The particulars: an 18G drawing needle · semaglutide · 8 mg/mL.
I am trying to choose between two options that are usually discussed as though only one exists.
I am not optimising for price, but I am not indifferent to it either.
Which axes does this decision turn on?
At 8 mg/mL a 1 mg dose is 0.125 mL — 12.5 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 18G scale a larger number is a finer needle, so an 18G drawing needle is coarse enough to draw quickly and coarse enough to cut a visible plug from the stopper. If you are drawing 12.5 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 8 mg/mL each microlitre is 8 µg.
The short version: 21G to 23G to draw, 29G to 31G to inject, and never the same needle for both.
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.
| Vial | Diluent | Concentration | 0.25 mg | 0.5 mg | 1 mg | 2.5 mg |
|---|---|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 5 mg | 2 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | 100 u |
| 10 mg | 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | 25 u |
| 10 mg | 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | 75 u |
Units are U-100 insulin units, where 1 unit = 0.01 mL. Divide dose by concentration for millilitres, then multiply by 100.
The relevant detail is that 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.
The Hagen–Poiseuille relation gives flow proportional to the fourth power of radius, which is the quantitative basis for every gauge recommendation here.
Flow goes as the fourth power of radius. That is why the difference feels so large.
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Browse resultsThe honest answer is that injection gauge is a comfort decision and drawing gauge is a stopper-coring decision.
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.
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.
The caveat is that no gauge choice makes a non-sterile preparation safe, and research-use compounds are not approved for human use.
Angle the bevel and insert gently to avoid coring the stopper.
edited 8 Apr 2026 by rota_site — added a caveat about sampling
Answer first: use the largest bore you tolerate for drawing and the smallest for injecting, because the two operations have opposite requirements.
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.
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.
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.
Length affects comfort more than gauge does at these volumes.
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.
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.
Nothing here is medical advice.
Gauge numbers run backwards. Higher number, thinner needle.
edited 10 Mar 2026 by u100_marks — added the placebo-arm figures
The relevant physics is the fourth-power dependence of flow on radius, which makes small gauge differences enormous in practice.
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.
Coring risk as a function of needle gauge and insertion technique is documented in pharmacy compounding guidance.
Big to draw, small to inject, never the same one twice.
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