An insulin pen is used to inject insulin for the treatment of diabetes. Insulin is a hormone produced by the pancreas. It is composed of an insulin cartridge (integrated or bought separately) and a dial to measure the dose, and is used with disposable pen needles to deliver the dose. It was introduced and marketed as Novopen by the Danish company Novo Nordisk in 1985.
Types of pens 
A number of companies make insulin pens including Novo Nordisk, Aventis and Eli Lilly. These companies produce pens for most of their insulins, including NovoLog/NovoRapid, Humalog, Levemir and Lantus.
There are two pen systems: durable and prefilled:
- A durable pen uses a replaceable insulin cartridge. When the insulin cartridge is empty, the empty cartridge is disposed of and a new one is inserted in the pen.
- A prefilled pen is entirely disposable. The pen comes pre-filled with insulin, and when the insulin cartridge or reservoir is empty, the entire unit is discarded.
Most brands of insulin are now available for use in pens, these include:
- NovoLog, NovoRapid and Levemir by Novo Nordisk
- Lantus and Apidra by Sanofi-Aventis
- Humulin and Humalog by Eli Lilly and Company
Global Patient Uptake 
Insulin pens are used by 95% of insulin-treated patients in Europe, Asia, and Scandinavia with excellent results. They are currently underutilized but growing in use in the United States.
Insulin pens offer several significant advantages over insulin syringes: ease of handling, accuracy, and they are more discreet to use.
To use an insulin pen 
- Screw or click on a new pen needle
- If necessary, prime the pen to remove any air from the needle
- Turn the knob on the end of the pen (or "dial") to the number of units needed
- Insert the needle into the skin
- Press the button on the end of the pen to deliver the dose
- Count to five
Insulin pens have a number of advantages:
- More convenient and easier to transport than traditional vial and syringe
- Repeatedly more accurate dosages
- Easier to use for those with visual or fine motor skills impairments
- Less injection pain (as polished and coated needles are not dulled by insertion into a vial of insulin before a second insertion into the skin)
It is important that proper injection sites on the body are used. A healthcare provider helps determine the best injection site for a patient. In general, recommended injection sites include the abdomen, parts of the buttocks and thigh areas.
Unlike with the traditional syringe, two different insulins cannot be mixed by the user in an insulin pen. On the other hand, some of the newest types of insulin cannot be mixed at all. In addition, using pens and pen needles is usually more expensive than using the traditional vial and syringe method; insurance coverage for insulin pens in the United States may vary widely.
Technology has not changed much since the launch of the first insulin pen over 25 years ago by Novo Nordisk. A number of independent companies have created accessories to help people using insulin pens better manage their insulin intake.
- Frio UK  offers a wallet to help keep insulin pens cool in warm temperatures.
- Timesulin  is a 'smart cap' that works with all major insulin pen types to show users how long it's been since their insulin pen injection to help avoid missed or accidental double doses.