What I have: 30 mg · 2 mL · sterile water for injection · semaglutide.
This is a planning question. I know what my options are; I do not know how to weigh them.
What I want is the minimum viable version, which I suspect is smaller than what I would design.
How do I make this decision on evidence rather than on feel?
It gives 15 mg/mL, and whether that is sensible depends on the dose you will draw from it. 30 ÷ 2 = 15 mg/mL in sterile water for injection. A 0.5 mg dose is then 3.3 units on a U-100 barrel and a 1 mg dose is 6.7 units. The smaller dose lands too low on the scale to read accurately — more diluent would buy resolution you cannot recover later.
Aim for a dose volume somewhere between about 10 and 30 units on a U-100 barrel and the reading problem disappears.
Worked example. A 10 mg vial reconstituted with 2 mL gives 5 mg/mL. A 0.5 mg dose is 0.5 ÷ 5 = 0.1 mL, which on a U-100 syringe is 10 units. Reconstitute the same vial with 1 mL and the concentration doubles to 10 mg/mL, the same dose becomes 0.05 mL, and you are now reading 5 units instead of 10 — the same dose at half the resolution.
| 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 |
The relevant detail is that vial headspace is the hard constraint. A nominal 2 mL vial typically holds a little over 2 mL to the shoulder; adding 3 mL is not an option and attempting it wastes the lot.
U-100 means 100 units per millilitre by definition, so 1 unit is 0.01 mL and volume in millilitres times one hundred gives units. Every conversion here reduces to that.
The caveat is that this arithmetic assumes the vial contains what the label says, and without a content assay it is precise about an unknown quantity.
Choose the volume that puts your largest intended dose between 10 and 30 units. Everything else follows.
edited 11 Jun 2026 by orla_ferriter — reworded for clarity after a comment
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe honest answer is that a wide range of volumes works and that the extremes at either end cause avoidable problems.
For a dose that will change during titration, choose the volume for the largest intended dose rather than the first, so the whole schedule fits on one barrel without a mid-vial recalculation.
Dead-space loss scales with the number of draws, not with the concentration, so a lower concentration spread over more draws loses proportionally less of the total peptide.
Measure a volume you can actually measure. Round numbers, real syringes.
Answer first: diluent volume sets concentration and therefore resolution on the syringe barrel, and resolution is free at reconstitution and impossible to recover afterwards.
The other direction: 10 mg in 3 mL is 3.33 mg/mL, and a 0.5 mg dose becomes 0.15 mL, or 15 units. More barrel, easier reading, and a larger fraction of the vial volume lost to dead space across the same number of draws.
Write the concentration and the resulting units-per-dose on the vial label at reconstitution. The arithmetic that is obvious now will not be obvious at six in the morning three weeks from now.
Do not change the diluent volume between vials of a titration without recalculating; it is the commonest source of a ten-fold error.
Write the concentration on the label at reconstitution, in units per dose.
The relevant detail is that the relevant arithmetic is concentration equals vial content divided by diluent volume, and content is not the same as label claim.
Content matters. If the same 10 mg vial assays at 94 per cent content, you have 9.4 mg. In 2 mL that is 4.7 mg/mL, and a nominal 0.5 mg draw of 10 units actually delivers 0.47 mg — a six per cent shortfall that no amount of careful drawing will fix.
Published content assay results across the independent testing services show nominal and measured content differing by one to ten per cent, which makes content the dominant term in dose error.
Check the vial can physically hold the volume before you draw it up.
Total vial volume is a physical constraint: most 2 mL vials will not take 3 mL of anything.
Round to a diluent volume you can measure accurately. Measuring 1.00 mL on a 1 mL syringe is reliable; measuring 1.37 mL on anything is not, and the error propagates into every dose.
Concentration equals content over volume, and content is not label claim.
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