Cartridges are sealed plastic containers that hold insulin for refillable insulin pens. Most contain 3ml (300U normally) of insulin. Cartridges are made to fit only certain types of pens; if you have a Novo Nordisk pen, neither Eli Lilly nor Aventis insulin cartridges will fit it.
In Europe, 3 ml cartridges may be more commonly sold than vials. Certain Analog, r-DNA/GE/GM, and some CP Pharma bovine and porcine insulins are available in cartridge form. Aventis' Lantus is available in Europe in packages of three, versus packages of five available in the U.S.
In the US, Levemir, Lantus, and some other brands are available in either cartridge or prefilled pen form. The reason for choosing this format is price. For cats and small dogs who seldom finish an entire 10ml vial, 3ml cartridges are much more economical to try to use up before they expire.
Cartridges (at least according to Novo Nordisk) are actually made of glass, and coated with plastic, to avoid interaction between the insulin and the plasticizer chemicals. So essentially they are small vials. Like a vial, you can extract insulin from them using a syringe for its greater dosage precision, though you need to remember an important rule:
- Don't replace a cartridge in a pen after using a syringe on it.
Prefilled disposable insulin pens such as the Novo Nordisk Flexpen  are essentially cartridges too, and may also be used with syringes as above. Again, don't use them as pens after using with a syringe.
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.
Until the advent of the Caninsulin VetPen, Lente-type insulins (Semilente, Ultralente, Lente) were not able to be dispensed in pen or cartridge form because the glass ball used to mix the insulin in these devices shattered the Lente crystals. 
- More Tips on Drawing Insulin From a Disposable Pen Diabetes Forums
- More Information on Using Syringes on Pen Cartridges TuDiabetes
- Using Insulin Pen Cartridges/Penfills with Syringes-Page 2 BC Childrens Hospital