Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative, which lets a multi-dose vial be punctured more than once (1). Hospira’s version is a prescription drug, and in August 2026 FDA told a peptide seller that its bacteriostatic water “is a drug” (2).
This page covers what the label says, where to look, how long an opened vial lasts, how it differs from sterile water, and how to spot a mislabeled vial. It lists no doses and no reconstitution steps.
Quick takeaways
| Topic | What the sources say |
|---|---|
| What it is | Water for injection with 0.9% (9 mg/mL) benzyl alcohol (1) |
| Prescription status | Hospira’s label is marked Rx only (1) |
| FDA position | August 2026 warning letter to a peptide seller: its bacteriostatic water is a drug (2) |
| After first puncture | Discard within 28 days unless the manufacturer states another date (3) |
| Peptide compatibility | Product-specific. Benzyl alcohol promoted rhG-CSF and interferon alfa-2a aggregation (4, 5) |
| Not for | Neonates, per the label warning (1) |
What is bacteriostatic water?
Bacteriostatic water for injection is water with 0.9% (9 mg/mL) benzyl alcohol, and Hospira also lists a 1.1% (11 mg/mL) version in glass vials. Its pH is 5.7, within a range of 4.5 to 7.0 (1).
Bacteriostatic means the preservative inhibits bacterial growth. It isn’t a sterilizer. The label describes the product as a diluent for drugs that require dilution before injection (1).
Is bacteriostatic water legal to buy?
The Hospira label is marked “Rx only” (1). That makes a prescriber and a pharmacy the route for the labeled product.
Peptide sellers also list bacteriostatic water, and FDA has addressed that directly. On 24 August 2026 it sent a warning letter to a peptide seller about its “Bacteriostatic Water 0.9% Benzyl Alcohol (10 ml).” FDA said the water was a drug because it gave customers the means to prepare an injectable drug for human use. The seller’s labels said “research use only,” and FDA said evidence from its website showed the products were intended as drugs for human use (2).
Where to buy bacteriostatic water
The labeled pharmaceutical product comes through a prescription. Sellers in the peptide market sit in the area FDA is enforcing, so check the label items below on any vial before you rely on it. You can compare sellers in our best peptide vendors guide.
Seller listings
Check the label before you rely on a vial
Look for the preservative and concentration, a named manufacturer, and a legible lot and expiry date.
How long does bacteriostatic water last?
An unopened vial is good until its printed expiration date.
Once punctured, a multi-dose vial should be dated and discarded within 28 days unless the manufacturer states another date. That’s CDC’s guidance, citing USP General Chapter 797, and the date should never run past the original expiration (3). Write the puncture date on the vial.
That’s a sterility rule for the vial. It says nothing about how long a peptide stays intact once dissolved, which depends on the peptide. Our guide on storing peptides covers the chemistry.
Bacteriostatic water vs sterile water
| Type | Preservative | Vial type | After first puncture |
|---|---|---|---|
| Bacteriostatic water | 0.9% or 1.1% benzyl alcohol | Multiple-dose vial | Discard within 28 days unless the manufacturer states another date (3) |
| Sterile water for injection | None added | Usually single-dose | Keep no leftovers (3) |
CDC says never to use a single-dose vial for more than one patient, and not to keep leftovers from one for later (3). Saline is a different product from either water.
Does benzyl alcohol affect peptides?
Benzyl alcohol is the most widely used antimicrobial preservative in multi-dose protein formulations. It can promote aggregation of some of them. At 0.9% it accelerated aggregation of recombinant human G-CSF at pH 7 (4). It induced interferon alfa-2a aggregation in a concentration-dependent way (5). For one acylated 31-residue peptide, it caused none, although m-cresol did (6).
Compatibility is product-specific. There’s no general rule that 0.9% is safe for a given peptide.
Why does the label say not for neonates?
Benzyl alcohol has been linked to a gasping syndrome in newborns. A 1982 report in the New England Journal of Medicine described benzyl alcohol poisoning in intensive-care newborns (7). The Hospira label carries a warning, “NOT FOR USE IN NEONATES” (1).
How to spot a mislabeled vial
Check these before you trust a vial. Our guide to peptide scams to avoid covers other fake-product patterns.
- Preservative. Benzyl alcohol is listed with its concentration, 0.9% or 1.1%.
- Manufacturer. A named manufacturer is on the label.
- Rx labeling. The Hospira product carries “Rx only” and the “NOT FOR USE IN NEONATES.” A vial without them isn’t that product.
- Lot and expiry. Both are legible and not stickered over.
- Condition. The closure is intact and the liquid is clear.
Can you make your own bacteriostatic water?
No. The formula is simple, but you can’t confirm sterility at home, and you can’t see contamination in a clear liquid.
Frequently asked questions
Can you use bacteriostatic water for peptides?
The Hospira label describes it as a diluent for drugs that require dilution before injection. Compatibility with any given peptide is product-specific, because benzyl alcohol can promote aggregation of some proteins.
Is bacteriostatic water the same as sterile water?
No. Bacteriostatic water contains 0.9% or 1.1% benzyl alcohol as a preservative and comes in multiple-dose vials. Sterile water has none added and is usually a single-dose product.
How long does bacteriostatic water last once opened?
CDC, citing USP General Chapter 797, says a multi-dose vial should be discarded within 28 days of opening unless the manufacturer states another date. An unopened vial is good until its printed expiration.
Do you need a prescription for bacteriostatic water?
The Hospira label is marked Rx only. In August 2026, FDA told a peptide seller that its bacteriostatic water is a drug, despite research-use-only labels.
Can you make your own bacteriostatic water?
No. You can’t confirm sterility at home, and you can’t see contamination in a clear liquid.
Is bacteriostatic water safe for newborns?
No. Benzyl alcohol has been linked to a gasping syndrome in newborns, and the label carries a warning: NOT FOR USE IN NEONATES.
What is in bacteriostatic water?
Water for injection with 0.9% (9 mg/mL) benzyl alcohol as a bacteriostatic preservative. Hospira also lists a 1.1% (11 mg/mL) version.
This page is information, not medical advice or instructions for human use. Bacteriostatic water is a prescription drug.
The author’s working paper on this topic is on Zenodo. It’s self-published and hasn’t been peer reviewed.
References
- Pfizer (Hospira). Bacteriostatic Water for Injection, USP: prescribing information.
- FDA. Warning letter to TXP Innovations LLC dba Tex Peptides, 24 August 2026 (reference 735067).
- CDC. Injection safety: clinical safety, multi-dose and single-dose vials.
- Thirumangalathu R, et al. Effects of pH, temperature, and sucrose on benzyl alcohol-induced aggregation of recombinant human granulocyte colony stimulating factor. J Pharm Sci. 2006;95(7):1480-1497. PMID 16729274.
- Bis RL, et al. Role of benzyl alcohol in the unfolding and aggregation of interferon alpha-2a. J Pharm Sci. 2015;104(2):407-415. PMID 25100180.
- Li M, et al. Molecular mechanism of antimicrobial excipient-induced aggregation in parenteral formulations of peptide therapeutics. Mol Pharm. 2022;19(9):3267-3278. PMID 35917158.
- Gershanik J, et al. The gasping syndrome and benzyl alcohol poisoning. N Engl J Med. 1982;307(22):1384-1388. PMID 7133084.