In this guide
What it actually is
Bacteriostatic water is sterile water with one addition: a small amount of a preservative, almost always 0.9% benzyl alcohol. That word "bacteriostatic" is the whole point — it means "stops bacteria from multiplying." The benzyl alcohol doesn't sterilize anything on contact, but it suppresses bacterial growth inside the vial, which is what lets you draw from the same vial more than once over a span of days instead of throwing it out immediately.
Bac water vs sterile vs distilled
Three "waters" get confused constantly. The differences matter:
| Type | Preservative? | Use pattern |
|---|---|---|
| Bacteriostatic water | Yes (benzyl alcohol) | Multi-use over ~28 days |
| Sterile water | No | Single use, then discard |
| Distilled water | No (and not for injection) | Lab/household, not reconstitution |
The key distinction is the preservative. Sterile water is pure and sterile but has nothing to keep bacteria from growing once it's opened, so it's meant for a single use. Distilled water is simply purified water — it isn't formulated or sterilized for injection and isn't the right choice for reconstitution at all. Bacteriostatic water is the multi-use option, which is why it's the standard for peptides that are drawn from over several sessions.
Why it's used for peptides
Research peptides arrive as a freeze-dried powder and have to be dissolved back into liquid before use — a step called reconstitution. Because a single vial of peptide is usually drawn from repeatedly over days or weeks, the liquid it's mixed into needs to resist bacterial growth for that whole window. That's exactly what bacteriostatic water is built for. Pair that with proper cold, dark storage and the reconstituted peptide stays stable for its usable life.
How much to use
This is the most-asked question and it has no single answer — on purpose. The amount of bacteriostatic water you add is what sets the concentration of the finished solution. Add more water and each unit of liquid contains less peptide (more dilute); add less and each unit contains more (more concentrated). So the "right" amount depends on how much peptide is in the vial and what concentration a given protocol calls for.
Rather than guess, run the numbers: our reconstitution calculator takes the peptide amount and your target and tells you exactly how much water to add, and the full walkthrough with the math is in How to Reconstitute Research Peptides. A practical tip: adding the water slowly down the inside wall of the vial, rather than blasting it onto the powder, helps the peptide dissolve gently.
How long it lasts once opened
An unopened vial of bacteriostatic water keeps for a long time, but the clock changes once you puncture the stopper. As a general rule, an opened vial is considered good for about 28 days when kept clean and refrigerated, after which any remainder is discarded. The benzyl alcohol slows bacterial growth — it doesn't make the vial last forever. Keep the stopper wiped with an alcohol pad before each draw, and never return liquid to the vial.
Stock up on bacteriostatic water. Available in convenient vial sizes alongside the syringes and prep pads you'll need to reconstitute.
Shop Bacteriostatic WaterFrequently asked questions
What is bacteriostatic water? Sterile water plus a preservative (typically 0.9% benzyl alcohol) that inhibits bacterial growth, allowing a vial to be used multiple times over a period of days.
How is it different from sterile water? Sterile water has no preservative and is single-use; bacteriostatic water's benzyl alcohol lets it be reused for roughly a month, which is why it's the standard for peptide reconstitution.
How much do I add? It depends — the water volume sets the concentration. Use a reconstitution calculator based on the peptide amount and your target concentration.
How long does it last once opened? Generally about 28 days, kept clean and refrigerated, then discard the rest.
All Patriot Labs products are sold strictly for in-vitro research and laboratory use only. Not for human consumption. This guide is general educational information about a laboratory reagent and is not medical advice.