Every peptide dosing mistake I have seen traced back to arithmetic has the same shape. The vial says 5 mg. The syringe is marked in units. Somewhere between those two numbers a decimal point moves, and the person ends up drawing twice or half what they meant to.
What the numbers on the vial actually mean
A lyophilized vial is labeled by mass, not by volume. A 5 mg vial holds 5 mg of dry powder and nothing else. It has no concentration until you add bacteriostatic water, and the concentration you get is entirely a function of how much you add.
Concentration in mg/mL is simply:
concentration = peptide mass in mg / diluent volume in mL
Reconstitute a 5 mg vial with 2 mL and you have 2.5 mg/mL. Reconstitute the same 5 mg vial with 1 mL and you have 5 mg/mL. The vial is unchanged. The number you have to divide by has doubled.
Why the syringe reads in units and not in mL
A U-100 insulin syringe is graduated for insulin, where 100 units equals 1 mL. That is the whole of the conversion:
1 mL = 100 units
0.1 mL = 10 units
0.01 mL = 1 unit
So once you know the concentration, the dose in units is:
units = (dose in mg / concentration in mg/mL) * 100
A worked example, both ways
Take a 5 mg vial and a target dose of 250 mcg, which is 0.25 mg.
Reconstituted with 2 mL, concentration is 2.5 mg/mL. Volume needed is 0.25 / 2.5 = 0.1 mL. On the syringe that is 10 units.
Reconstituted with 1 mL, concentration is 5 mg/mL. Volume needed is 0.25 / 5 = 0.05 mL. On the syringe that is 5 units.
Identical vial, identical dose, and the mark you draw to is 10 on one and 5 on the other. This is the single most common source of a doubled or halved dose, and it happens most often when someone reconstitutes a new vial with a different volume than the last one and keeps drawing to the mark they remember.
Rounding rules that matter
A U-100 syringe resolves to 1 unit, which is 0.01 mL. At 5 mg/mL that single smallest mark is 50 mcg of peptide. If your titration steps are 25 mcg, the syringe cannot express them and no amount of squinting will fix it. The fix is on the reconstitution side: add more diluent so the same dose occupies more volume and lands on a readable mark.
Three rules worth writing on the box:
- Choose the reconstitution volume so your intended dose falls between 10 and 40 units. Below 10 the rounding error is a large fraction of the dose; above 50 you are fighting syringe capacity.
- Round to whole units, never to half marks you cannot actually see.
- Recompute from scratch whenever a new vial is mixed, even if it is the same product at the same strength.
What a dose log should record
A log that only stores "250 mcg" cannot reconstruct anything. The fields that make a record verifiable later are the vial mass, the diluent volume, the derived concentration, the dose in mg, the volume in units, the date, and the site. With those seven you can reconstruct the arithmetic months later and catch a drift that a bare dose number hides completely. I keep this in a Peptide Tracker & Calculator rather than a notebook precisely because the derived fields should be computed, not copied.
Common failure cases
Assuming the concentration is a property of the product. It is a property of your reconstitution. Two people using the same product can hold vials that differ by a factor of four.
Converting mcg to mg by feel. 1 mg is 1000 mcg. A 250 mcg dose is 0.25 mg, not 2.5 mg. This error is a factor of ten and it is silent.
Reading a U-40 or a 0.3 mL syringe as if it were U-100. Check the barrel marking before you check anything else. The unit-to-mL relationship is a property of the syringe, not of the drug.
Topping up a partly used vial. Adding diluent to a vial that already contains solution changes the concentration of everything remaining in it, and the new concentration is not the one printed on any of your notes.
Summary
The whole calculation is two divisions and one multiplication, and it fails almost exclusively at the step where people assume the vial has a fixed concentration. It does not. Compute mg/mL every time you mix, choose a diluent volume that puts your dose in a readable range on the syringe, round to whole units, and log the inputs rather than the answer.