Reconstituting a lyophilized peptide is one subtraction and one division, but the division is where most people go wrong, because the syringe is marked in insulin units and the vial is labelled in milligrams.
The only formula you need
concentration (mg/mL) = peptide in vial (mg) / bacteriostatic water added (mL)A 5 mg vial with 2 mL of bacteriostatic water is 2.5 mg/mL. Add 1 mL instead and it is 5 mg/mL. The powder itself adds no meaningful volume, so you do not need to correct for it.
Turning mg/mL into units on a U-100 syringe
A U-100 insulin syringe has 100 units per 1 mL, so one unit is 0.01 mL.
units to draw = desired dose (mg) / concentration (mg/mL) * 100At 2.5 mg/mL, a 0.25 mg dose is 0.1 mL, which is 10 units. Same vial, same dose, but filled with 1 mL of water instead: 5 mg/mL, so 0.25 mg is 5 units. The dose did not change. The number on the syringe halved. That is the single most common dosing error.
Handy reference at 2.5 mg/mL:
0.25 mg -> 10 units
0.50 mg -> 20 units
1.00 mg -> 40 units
1.25 mg -> 50 unitsDoses per vial
doses per vial = vial mg / dose mg5 mg at 0.25 mg per week is 20 weeks on paper. In practice you lose a little to dead space in the needle hub and to the drop left at the bottom, so plan on the whole number below. If you would rather not redo the arithmetic every time you change strength, the Peptide Tracker & Calculator does the mg/mL and unit conversion in one place and keeps the numbers with the vial they belong to.
Handling and storage notes worth writing down
- Add the water slowly against the inner wall of the vial. Do not squirt it onto the powder and do not shake. Swirl gently and let it dissolve on its own.
- Bacteriostatic water contains 0.9 percent benzyl alcohol as a preservative, which is why a reconstituted vial keeps for weeks refrigerated. Sterile water has no preservative and is a single-use proposition.
- Keep reconstituted vials at 2 to 8 C, upright, away from the freezer wall. Do not freeze a reconstituted peptide.
- Write the reconstitution date and the resulting mg/mL straight onto the vial label. Future you will not remember whether it was 1 mL or 2 mL of water.
Keep a dose log
The fields that actually turn out to matter later are: date, compound, dose in mg, the concentration it was drawn from, injection site, and a short note on side effects. Six columns in a spreadsheet is enough. Recording units without recording the concentration makes the log useless the moment you reconstitute the next vial differently, which is the mistake worth avoiding from day one. If you want the log and the maths in the same place rather than in two, glp1.app is built around exactly that pairing.
None of this is medical advice. Doses, titration schedules, and whether a compound is appropriate at all are questions for a clinician.