An insulin pen is a device used to inject insulin. There are many types of insulin pens available. Most are disposable. A disposable pen contains a prefilled amount of insulin. When this type of pen is empty, it is thrown away. A few types are reusable pens. A reusable pen contains an insulin cartridge that can be replaced. When the cartridge is empty, it is thrown away. Then a new, prefilled cartridge is put in. Always use a new needle every time you inject insulin.