Conditions: semaglutide · 2 mg/mL.
The failure mode I am trying to avoid is making this decision emotionally.
I have twelve months in view and I would like the plan to survive that long.
What should I decide now, and what should I defer?
At 2 mg/mL a 0.25 mg dose is 12.5 units on a U-100 barrel and a 1 mg dose is 50 units. Volume is dose divided by concentration and one unit is 0.01 mL, so the unit count is dose ÷ 2 × 100. Both land in a readable part of the barrel, which is what choosing the volume deliberately buys you.
Total vial volume is a physical constraint: most 2 mL vials will not take 3 mL of anything.
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.
| 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.
Mechanically, 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.
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.
A concentration calculated to three decimal places from a diluent volume measured to one is false precision.
Check the vial can physically hold the volume before you draw it up.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThis is the one decision in the whole preparation sequence that cannot be revised later, which is why it is worth thirty seconds of arithmetic.
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.
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.
Do not change the diluent volume between vials of a titration without recalculating; it is the commonest source of a ten-fold error.
Concentration equals content over volume, and content is not label claim.
Answering this needs the syringe you actually own, because the barrel graduations decide what "readable" means.
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.
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.
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.
Choose the volume that puts your largest intended dose between 10 and 30 units. Everything else follows.
It helps to be literal here: the relevant arithmetic is concentration equals vial content divided by diluent volume, and content is not the same as label claim.
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.
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.
Measure a volume you can actually measure. Round numbers, real syringes.
The short version: more diluent means a lower concentration, a larger volume per dose and a finer reading; less means the opposite.
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.
Nothing here is medical advice, and research-use material is not approved for human use.
Write the concentration on the label at reconstitution, in units per dose.
edited 2 Jun 2026 by orla_ferriter — clarified the distinction between purity and content
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