What is an insulin pen?
· 2 min
A pen is a tool that joins carrying insulin and measuring it in one body; the description is that short. What’s interesting is why the parts are arranged this way. Renewing the tip every time, the body staying with one person, two separate pen families existing: none of these are independent decisions.
Before the pen came two separate pieces: a glass vial holding the medicine and a syringe prepared separately for each use. The pen gathered these into one body, and took the measuring job inside the body too. Instead of drawing by eye you turn a dial, clicking at every step. That was the design’s starting point: measuring moved from eye to hand.
Inside the body sits the cartridge, the thin glass cylinder holding the medicine. Outside, a dial sets the amount, clicking step by step as it turns. The clicks aren’t decoration: for someone who can’t see well, they’re a separate feedback channel. On some pens the dial moves in finer steps, on others coarser. Fingertips hear that difference. The pen doesn’t carry the tip; it goes on later, sold separately in its own package.
Why the needle tip is single-use isn’t visible to the naked eye. Under magnification a once-used tip’s edge comes out blunted and nicked. In people who reuse the same tip, more pain is reported; the blunting edge is the known counterpart. Hardening under the skin is another matter. There the main share is insulin’s, not the tip’s; a separate article covers it. A tip left on the pen lets air into the cartridge, and the dial’s number stops matching what comes out.
The same reasoning holds for the body itself. Even with a new tip, microscopic tissue can slip back into the cartridge mid-injection, making the pen a one-person device. So a pen doesn’t pass between two people.
Pens fall into two families by one measure alone: is the cartridge embedded in the body?
| Feature | Disposable pen | Refillable pen |
|---|---|---|
| Cartridge | Embedded in the body, doesn’t come out | Replaced with a new one when empty |
| Body | Thrown out when the medicine runs out | Same for years |
| Needle tip | Sold separately, single-use | Sold separately, single-use |
| Body material | Mostly plastic, light | Mostly metal, slightly heavy |
| Waste left behind | One body every time | Only cartridge and tip |
A pen isn’t a pump. It doesn’t decide on its own, and doesn’t measure blood sugar either. A new generation goes a step beyond this: connected pens log the last amount and time and send it to the phone. For that, a battery and pairing circuit go into the body, so the pen now carries not just the medicine but its record. So the body itself remembers how long since the last injection.
The ADA puts the pen ahead of the syringe in treatments needing many injections. In people with reduced hand dexterity or vision trouble, the preference is more pronounced. So the pen isn’t the treatment itself; it’s a tool that makes treatment portable. Which pen suits whom changes with hand, eye, and daily routine.
Sources
- American Diabetes Association Professional Practice Committee. 7. Diabetes technology: Standards of Care in Diabetes-2026. Diabetes Care. 2026;49(Suppl 1):S150-S165. doi:10.2337/dc26-S007. PMCID: PMC12690173.
- Sparre T, Hammershøy L, Steensgaard DB, et al. Factors Affecting Performance of Insulin Pen Injector Technology: A Narrative Review. Journal of Diabetes Science and Technology. 2022;17(2):290-301. PMCID: PMC10012375.
- Zabaleta-del-Olmo E, Vlacho B, Jodar-Fernández L, Urpí-Fernández AM, Lumillo-Gutiérrez I, Agudo-Ugena J, Morros-Pedrós R, Violán C. Safety of the reuse of needles for subcutaneous insulin injection: A systematic review and meta-analysis. International Journal of Nursing Studies. 2016;60:121-132. PMID: 27297374.
- Klonoff DC, Berard L, Franco DR, et al. Advance Insulin Injection Technique and Education With FITTER Forward Expert Recommendations. Mayo Clinic Proceedings. 2025;100(4):682-699. PMID: 40180487.