Almost every dosing mistake I have seen with lyophilized peptides comes from the same place: the vial is labelled in milligrams, the syringe is labelled in units, and nothing on either label tells you the ratio between them. That ratio is entirely a consequence of how much bacteriostatic water you added, which is a number nobody else knows. So it has to be recomputed by hand every single time the vial size or the diluent volume changes.
A U-100 insulin syringe is marked so that 100 units equals 1 mL. That is the only fact you need to bridge the two labels.
concentration (mg/mL) = vial strength (mg) / diluent added (mL)
volume to draw (mL) = target dose (mg) / concentration (mg/mL)
syringe units = volume to draw (mL) * 100Collapsed into one step, and this is the form worth memorising:
units = target dose (mg) * 100 * diluent (mL) / vial strength (mg)Take a 5 mg vial reconstituted with 2 mL of bacteriostatic water. The concentration is 5 / 2 = 2.5 mg/mL. For a 0.25 mg dose, the volume is 0.25 / 2.5 = 0.1 mL, which is 10 units on a U-100 syringe. That is a comfortable, readable mark.
Now reconstitute the same 5 mg vial with only 1 mL. Concentration doubles to 5 mg/mL, and the same 0.25 mg dose is now 0.05 mL, or 5 units. Still readable, but you have halved the distance between a correct dose and a double dose. If you then move up to a 1 mg dose you are drawing 20 units in the first case and 10 in the second.
| Vial | Diluent | Concentration | 0.25 mg dose | 1 mg dose |
|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 5 units | 20 units |
| 5 mg | 2 mL | 2.5 mg/mL | 10 units | 40 units |
| 10 mg | 2 mL | 5 mg/mL | 5 units | 20 units |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 units | 30 units |
The useful trick is to work backwards. Decide what you want your usual dose to read as on the syringe, then solve for the diluent volume. Aiming for something in the 10 to 40 unit range keeps you away from both ends of the barrel, where the markings are hardest to read and the dead space in the hub matters most as a proportion of what you drew.
If you would rather not do this on paper each time, the Peptide Tracker & Calculator solves the same equation in both directions, so you can enter the units you want to see and get the water volume back out.
Doses usually move on a schedule, and the vial usually does not move with them. A common pattern is four weeks at one dose, then a step up, with the step landing partway through a vial. That is the moment where people carry the old unit count forward out of habit. Writing down the concentration on the vial cap in marker, rather than the dose, removes most of that risk, because the concentration is the thing that stays constant until the vial is finished.
Keeping a per-injection record also makes the awkward questions answerable later: how many milligrams are actually left in this vial, when was the site last rotated, and whether a missed week means resuming at the same step or dropping back one. I keep mine in a dose log built for this rather than a spreadsheet, mostly because the running vial balance is calculated instead of typed.
None of this is medical advice. It is arithmetic, and arithmetic is only as good as the numbers you put into it, so confirm the vial strength and the schedule with whoever prescribed them.
Written up because I kept re-deriving it. Calculator and log: glp1.app.