Accepted answer
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.
On the detail: 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.
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 |
Specifically, 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.
Gauge numbers run backwards. Higher number, thinner needle.