How reconstitution works
A lyophilised vial is freeze-dried powder. The milligram figure on the label is fixed and never changes. All you decide is how much liquid to dissolve it in, and that decision sets every other number.
Concentration is one division - the peptide in the vial divided by the water you add. Once you know the concentration, the volume for any dose is another division.
concentration = vial mg ÷ water mL
volume = dose mg ÷ concentration
A 5 mg vial with 2 mL of water gives 2.5 mg/mL. A 250 mcg dose is 0.25 mg, so 0.25 ÷ 2.5 = 0.1 mL, which is 10 units on any insulin syringe.
Choosing how much water to add
There's no single correct amount. It's a trade-off between how precisely you can measure and how many doses you can get out of a vial before it's exhausted.
Add too little and your dose might land at 2 or 3 units, where being one mark out is a 30% dosing error. Add too much and you'll be drawing 60 or 70 units for a single dose, which won't fit a small syringe and drains the vial faster in volume terms.
The comfortable range is roughly 10 to 40 units per dose. If your figure sits outside that, change the water volume and watch the units move. The calculator flags it when you're outside that band.
| 5 mg vial, 250 mcg dose | Concentration | Volume | Units |
|---|---|---|---|
| 1 mL water | 5.00 mg/mL | 0.05 mL | 5 |
| 2 mL water | 2.50 mg/mL | 0.10 mL | 10 |
| 3 mL water | 1.67 mg/mL | 0.15 mL | 15 |
| 5 mL water | 1.00 mg/mL | 0.25 mL | 25 |
Every row delivers exactly the same 250 mcg. Only the ease of measuring it changes.
Peptide reconstitution questions
Does adding more water make the dose weaker?
It makes the solution weaker but the dose identical, provided you adjust the volume you draw. That's the whole point of the calculation. Doubling the water halves the concentration and doubles the volume for the same dose, so the amount of peptide entering you is unchanged.
Why bacteriostatic water rather than sterile water?
Bacteriostatic water contains a small amount of benzyl alcohol, which suppresses bacterial growth and lets a vial be punctured multiple times over a period of days or weeks. Plain sterile water has no preservative, so a reconstituted vial should be treated as single-use.
How many doses will a vial give me?
Divide the vial contents by the dose, both in the same unit. A 5 mg vial at 250 mcg per dose is 5000 ÷ 250, so 20 doses. Note this is set entirely by the vial and the dose - the amount of water makes no difference to it.
My dose is under 5 units. Is that a problem?
It's harder to measure accurately. At 3 units, being one mark out is a 33% error. Reconstituting with more water spreads the same dose across more units, so the same physical imprecision becomes a much smaller proportional error.
What if my volume is more than the syringe holds?
Either use a larger syringe or reconstitute with less water. The calculator warns you when the dose exceeds the capacity of the syringe you selected, since that's a case where you'd otherwise have to inject twice.
Does this work for any peptide?
The arithmetic is identical for anything supplied as a lyophilised powder measured in milligrams, because it's pure unit conversion. What it can't tell you is anything about a specific compound - its stability once mixed, how it should be stored, or whether the dose you entered is sensible.