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.
Plastic 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.