Diabetes in Pets
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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. They are less prone to dosage error, too. They can be a big help if you are new to diabetes and a bit unsure about things.

See also injecting insulin.

InsulinPenDial

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


Refillable vs. disposable

Insulin pens come in 2 basic styles: prefilled which you discard after the insulin is used, or refillable. Refillable pens are much like fountain pens; insulin cartridges are inserted and changed when empty. Refillable pens are usually cheaper in the long run, and also more accurate -- the plunger mechanism on the disposable pens is a bit flimsy, and can give inaccurate doses by half a unit or so.

Pens vs. vials

Availability

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.

Cost

The initial outlay for a refillable pen may cost more (around $50-60), and cartridges or refillable 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.

Precision

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 allowing doses that include half-units, and they only work with Novo insulins.

Pen usage

  • 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, may pen makers specify leaving the pen under the skin for 6 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.
  • 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.
  • Most pens are now designed to hold 300ml 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.

Needles

InsulinPenandNeedles

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 and ReliOn 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 (5/16")

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.

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