What are Insulin Needles and How Do They Work
, by Andrew Odgers, 11 min reading time
Fundamentals
What Are Insulin Needles and How Do They Work
Understanding what insulin pen needles are made of, how they attach to pen devices, and what happens from insertion to insulin absorption gives patients a clearer foundation for using them correctly and for understanding why the technique details their diabetes nurse has taught them actually matter.
Design and construction
What insulin needles are made of and how they are built
Components of an insulin pen needle
An insulin pen needle has three main components: the needle cannula, the hub, and the protective caps. The cannula is a hollow steel tube, typically made from medical-grade stainless steel, with a precision-ground bevel at the patient-facing tip and an inner end that pierces the insulin cartridge seal when the needle is attached to the pen. The hub is a plastic body that connects the cannula to the pen device, typically via a screw thread or a push-fit connection. The outer protective cap covers the full length of the needle during storage; the inner cap, which is smaller, covers only the inner end and is removed immediately before attaching the needle to the pen.
The double-ended design
Unlike a standard hypodermic needle, which has one sharp end, an insulin pen needle has two functional ends. The outer tip that enters the patient's skin during injection is the visible needle end. The inner end, typically shorter and finer, pierces the rubber seal of the insulin cartridge when the hub is screwed onto the pen. Both ends must be sharp and intact for the needle to function correctly. This is one of the reasons pen needles must be single-use: after use the outer tip is blunted and the inner end may be contaminated with insulin from the cartridge.
Gauge and length specifications
Pen needles are specified by two dimensions: gauge and length. Gauge follows the Standard Wire Gauge system, in which higher numbers indicate a narrower outer diameter. Common insulin pen needle gauges are 29, 30, 31, and 32 gauge, with 31 and 32 gauge being the finest commonly available. Length options are typically 4 mm, 5 mm, 6 mm, and 8 mm. Most adults in the UK are advised to use 4 mm needles as these reliably reach the subcutaneous tissue in most body compositions without risking intramuscular injection. Longer lengths may be needed in some patients; your diabetes nurse will advise on the appropriate specification for you.
The lubricant coating
The outer tip of a new pen needle is coated with a fine layer of silicone lubricant that reduces friction as the needle passes through the skin surface. This coating is responsible for the near-painless insertion that characterises a fresh needle and is one of the main reasons why reusing needles causes more discomfort: a single use removes the lubricant coating entirely, exposing the bare steel surface on all subsequent insertions.
How the injection works
What happens from needle attachment to insulin delivery
Attaching the needle and priming
The pen needle is attached by screwing the hub onto the pen device's cartridge cap thread, or by a push-fit mechanism depending on the device. Once attached, the inner end of the needle has pierced the cartridge seal, connecting the needle lumen to the insulin in the cartridge. Before the first injection from a new cartridge, the pen is primed by dialling two units and pressing the plunger until a small drop of insulin appears at the needle tip. This removes any air from the cartridge-to-needle path and confirms the needle is clear.
Dose dialling and delivery
The prescribed dose is set by dialling the pen's dose selector until the required number of units appears in the dose window. The needle is then inserted subcutaneously at the injection site and the plunger is pressed fully until the dose window returns to zero. The insulin flows from the cartridge through the needle lumen and is deposited as a small depot in the subcutaneous tissue. Holding the needle in place for ten seconds after the plunger reaches zero allows the depot to disperse before the needle is withdrawn, preventing insulin from tracking back out along the needle path.
Absorption from subcutaneous tissue
Once deposited in the subcutaneous fat layer, insulin is absorbed into the surrounding capillaries and distributed via the bloodstream. The rate of absorption depends on the insulin type, the injection site, the depth of delivery, blood flow to the area, and individual factors. Subcutaneous delivery from healthy, non-lipohypertrophic tissue produces a consistent and predictable absorption profile. Intramuscular injection produces faster and less predictable absorption; lipohypertrophic tissue produces slower and more variable absorption. Both deviations from the intended subcutaneous target are important reasons to maintain correct technique and healthy injection sites.
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For a step-by-step injection guide using what you have learned here, see How to Prepare for Your First Insulin Injection.
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Back to the Insulin Needles Knowledge Hub
This article is part of our complete insulin needle knowledge base, covering injection technique, needle selection, pain reduction, site care, disposal, travel, and everything patients managing insulin at home need to know.
Keep reading
Related guides in this hub
How to Prepare for Your First Insulin Injection applies the device knowledge in this guide to the full injection process. Insulin Pen Needles vs Syringe Needles explains the differences between the two delivery systems. And Common Mistakes When Using Insulin Needles covers the errors that understanding the device design helps you avoid.
Frequently asked
Insulin needle fundamentals answered
Why is an insulin pen needle double-ended?
One end enters the patient's skin during injection; the other end pierces the rubber seal of the insulin cartridge when the needle is attached to the pen device. Both ends must function correctly for the needle to deliver insulin from the cartridge to the subcutaneous tissue. This design means the needle is part of the flow path from the cartridge, which is why it must be changed for every injection and why leaving a needle attached between doses allows air and insulin to move through the needle, which can affect subsequent doses.
What happens if I leave the needle on the pen between injections?
Leaving a needle attached between injections allows air to enter the cartridge via the needle, which can cause air bubbles that lead to underdosing. It also allows insulin to crystallise at the needle tip, potentially blocking it. Remove the needle immediately after each injection and attach a fresh one before the next dose.
Why does reusing a needle hurt more?
After a single injection the silicone lubricant coating on the needle tip is completely stripped. The bare steel surface causes more friction as it passes through the skin surface, producing more noticeable insertion pain. The tip also becomes microscopically bent or barbed from the first use, which further increases tissue disruption on subsequent insertions.
What is the difference between 4 mm and 8 mm insulin needles?
The length determines the depth of insertion at a 90 degree angle. A 4 mm needle reaches the subcutaneous tissue reliably in most adult body compositions and is the standard recommendation for most adults in current UK guidelines. An 8 mm needle penetrates deeper and carries a higher risk of intramuscular injection in lean patients. Longer needles may be appropriate for some patients with more subcutaneous tissue, confirmed with the diabetes nurse based on the individual patient's anatomy.