What bacteriostatic water actually is
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol stops most bacteria from growing, which is why a reconstituted peptide vial can be withdrawn from multiple times over several weeks without becoming contaminated immediately.
It is not the same as plain sterile water, which has no preservative and is meant for single-use only. It is also different from bacteriostatic sodium chloride, which contains salt and is used for some injectable medicines but is not the standard diluent for most lyophilised peptides.
The peptide community calls it 'bac water' for short. Whatever you call it, the important point is that the preservative is what makes multi-dose storage possible after reconstitution.
How much bacteriostatic water to add
There is no single correct volume. The right amount depends on the milligrams of peptide in the vial and the concentration you want the finished solution to have. The formula is simple: concentration = peptide mass ÷ diluent volume.
A 5 mg vial mixed with 2 mL of bacteriostatic water gives 2.5 mg/mL. A 10 mg vial mixed with 2 mL gives 5 mg/mL. The same 10 mg vial mixed with only 1 mL gives 10 mg/mL, meaning each millilitre holds twice as much peptide but the volume you draw for a given milligram figure is half as much.
Lower concentrations are usually easier to measure accurately with standard insulin syringes. Higher concentrations mean less liquid per dose, which some people prefer, but small volume errors become larger dose errors. For most research vials, 1–2 mL of diluent per 5–10 mg of peptide is a common starting point.
How volume changes concentration and syringe readings
The mass of peptide in the vial is fixed the moment you open it. Adding more water does not create more peptide; it only spreads the same mass through a larger volume. That is why concentration falls as diluent rises.
If you reconstitute a 10 mg vial with 2 mL, the concentration is 5 mg/mL and a 1 mg draw is 0.2 mL, which reads as 20 units on a U-100 insulin syringe. If you instead add 4 mL, the concentration drops to 2.5 mg/mL, the same 1 mg draw becomes 0.4 mL, and the U-100 reading becomes 40 units.
Changing the diluent volume changes every downstream number: the volume per milligram, the units per draw, and how many equal draws the vial theoretically contains. This is why it is worth writing the concentration on the vial immediately after reconstitution — the arithmetic you did at mixing time is the only way to interpret the syringe later.
What to avoid
Tap water, bottled water and unsterile vials should never be used. They can introduce bacteria and endotoxins. Plain sterile water is acceptable only if you intend to use the entire vial in one go, because without benzyl alcohol the solution will not resist microbial growth on repeat withdrawals.
Do not shake the vial to dissolve the powder. Aim the stream of bacteriostatic water down the inside wall of the vial and let the powder dissolve on its own. Swirl gently if needed. Shaking can shear peptide chains and cause aggregation.
After reconstitution
Once mixed, store the vial in the refrigerator and protect it from light and heat. Most reconstituted peptides keep for around four weeks under refrigeration, though some are more fragile and some last longer. Label the vial with the peptide name, concentration, reconstitution date, and the volume of bacteriostatic water added.
If the solution becomes cloudy, discoloured, or develops particles, discard it. A few peptides have a natural colour — GHK-Cu is genuinely blue — but most clear solutions should stay clear.
Tools that do the arithmetic for you
The calculator and chart below run the same concentration and unit conversions described above. Enter your actual vial size and diluent volume to see the exact concentration, volume per milligram, and U-100, U-50 or U-40 syringe-unit readings.