The case in front of me: a 25G drawing needle · liraglutide · 10 mg/mL.
I would like the axes of comparison first and the recommendation second.
I have tried the first option and it works; the question is whether the second is better rather than merely different.
Under what conditions does the answer flip?
At 10 mg/mL a 1 mg dose is 0.1 mL — 10 units on a U-100 barrel — and no needle gauge changes that number. Gauge changes three other things: how long the draw takes, how much stays behind in the hub, and how much rubber you core out of the stopper. On the 25G scale a larger number is a finer needle, so a 25G drawing needle is coarse enough to draw quickly and coarse enough to cut a visible plug from the stopper. If you are drawing 10 units at a time, the dead space matters more than the bore: a fixed-needle barrel loses microlitres, a luer hub loses tens of them, and at 10 mg/mL each microlitre is 10 µg.
Start with the fact that gauge numbers run backwards — a higher number is a thinner needle — which is the source of half the confusion in this tag.
Very fine needles are more prone to bending and to blocking with any particulate, which is a practical argument for inspecting the solution before drawing.
| Appearance | Interpretation | Action |
|---|---|---|
| Intact opaque puck, proud of base | Cycle ran correctly | Proceed |
| Slumped to one side | Shipped before fully dry, or vibration | Usually usable; note it |
| Glassy translucent film | Collapse above glass transition | Test before use |
| Melt-back ring at stopper | Thermal excursion in transit | Test before use |
| No visible cake at all | Very low fill, or nothing there | Weigh it; query the supplier |
A 30G or 31G needle through a butyl stopper leaves a track that reseals, which is why fine-gauge repeated entry is tolerable and coarse-gauge repeated entry is not.
Butyl rubber closures are specified for resealing after piercing up to a stated gauge, which is the basis for the fine-gauge repeated-entry practice.
Nothing here is medical advice.
Angle the bevel and insert gently to avoid coring the stopper.
edited 1 Apr 2025 by esben_lykke — updated for the 2026 guidance change
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Browse resultsThe relevant physics is the fourth-power dependence of flow on radius, which makes small gauge differences enormous in practice.
Stopper coring — punching a disc of rubber into the solution — is a large-bore phenomenon. An 18G or 21G needle inserted straight and fast is the classic way to do it; inserting at a slight angle with the bevel up reduces the risk.
On the detail: drawing a viscous or foamy solution through a fine needle takes long enough that people rush the plunger, which causes more foaming. Using a wider drawing needle is the fix.
Big to draw, small to inject, never the same one twice.
Answering this needs to know the viscosity of what is being drawn, since a viscous solution through a fine needle is slow enough to encourage bad technique.
Fixed-needle insulin syringes are supplied in 29G to 31G and cannot be swapped for drawing, which is the trade-off against their much lower dead space.
Flow through a needle scales with the fourth power of the internal radius under the Hagen–Poiseuille relation. Halving the radius reduces flow sixteen-fold at the same pressure, which is why a 31G needle draws so much more slowly than a 21G.
The Hagen–Poiseuille relation gives flow proportional to the fourth power of radius, which is the quantitative basis for every gauge recommendation here.
Length affects comfort more than gauge does at these volumes.
Coring the stopper with a large-bore needle is the risk at the drawing end, and it is real.
Typical outer diameters: 21G is about 0.82 mm, 23G about 0.64 mm, 25G about 0.51 mm, 29G about 0.34 mm and 31G about 0.26 mm. The gauge number and the diameter move in opposite directions.
Higher gauge is not automatically better; it is thinner, which has costs as well as benefits.
Flow goes as the fourth power of radius. That is why the difference feels so large.
In practice, this is a straightforward answer that people over-complicate because the numbering is counter-intuitive.
For injecting, 29G to 31G is the usual range and the difference in perceived discomfort between them is small. Needle length matters more than gauge for comfort at these volumes.
Needle gauge to outer diameter correspondence is standardised and published; the inverse relationship between gauge number and diameter is the reason for the counter-intuitive labelling.
Gauge numbers run backwards. Higher number, thinner needle.
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