Is a 27G needle the right choice for drawing cagrilintide at 4 mg/mL?

Question

Stated plainly: a 27G needle · cagrilintide · 4 mg/mL. I suspect the honest answer is that it depends, in which case I would like to know on what. Assume I can obtain either option without difficulty, so availability is not the deciding factor. Under what conditions does the answer flip?

Dr_Ingrid_Baumgartner · Accepted Answer

At 4 mg/mL a 1 mg dose is 0.25 mL — 25 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 27G scale a larger number is a finer needle, so a 27G needle is fine enough that a viscous solution draws slowly and a hurried draw pulls bubbles. If you are drawing 25 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 4 mg/mL each microlitre is 4 µg. On the detail: the relevant physics is the fourth-power dependence of flow on radius, which makes small gauge differences enormous in practice. 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. Concentration and unit conversion at a glance 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. 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. 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.

stopper_core · Answer

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.

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 part that matters: 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.

Coring risk as a function of needle gauge and insertion technique is documented in pharmacy compounding guidance.

Angle the bevel and insert gently to avoid coring the stopper.

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answeredstopper_core28k12714 Nov 2025

6Thank you — the worked example is what makes this usable. – kirsi_lahtinen 2 months ago

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Dr_Ilse_Vandenberg · Answer

Stated carefully, this is a straightforward answer that people over-complicate because the numbering is counter-intuitive.

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.

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.

Nothing here is medical advice.

Flow goes as the fourth power of radius. That is why the difference feels so large.

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answeredDr_Ilse_Vandenberg113k2486 Dec 2025

seven_day_half · Answer

Coring the stopper with a large-bore needle is the risk at the drawing end, and it is real.

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.

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.

The caveat is that no gauge choice makes a non-sterile preparation safe, and research-use compounds are not approved for human use.

Big to draw, small to inject, never the same one twice.

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answeredseven_day_half31k13818 Jan 2026

5I have added the label-the-vial suggestion to my own notes. Obvious in hindsight. – orla_ferriter 7 months ago

4Confirming: I did the wrong thing here once and got exactly the predicted result. – v_ramaswamy 5 months ago

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laminar_bench · Answer

The honest answer is that injection gauge is a comfort decision and drawing gauge is a stopper-coring decision.

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.

If a solution will not draw through a 25G needle, the problem is the solution rather than the needle.

Length affects comfort more than gauge does at these volumes.

edited 15 Dec 2025 by laminar_bench — fixed an arithmetic slip in the third paragraph

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answeredlaminar_bench69k5725 Nov 2025

Is a 27G needle the right choice for drawing cagrilintide at 4 mg/mL?

needle-gauge

injection-technique

dead-space

amylin

5 Answers

Concentration and unit conversion at a glance

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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