Conventional pen needles (without safety engineering) used by self-injecting patients. Pen needles are sterile and can only be used once.

Safety-engineered pen needles used by healthcare providers (this example shows protective shields at both ends of the pen needle after use).

Medication pens can be used with only one person — “one pen, one patient.”
Medication pens, often the size and shape of a large marker, carry medication in self-contained cartridges. The most commonly used pens contain insulin. Intended primarily for self-injection, medication pens are now also used in healthcare settings.
Another type of medication pen is single use with a pre-attached needle.

Insulin pens: after attaching the pen needle, the needle needs to be always primed (recommend 2 units and repeat until fluid is seen at the needle tip). If the fluid is not seen after repeated priming attempts, replace with a new needle and repeat priming procedure.
Insulin pens fall into one of two groups: reusable pens and disposable. Reusable insulin pens have cartridges of insulin that are replaced when empty. Disposable insulin pens (used in healthcare settings) come pre-filled with insulin and are thrown away when empty.
Insulin pens are used with pen needles that are sold separately. NEVER reuse pen needles.

Anatomy of the Medication Pen