Bacteriostatic Water Explained
Bacteriostatic water is sterile water with about 0.9% benzyl alcohol added to inhibit bacterial growth, making it suitable for multi-dose vials. It is the standard diluent for reconstituting lyophilized peptides. The volume you add sets the concentration and how many units you draw, but never changes the milligram dose itself.
For educational and research purposes only. Not medical advice. Always consult a licensed healthcare professional before starting any protocol.
What makes it bacteriostatic
Bacteriostatic water is ordinary sterile water for injection with a small amount of benzyl alcohol, usually around 0.9%, added as a preservative. That preservative stops bacteria from multiplying in the vial, which is what lets you puncture the stopper and draw from the same vial many times over several days or weeks.
Plain sterile water and saline have no preservative, so they are intended for single use. For a multi-dose peptide vial you will draw from repeatedly, bacteriostatic water is the standard choice.
How much to add
There is no fixed amount. The volume you add sets the concentration, and the concentration determines how many units you draw for a given dose. A common approach is to work backward from an easy-to-read draw: pick the unit mark you want your dose to land on, then solve for the water volume that produces it.
As a rule of thumb, most people add 1 to 3 mL per vial. More water makes the solution weaker and the draw larger and easier to measure precisely; less water makes it stronger and the draw smaller. Very small draws are harder to measure accurately, so err toward a slightly larger volume when the dose is tiny.
Why it never changes your dose
Reconstitution changes concentration, not quantity. If you have a 5 mg vial, it contains 5 mg of peptide whether you dissolve it in 1 mL or 3 mL. Adding more water simply spreads that same 5 mg across more liquid, so you draw a larger volume for the same milligram dose. Always think in milligrams first, then convert to units for the syringe.
Frequently asked questions
Is bacteriostatic water the same as sterile water?
No. Both are sterile, but bacteriostatic water adds benzyl alcohol as a preservative so the vial can be used multiple times. Plain sterile water has no preservative and is meant for single use.
How much bacteriostatic water do I add to a 5 mg vial?
Most people add 1 to 3 mL. Adding 2 mL gives 2.5 mg/mL, a clean ratio where a 0.25 mg dose is 10 units. The exact best volume depends on the dose and the unit mark you want to hit.
Can I use more bacteriostatic water than the vial seems to hold?
A typical vial holds only 2 to 3 mL of headspace, so very large volumes may not fit. If your target needs more water than the vial can hold, choose a larger unit target or a smaller dose per draw.
Does the benzyl alcohol affect the peptide?
At the small concentrations used, benzyl alcohol is a well-established preservative for injectable multi-dose products. Some people are sensitive to it, which is a separate question to raise with a licensed professional.
Related tools
Bacteriostatic Water Calculator
Free bacteriostatic water calculator: enter your vial strength and dose, then get the exact amount of bac water to add so your dose lands on the units you want, shown on a to-scale insulin syringe.
Peptide & GLP-1 Dose Calculator
Free lab-grade calculator for peptide reconstitution and GLP-1 dosing. Calculate your exact dose and see it rendered on a to-scale insulin syringe — semaglutide, tirzepatide, BPC-157 and more.