What Is a 31G Syringe and When Should It Be Used
, by Andrew Odgers, 9 min reading time
Fine Gauge
What Is a 31G Syringe and When Should It Be Used
A 31 gauge syringe typically refers to an insulin syringe or pen needle with a 31 gauge pre-attached needle, one of the finest gauges used in routine clinical practice. The very fine needle minimises insertion sensation for patients who inject frequently, making it the gauge of choice for insulin delivery and some other subcutaneous self-injection therapies. This guide explains what 31G means, when it is the right choice, and when a wider gauge is more appropriate.
What 31G means
Understanding the 31 gauge specification
The gauge number explained
31 gauge refers to the outer diameter of the needle attached to the syringe, not to the syringe barrel itself. In the Standard Wire Gauge system, a higher number indicates a narrower needle. A 31 gauge needle has an outer diameter of approximately 0.26 mm, making it one of the finest needle gauges used in routine clinical practice. When people refer to a 31G syringe, they typically mean a syringe with a 31 gauge pre-attached needle, as seen in many insulin syringe products.
The syringe and needle combination
A 31 gauge syringe is most commonly an insulin syringe with a pre-attached 31 gauge needle. The barrel is typically 0.3 ml, 0.5 ml, or 1 ml, calibrated in insulin units for U100 insulin. The 31 gauge needle at 5 to 8 mm length is designed to deliver insulin subcutaneously with minimal insertion sensation. These products are also referred to as insulin pen needles at 31 gauge when they are designed for insulin pen devices rather than syringe barrels.
Clinical applications
When 31 gauge is the right choice
Insulin delivery
31 gauge insulin syringes and pen needles are among the most widely used gauges for insulin delivery in the UK. The very fine gauge is well tolerated by patients who inject multiple times daily, reducing the cumulative discomfort of frequent injections at the same sites. The short length options of 4 to 8 mm are appropriate for most adults and children using insulin, including those with minimal subcutaneous tissue who would risk intramuscular injection with longer needles.
Subcutaneous injections of other medications
For medications other than insulin that are administered subcutaneously in very small volumes, a 31 gauge needle on a standard 1 ml luer syringe provides minimal insertion discomfort. Patients self-administering subcutaneous biologics, growth hormone, or fertility medications on a long-term basis often prefer the finest available gauge within the viscosity constraints of the medication. Confirm that the medication flows adequately through a 31 gauge lumen before committing to this gauge, as some subcutaneous preparations are too viscous for 31 gauge delivery at acceptable plunger force.
Paediatric subcutaneous injections
In paediatric settings, the finest gauge that adequately delivers the medication is preferred to minimise needle aversion and procedure distress. 31 gauge at short lengths is used for subcutaneous injections in children and infants where the medication viscosity and volume permit. As with adult use, confirm that the specific preparation flows adequately at this gauge.
When 31 gauge is not appropriate
31 gauge is not suitable for intramuscular injections, for medications with viscosity above what the narrow lumen can accommodate at clinical plunger forces, or for draws from vials where the fine lumen makes draw-up impractically slow. For viscous medications, 27 to 29 gauge is typically the minimum practical gauge. For intramuscular use, 21 to 25 gauge is appropriate. The 31 gauge specification is specifically for fine-gauge subcutaneous delivery of low-viscosity preparations in small volumes.
Fine gauge syringes in stock
31G insulin syringes and pen needles with next-day UK delivery
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For the complete gauge and size reference, see Why Gauge Size Affects Injection Comfort and Precision.
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Related guides in this hub
Why Gauge Size Affects Injection Comfort and Precision explains the full gauge-comfort-flow relationship. The Difference Between Insulin Syringes and Standard Syringes covers the insulin syringe category in full. And How to Choose the Right Syringe Size for Injections covers barrel volume selection alongside gauge.
Frequently asked
31G syringe questions answered
Is 31G the finest syringe needle available?
31 gauge is among the finest gauges in routine clinical use. 32 and 33 gauge needles exist for specialist applications including mesotherapy and some aesthetic treatments. For routine insulin and subcutaneous injection use, 31 gauge represents the finest end of the commonly available clinical range.
Can a 31G needle be used for intramuscular injections?
No. 31 gauge is too fine for intramuscular injections. The narrow lumen cannot deliver the volumes used in most IM injections at acceptable plunger pressure, and the short lengths available at this gauge are not appropriate for reliable muscle penetration in most adult patients. Use 21 to 25 gauge for intramuscular injections.
Is 31G appropriate for all subcutaneous medications?
No. 31 gauge is appropriate for low-viscosity preparations in small volumes, primarily insulin and very dilute subcutaneous medications. More viscous preparations including some concentrated biologics and some hormone therapies require a wider gauge of 27 to 29 gauge to allow adequate flow at acceptable plunger pressure. Always confirm the medication flows adequately before committing to a very fine gauge.
What is the difference between a 31G insulin syringe and a 31G pen needle?
A 31G insulin syringe has a standard syringe barrel with a pre-attached 31G needle for draw-and-inject technique. A 31G pen needle screws onto an insulin pen device cartridge for pen-based insulin delivery. The needle gauge is the same but the delivery system is entirely different. Use the type specified for your insulin delivery device.