A pen injector is a device that holds a small vial of insulin and a
disposable needle, and injects an amount measured with a dial.
Advantages include being compact, convenient, easy to use circumspectly
in public, and accurate and simple in dose measurement. The pen device
clicks for each unit (or two depending on the manufacturer) dialed;
this can help those with impaired vision.

Some pen units only allow setting a multiple of two units of insulin,
which many find inadequate. Get a model which measures a multiple of
one unit, which should be easy to find among current models.

The primary disadvantage is cost, up to twice as much per unit of
insulin compared with standard vials. The special vials may be
difficult to obtain in remote areas, and widespread shortages have
occurred occasionally. Falling back to a standard syringe is always an
option.

Also, the special vial can be refilled from a standard vial using a
syringe, making sure the rubber stopper is not damaged, though the
manufacturer will not recommend this. If you do refill, make sure to
use the same concentration of insulin. This is not a problem in the US,
where only U100 concentration is used. In some parts of the world, U40
concentration is common, but pen refills are always U100. Make sure to
match the concentration.

Pens are more popular in Europe than in the US, but are being heavily
promoted in the US.