Guide
Bacteriostatic water (bac water) explained
“Bac water” is the standard liquid for dissolving lyophilised peptides because its preservative lets one vial be used many times.
What is in bac water?
Bacteriostatic water is sterile water for injection with 0.9% benzyl alcohol (9 mg/mL) added as a preservative. The benzyl alcohol does not kill all bacteria outright — it stops them multiplying (“bacteriostatic”), which keeps a vial usable after its stopper has been punctured.
Bac water vs sterile water vs saline
| Bacteriostatic water | Sterile water | 0.9% saline | |
|---|---|---|---|
| Preservative | 0.9% benzyl alcohol | None | None (unless bacteriostatic saline) |
| Multi-use? | Yes — up to 28 days | No — single use | Usually single use |
| For peptides | Standard choice | OK for same-day use | Some peptides less soluble |
| Not suitable for | Neonates; benzyl-alcohol sensitivity | — | — |
How long does bac water last?
Unopened, bac water lasts until its printed expiry date when stored at controlled room temperature. Once the stopper has been punctured, manufacturer labelling (for example Hospira's) says to discard 28 days after first use. Peptides reconstituted with it should be kept in the fridge.
Does bacteriostatic water need to be refrigerated?
Unopened bac water does not need refrigerating — store at 20–25 °C away from light. After opening it can be kept at room temperature or refrigerated for the 28-day window. Once it is mixed into a peptide vial, the peptide needs refrigeration.
Hospira and other brands
Hospira (now part of Pfizer) is the best-known US manufacturer, which is why “Hospira bac water” is so widely searched. In the UK, bacteriostatic water is supplied by research-chemical and laboratory suppliers in 10 mL and 30 mL vials. What matters is that it is sterile, contains 0.9% benzyl alcohol, and comes in a sealed vial with a batch number.
Frequently asked questions
What is in bac water?
Sterile water with 0.9% benzyl alcohol as a preservative.
How long does bacteriostatic water last once opened?
Up to 28 days after the stopper is first punctured, per manufacturer labelling.
Is bacteriostatic water the same as sterile water?
No. Sterile water has no preservative and should be used once; bacteriostatic water can be used repeatedly for 28 days.
Can you use bac water for all peptides?
It is suitable for most. A few peptides are better dissolved in other solvents (for example dilute acetic acid for some hydrophobic peptides); check the supplier's guidance.