Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. That preservative is the whole point: it lets a vial be punctured and drawn from repeatedly, which is why it's the default solvent for turning a lyophilised research-peptide cake back into a usable solution. This guide covers what it is, how it differs from sterile water, exactly how much to add, and how to store it. Research use only — not for human use.
It's water for injection, manufactured to USP sterility standards, with 0.9% (v/v) benzyl alcohol. Benzyl alcohol is bacteriostatic — it stops bacteria from replicating — so the solution resists contamination across multiple needle punctures. Plain sterile water has no such protection. That single difference is what makes bacteriostatic water the standard for multi-dose research vials.
| Bacteriostatic water | Sterile water (SWFI) | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Punctures | Multi-use (repeated draws) | Single-use |
| Once opened | ~28 days refrigerated | Use immediately / discard |
| Best for | Multi-dose research peptide vials | Protocols that must avoid benzyl alcohol |
For almost all research peptide work the answer is bacteriostatic water. Reach for preservative-free sterile water only when the protocol specifically needs to exclude benzyl alcohol.
This is the question everyone asks, and the key insight is: the volume of water sets the concentration, not the dose. The amount of peptide in the vial is fixed; adding more or less water just makes the solution more dilute or more concentrated.
The math is simply:
Concentration (mg/mL) = peptide mass in vial (mg) ÷ bacteriostatic water added (mL)
Choose a volume that puts your intended measurements on easy-to-read syringe markings, then let our reconstitution calculator do the arithmetic. Our step-by-step reconstitution guide walks through the physical technique — angling the stream down the vial wall, never injecting water directly onto the peptide cake, and swirling rather than shaking.
Unopened bacteriostatic water is fine at room temperature until its labelled expiry. Once you've punctured it, refrigerate at 1–6 °C and treat it as good for roughly 28 days — the window the benzyl alcohol reliably protects. The reconstituted peptide solution should also be refrigerated and shielded from light; most peptides degrade only a few percent per year under those conditions, but always follow the stability window for the specific compound.
New-U supplies bacteriostatic water (and preservative-free sterile water) as a reconstitution solvent alongside the research-peptide catalogue, so the compound and its solvent can be sourced together — all for laboratory research use only.
What is bacteriostatic water?
Sterile water for injection containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol stops bacterial growth, which lets a vial be drawn from repeatedly — making it the standard solvent for reconstituting lyophilised research peptides. Research use only.
Bacteriostatic vs sterile water — what's the difference?
Both are sterile; only bacteriostatic water has the 0.9% benzyl alcohol preservative, so it stays usable for ~28 days across multiple punctures. Sterile water is single-use. For multi-dose research vials, bacteriostatic water is the usual choice.
How much do I add to a peptide vial?
The water sets the concentration, not the dose: concentration (mg/mL) = vial mass (mg) ÷ water added (mL). A 5 mg vial + 2 mL = 2.5 mg/mL. Pick a convenient volume and use a reconstitution calculator. Lab preparation math only.
How much to mix with 10mg semaglutide or tirzepatide?
10 mg vial + 1 mL = 10 mg/mL; + 2 mL = 5 mg/mL. The water volume only changes concentration so syringe markings are easy to read — it doesn't change what's in the vial. Research preparation math only.
Does it need refrigerating, and how long does it last?
Unopened: room temperature until expiry. Opened: refrigerate at 1–6 °C and treat as good for ~28 days. After that, discard and use a fresh vial.
New-U supplies >99% HPLC-verified research compounds with a per-batch Certificate of Analysis, plus the bacteriostatic and sterile water to reconstitute them. Research use only — not for human consumption.
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