Conditions: 2.5 mg · 0.5 mL · 0.9% sodium chloride · a GLP-1 receptor agonist.
I want to decide this in advance so that I am not deciding it under pressure later.
Assume I will follow the plan I write down, so I would like it to be a good one.
What would you do, and what would make you change course?
It gives 5 mg/mL, and whether that is sensible depends on the dose you will draw from it. 2.5 ÷ 0.5 = 5 mg/mL in 0.9% sodium chloride. A 0.5 mg dose is then 10 units on a U-100 barrel and a 1 mg dose is 20 units. Both land in a readable part of the barrel, which is the whole point of choosing the volume deliberately.
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.
| 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.
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.
Nominal vial volumes in the standard 2R and 3R glass sizes have published brimful capacities well above the nominal fill, but the usable volume is bounded by the stopper displacement.
Nothing here is medical advice, and research-use material is not approved for human use.
Concentration equals content over volume, and content is not label claim.
edited 12 Jul 2026 by gel_pack_warm — reworded for clarity after a comment
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemAim for a dose volume somewhere between about 10 and 30 units on a U-100 barrel and the reading problem disappears.
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.
Choose the volume that puts your largest intended dose between 10 and 30 units. Everything else follows.
Start from the dose you intend to draw and work backwards to the volume that puts it in a readable part of the barrel.
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.
Put another way, 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.
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.
A concentration calculated to three decimal places from a diluent volume measured to one is false precision.
Write the concentration on the label at reconstitution, in units per dose.
Total vial volume is a physical constraint: most 2 mL vials will not take 3 mL of anything.
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.
Check the vial can physically hold the volume before you draw it up.
Answering this needs the syringe you actually own, because the barrel graduations decide what "readable" means.
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.
Insulin syringe barrel graduations are typically 1 unit on a 0.3 mL barrel, 1 unit on a 0.5 mL barrel and 2 units on a 1 mL barrel, which is why the barrel size changes what is readable.
Measure a volume you can actually measure. Round numbers, real syringes.
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