Bacteriostatic water (BAC) is sterile water with 0.9% benzyl alcohol added as a preservative. That preservative lets a multi-use vial stay usable for about 28 days after opening — which is why it's the go-to for reconstituting freeze-dried research peptides. You add it to the powder, swirl gently, and the peptide dissolves into a liquid you can measure.
What it actually is
Lyophilized peptides ship as a dry powder for stability. To work with them you have to reconstitute — dissolve the powder into a liquid of known concentration. Bacteriostatic water is the preferred diluent because the 0.9% benzyl alcohol suppresses bacterial growth, so a vial can be entered more than once over several weeks without spoiling. Plain sterile water has no preservative, so it's single-use. This is why bac water for peptides — rather than plain sterile water — is the standard choice for multi-use research vials.
Bac water vs. sterile water vs. SWFI
| Type | Preservative | After opening | Best for |
|---|---|---|---|
| Bacteriostatic (BAC) | 0.9% benzyl alcohol | ~28 days, refrigerated | Multi-use research vials |
| Sterile water | None | Use immediately | Single-use only |
| SWFI (sterile water for injection) | None | Use immediately | Single-use only |
The practical takeaway: if a vial will be opened more than once, bacteriostatic water is the standard choice. Check the label for 0.9% benzyl alcohol — that's what makes it bacteriostatic.
How much to add
There's no single "right" amount — it sets your concentration, and concentration is just:
concentration (mg/ml) = peptide mg ÷ bacteriostatic water ml
More water = more dilute = larger, easier-to-measure draws; less water = more concentrated. Run your exact numbers with our peptide calculator — it converts mg, ml, and target amount into concentration and syringe units instantly. This is lab math, not dosing guidance.
Adding it without wrecking the peptide
Storage & shelf life
Unopened BAC water keeps until its printed date. Once punctured, it's good for roughly 28 days refrigerated. A reconstituted peptide vial follows the shorter of the two dates — keep it cold (2–8 °C), out of light, and label it with the date you mixed it.
Where to buy bacteriostatic water
Wondering where to buy bacteriostatic water? Most research-peptide vendors sell it alongside vials and syringes, and it's also stocked by pharmacies and medical suppliers. When buying, check for an intact seal, 0.9% benzyl alcohol on the label, and a clear expiration date. Compare peptide vendors on our directory — many bundle supplies. For the short-answer versions of common questions, see our bac water answers.
For research use only. This article is educational and makes no health, dosing, or usage claims of any kind.