Reconstitution is the process of turning a freeze-dried (lyophilized) peptide back into a liquid you can measure and inject. In plain terms: you add water to a powder, and the amount of water you choose is the only real decision — it sets how strong the solution is. It is arithmetic, not chemistry: the compound itself doesn't change, only its concentration.
The three numbers that matter
Every reconstitution comes down to three quantities:
- Peptide mass — how much compound is in the vial, printed on the label (e.g.
5 mg). This is fixed. - Diluent volume — how much bacteriostatic water you add (e.g.
2 mL). This is your choice. - Concentration — the result:
concentration = mass ÷ volume. Five milligrams in two millilitres gives2.5 mg/mL.
In plain terms: the mass is set by the manufacturer, the water is up to you, and the concentration falls out of the two. Concentration is the bridge between "how much compound" and "how many units to draw."
The formulas
`` concentration (mg/mL) = peptide mass (mg) ÷ water (mL) draw volume (mL) = dose ÷ concentration insulin units (U-100) = draw volume (mL) × 100 ``
A U-100 insulin syringe holds 100 units per millilitre, so units are just the draw volume scaled by 100. On a U-50 syringe multiply by 50, on a U-30 by 30.
Worked example
Say a vial holds 5 mg and you add 2 mL of bacteriostatic water. Concentration is 2.5 mg/mL. For a 250 mcg reference dose:
250 mcg = 0.25 mg- draw volume =
0.25 ÷ 2.5 = 0.1 mL - units =
0.1 × 100 = 10 unitson a U-100 syringe
The same vial holds 5 ÷ 0.25 = 20 such doses.
Same vial, different water — same dose
This table is the whole point of the article in one picture. A 5 mg vial, a 250 mcg reference dose, three different water volumes:
| Water added | Concentration | 250 mcg draw (U-100) |
|---|---|---|
| 1 mL | 5 mg/mL | 5 units |
| 2 mL | 2.5 mg/mL | 10 units |
| 3 mL | 1.67 mg/mL | 15 units |
All three deliver the identical amount of compound. Only the number of units on the barrel changes. In plain terms: more water spreads the same dose across more marks, which makes small draws easier to read accurately.
Why bacteriostatic water
The standard diluent is bacteriostatic water — sterile water with about 0.9% benzyl alcohol added as a growth-inhibiting (bacteriostatic) preservative, so the vial can be punctured and drawn from repeatedly over days or weeks1. Plain sterile water has no preservative and is intended for single use.
The physical steps
- Let both the vial and the bacteriostatic water reach room temperature.
- Wipe both rubber stoppers with an alcohol swab.
- Draw your chosen volume of bacteriostatic water into a syringe.
- Add the water slowly, down the inside wall of the vial — aiming the stream directly onto the powder pellet can shear fragile peptides.
- Do not shake. Swirl gently, or let it sit; most peptides dissolve within a few minutes.
- Label the vial with the concentration and the date you reconstituted it.
Peptides are delicate proteins. Heat, light, and the foaming caused by shaking are the common ways they degrade2. Gentle handling preserves them.
Why the water volume is a choice, not a rule
Because dose and concentration are independent, two people using the same compound at the same dose can add completely different amounts of water and both be correct — they will simply draw a different number of units for the identical dose. The practical goal is to choose a concentration that lands your typical draw somewhere readable on the syringe: not 1–2 units, not off the end of the barrel.
Use the reconstitution calculator below to try your own numbers, then read how to read an insulin syringe if the units-versus-millilitres distinction is still fuzzy.