Bacteriostatic Water: What It Is and Why It Matters
Sterile water with a benzyl alcohol preservative, and the reason every reconstitution calculator names it. What the labels say, and how it differs.
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Plain English
Bacteriostatic water is sterile water with a preservative, usually benzyl alcohol, so a multi-dose vial can be entered more than once under proper conditions. It is not the same as plain sterile water, and the difference is why calculators name it specifically.
- It is water plus a bacteriostatic preservative - different from unpreserved sterile water.
- Labels specify strength and limits (including neonate warnings on some products).
- We explain maths and product names for education, not reconstitution instructions for human use.
Bacteriostatic water shows up in every reconstitution forum thread. Here is the label-level picture - without turning this page into a how-to for unapproved injectables.
What it is
Bacteriostatic Water for Injection, USP is sterile water that contains benzyl alcohol as a bacteriostatic preservative. The preservative is why a multi-dose vial can be entered more than once under appropriate pharmaceutical conditions.
Per Pfizer’s prescribing information for Bacteriostatic Water for Injection, USP:
- Benzyl alcohol is present at 0.9% (9 mg/mL) or 1.1% (11 mg/mL) depending on the presentation - so 0.9% is common, not universal.
- Approximate pH 5.7 (labelled range 4.5–7.0).
- Supplied as multi-dose vials (for example 30 mL presentations described on the label).
- Boxed: NOT FOR USE IN NEONATES.
The same label discusses animal-derived estimates around benzyl alcohol exposure; that is not a green light for unlimited human IV use. Read the full label for any product you handle.
Not the same as sterile water
| Product | Preservative | Typical framing |
|---|---|---|
| Sterile Water for Injection | None | Single-use reconstitution contexts |
| Bacteriostatic Water for Injection | Benzyl alcohol | Multi-dose vial contexts per label |
Swapping them casually is how people invent rules that labels never wrote.
Why peptide calculators mention it
Reconstitution maths needs a diluent volume (mL). Calculators use that volume to compute concentration and draw volume. They do not magically make an unapproved powder into a medicine.
- More diluent → lower concentration → larger volume per dose
- More diluent does not create more peptide - doses per vial depend on total mass and dose size, not how much water you poured
Worked arithmetic: How to reconstitute peptides · Tool: Dosage calculator · SEO twin: Peptide dosage calculator
Legal note for research peptides
If the powder is sold for research use only, reconstitution for human self-administration is outside the approved-drug pathway. FDA looks at intended use, not just a disclaimer sticker. See research peptides and the law.
References
Bibliographic detail is fetched from PubMed, not written by us - so a citation here cannot drift from the paper it names. Study design comes from PubMed's own tags rather than our judgement.
- 1.Pfizer - Bacteriostatic Water for Injection, USP prescribing information: benzyl alcohol 0.9% or 1.1%, pH range, multi-dose presentation, and NOT FOR USE IN NEONATES boxed warning language.- Pfizerlabel
- 2.DailyMed - HUMULIN R U-100 label language on units per mL and correct syringe type (context for why unit scales matter when measuring liquids).- DailyMed / FDA label repositorylabel