confidence, or help if you have visual or dexterity impairments[1]. They are less prone to dosage error, though a pen is not accurate when dosage is 3 units or less. They can be a big help if you are new to diabetes and a bit unsure about things. Pens are the predominant insulin delivery system in most of the world, except the United States, where syringes and insulin vials still dominate.

See also injecting insulin.


Dialing a dose on an insulin pen. Click for large image.

Refillable vs. disposableEdit

Insulin pens come in 2 basic styles: prefilled which you discard after the insulin is used, or refillable. Refillable pens (Like the Novopen Junior[2]) are much like fountain pens; insulin cartridges are inserted and changed when empty. Disposable pens (like the Levemir FlexPen[3]) 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.

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. Lente, Ultralente, and PZI insulins do not, as they are not able to be properly resuspended for injecting in pen form. 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 Novopem 3 Demi[4] and Novopen Junior[5] (refillable) pens seem to be the only ones on the market allowing doses that include half-units, and they only work with Novo insulins.

This 1998 study[6], 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.

For more on this, see Fine doses.

Pen usageEdit

  • 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[7][8]. 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.
  • All "cloudy" (NPH, NPH Mixes, Analog Mixes) insulins available in pen form need to be re-suspended before use, just like their vialed counterparts. See the manufacturer's instructions regarding proper re-suspension technique for your brand of pen/type of insulin.

Pens with syringesEdit

Pen cartridge with a syringeEdit

  • If 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.



A prefilled Novo insulin pen, with needles. Click to zoom.

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.

Both BD[9] and ReliOn[10] produce "universal" pen needles which fit all major brands of pens in the US.

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.

Further ReadingEdit




  1. BD Diabetes-Insulin injection Tips-Pens
  2. Novo Nordisk-Novopen Junior
  3. Novo Nordisk-Levemir FlexPen
  4. Novo Nordisk-Novopen 3 Demi
  5. Novo Nordisk-Novopen Junior
  6. BMJ Journals-Accuracy & Reproducibility of Low-Dose Insulin Administration Using Pen-injectors & Syringes
  7. Joslyn Diabetes Center-Tips for Injecting Insulin
  8. BD Diabetes-Insulin Injection Tips-Pens
  9. BD Website: Insulin Pen Needles
  10. ReliOn Website-Insulin Pen Needles
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