To use an insulin pen, you screw a special pen needle onto one end of the pen and dial in an insulin dose at the other end. Then you insert the needle into the skin and press a button to inject the insulin from a cartridge inside the pen. Needles should be discarded after each use.

When selecting a pen, there are a few important points to consider. Each pen is compatible only with certain types of insulin. In the United States, three companies make insulin: Eli Lilly, Novo Nordisk, and Sanofi-Aventis. These manufacturers and another company, Owen Mumford, also make insulin pens. 

There are two basic varieties of insulin pen: reusable and disposable. Disposable pens come preloaded with insulin and should be stored in the refrigerator until use; once opened, they are kept at room temperature. When disposable pens are out of insulin, they can be thrown away. Reusable pens employ insulin cartridges that are purchased separately. These pens should never go in the refrigerator, but the cartridges should be stored in the fridge until they are loaded into the pen.

Insulin pens administer doses in increments ranging from half a unit to two units. Children and people who are very sensitive to insulin may want to look for pens that can dose insulin in half units for extra precision and control.