Accepted answer
At 6.67 mg/mL a 1 mg dose is 0.15 mL — 15 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 15 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 6.67 mg/mL each microlitre is 6.67 µg.
The relevant detail is that for a 4 mm pen-style needle the gauge options are narrow and the choice is nearly made for you.
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.
Dead space by syringe type
| Configuration | Dead volume | Loss at 5 mg/mL | Over 20 draws |
|---|
| Fixed-needle insulin syringe | 3–5 µL | 15–25 µg | 0.3–0.5 mg |
| Low-dead-space, detachable | <2 µL | <10 µg | <0.2 mg |
| Standard luer-lock + 30G | 35–60 µL | 175–300 µg | 3.5–6 mg |
| Luer-lock + 21G drawing needle | 70–100 µL | 350–500 µg | 7–10 mg |
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.
Nothing here is medical advice.
Gauge numbers run backwards. Higher number, thinner needle.
edited 14 Dec 2025 by fiadh_cronin — added the citation requested in comments