What Is Bacteriostatic Water? (A Guide for Research Peptide Use)
Bacteriostatic water is the standard solvent for reconstituting research peptides. Here is what makes it different from sterile water, why the preservative matters, and how to store it.
Bacteriostatic water is the solvent almost every research peptide is reconstituted in. It looks identical to plain sterile water in the vial, but the chemistry inside is different — and that difference is what makes a multi-dose research vial workable.
This is a short guide to what bacteriostatic water actually is, how it differs from the other clear liquids it sits next to on a supply shelf, and what to look for when sourcing it.
For laboratory research use only.
What is in the vial
Bacteriostatic water — often written BAC water — is sterile water for injection that contains 0.9% benzyl alcohol as a preservative. Benzyl alcohol is a bacteriostatic agent, meaning it suppresses the growth of bacteria rather than killing them outright. The concentration is low enough to be compatible with most peptide chemistry and high enough to keep a punctured vial usable across a multi-dose window.
The standard format in the research peptide market is a 30 mL multi-dose vial with a rubber septum on top. The vial is designed to be punctured repeatedly with a fresh syringe each time, drawing out small volumes of solvent over the 28 to 30 days after the first puncture.
That multi-dose use case is the whole point. A typical 5 mg research peptide vial is reconstituted into 2 mL of solvent and then drawn from across approximately 30 days. Without a preservative in the solvent, every draw after the first one would carry a contamination risk. Benzyl alcohol is what makes the 30-day reconstituted window viable.
Bacteriostatic vs sterile water vs saline
The three clear liquids that show up next to each other in this conversation are not interchangeable.
| Solvent | Preservative | Use case |
|---|---|---|
| Bacteriostatic water | 0.9% benzyl alcohol | Multi-dose vials; standard for research peptide reconstitution |
| Sterile water for injection | None | Single-use only; vial must be discarded after one puncture |
| 0.9% sodium chloride (saline) | None or preservative-dependent | Specific clinical applications; not standard for peptide reconstitution |
The most common mistake when buyers source solvent for the first time is grabbing a vial labeled "sterile water for injection" because it looks the same and is cheaper. It is not the same. A peptide reconstituted in plain sterile water is, by the rules of the solvent, a single-use vial — every draw after the first carries an open contamination risk.
For the full reconstitution procedure and why preservative chemistry matters, see our peptide reconstitution guide.
Storage and sourcing
Unopened bacteriostatic water is stable at room temperature for the duration of its labeled shelf life — typically 2 to 3 years from manufacture. Once the septum is punctured, refrigerate between uses and observe the 28 to 30 day open-vial window printed on the label.
Sources commonly used by the research community include US compounding pharmacies, research-supply vendors, and some peptide suppliers that ship a BAC water vial as an add-on alongside the peptide order. The supplier add-on is the most convenient option for new buyers because it removes one sourcing step.
Two checks when buying:
Read the label. The product must specify "Bacteriostatic Water for Injection" and "0.9% benzyl alcohol." Anything labeled only "sterile water" is the wrong product, even from a legitimate vendor.
Check the volume. Standard research use is a 30 mL multi-dose vial. Single 10 mL ampoules are sold but are typically intended as single-use units and are not ideal for the 30-day reconstituted peptide window.
For research use only. Not for human consumption.
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