Bacteriostatic Water: Complete Guide to Mixing, Ratios & the 28-Day Rule
Bacteriostatic vs sterile water vs saline, how much to use per vial, concentration tables, supplier checks, technique, and storage rules.
By PX1 Research Team · August 4, 2026
What bacteriostatic water actually is
Bacteriostatic water is sterile water for injection with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol is what makes a vial multi-use: it suppresses microbial growth after the stopper is punctured, which is why bacteriostatic water carries a ~28-day in-use window while plain sterile water is single-use only.
Bacteriostatic vs sterile vs saline
| Diluent | Preservative | Multi-use | Typical use |
|---|---|---|---|
| Bacteriostatic water | 0.9% benzyl alcohol | Yes (~28 days) | Multi-dose lyophilized vials |
| Sterile water for injection | None | No | Single-use reconstitution |
| 0.9% sodium chloride | None (unless bacteriostatic NaCl) | No | Isotonic dilution |
How much to use
The volume you add sets the concentration — it does not change the amount of compound in the vial. Common ratios:
| Vial | Diluent | Resulting concentration |
|---|---|---|
| 5 mg | 2 mL | 2.5 mg/mL |
| 10 mg | 2 mL | 5 mg/mL |
| 30 mg | 3 mL | 10 mg/mL |
| 50 mg | 5 mL | 10 mg/mL |
Use the peptide reconstitution calculator to convert any combination into insulin units.
How to judge a bacteriostatic water supplier
- USP-grade sterile fill with a stated manufacturer, not an unlabeled bottle.
- Lot number and expiry printed on the vial, not just the carton.
- 0.9% benzyl alcohol stated on the label — if the label omits the preservative concentration, it may be plain sterile water.
- Intact flip-top seal and a clear, particulate-free solution.
- 10 mL and 30 mL options — 30 mL is more economical for multi-vial work, but the 28-day clock starts at first puncture regardless of vial size.
Storage and the 28-day rule
Store unopened vials at room temperature away from light. Once punctured, date the vial and refrigerate; discard at 28 days even if solution remains. Never re-sterilize or top up a used vial.
Technique
Wipe the stopper with 70% isopropyl alcohol, inject diluent slowly against the vial wall so the stream does not hit the lyophilized cake directly, and swirl gently until clear. Shaking shears peptide chains and causes foaming. If the solution stays cloudy or shows particulate after several minutes, do not use it — contact the supplier with the lot number.
See also: How to start with peptides and Who has the best bacteriostatic water.
All figures below are reported from published laboratory literature and are provided for informational and laboratory-research context only. PX1 Research supplies compounds for laboratory and research use only — not for human or veterinary use.