Bac water (bacteriostatic water): the complete guide
This is the complete guide to bac water: what bacteriostatic water is made of, how it works, what it's used for, how it differs from sterile water, how to store it, and the safety points from the official label. Every fact here is tied to a primary source and reviewed by a pharmacist.
What bac water is
Bac water and bacteriostatic water are two names for the same product: bacteriostatic water for injection, USP. It's sterile water with a small amount of benzyl alcohol added.1 "Bac" is simply short for "bacteriostatic," which describes what the added preservative does: it holds back the growth of bacteria. Because of that preservative, the vial can be entered and drawn from repeatedly, so it's supplied as a multiple-dose container.1
What bacteriostatic water is made of
The formula is simple. It's Water for Injection, USP, with 0.9% (9 mg/mL) benzyl alcohol as the bacteriostatic preservative.1 Here's the full specification from the official label.
| Preservative | Benzyl alcohol 0.9% (9 mg/mL)1 |
|---|---|
| Base | Water for Injection, USP (H2O)1 |
| pH | 5.7 (range 4.5 to 7.0)1 |
| Sterility | Sterile, non-pyrogenic1 |
| Container | Multiple-dose vial for repeated withdrawals1 |
| Storage | 20–25 °C (68–77 °F)3 |
How bacteriostatic water works
The benzyl alcohol is what does the work. It suppresses or stops the growth of most bacteria that could contaminate the vial, so the container can be re-entered multiple times with a sterile needle.2 Plain sterile water has no such preservative, so once it's opened it can't safely be kept for repeated use. That single ingredient is the reason bac water exists as a multi-dose product.
Is the benzyl alcohol safe?
Benzyl alcohol is a widely used pharmaceutical preservative, and at the 0.9% concentration in bacteriostatic water it's considered suitable for its intended drug-diluent use.1 There's one important exception on the label, covered in the safety section below. As with any preservative, a small number of people can be sensitive to it, so follow a healthcare professional's guidance for your situation.
What bac water is used for
Bacteriostatic water is a diluent. The label indicates it only for diluting or dissolving drugs for intravenous, intramuscular, or subcutaneous injection, following the instructions of the manufacturer of the drug being administered.2 The amount of water it adds is small, so when it's used only to dilute or dissolve a drug it's unlikely to affect the body's fluid balance.2 How much you use depends entirely on the drug's own instructions.
Bacteriostatic water vs sterile water
The difference is the preservative. Sterile water for injection contains no bacteriostatic agent, so it's usually a single-dose product that's discarded after one use. Bacteriostatic water's benzyl alcohol allows repeated withdrawals from one vial.1 They aren't interchangeable, so the right one depends on the drug and its labeling. We cover this in depth on the dedicated comparison page.
Storage and shelf life
Store bac water at controlled room temperature, 20 to 25 °C (68 to 77 °F); the label gives no refrigeration instruction, so a fridge isn't needed.3 An unopened vial keeps until its printed expiry date. Once you first puncture it, the common practice is to use it within about 28 days and then discard it — that window is USP <797> beyond-use practice for a preserved multi-dose vial, not a figure on the manufacturer's label.5 Use aseptic technique on every withdrawal, and don't use the product unless the solution is clear and the seal is intact.2
Safety and warnings
Used as directed for diluting drugs, bac water has few side effects of its own; most reactions relate to the drug added or to injection technique.4 The label carries several important warnings worth knowing:
From the official label. Bacteriostatic water is for drug-diluent use only. It's not for use in neonates, because of the potential toxicity of benzyl alcohol. It must be made approximately isotonic before use, and preparations containing benzyl alcohol should not be used in epidural or spinal anesthesia. Injecting it intravenously without a solute may cause red blood cell breakdown (hemolysis) because it isn't isotonic on its own.2,4
Always follow a healthcare professional's guidance and the drug manufacturer's instructions.
Sizes and where to buy
Bac water commonly comes in 10 mL and 30 mL multi-dose vials, and the best-known manufacturer is Hospira, a Pfizer company. If you're deciding where to buy, chain pharmacies stock it inconsistently and marketplace quality varies, so it's worth checking the manufacturer, lot, and expiry on whatever you get.
Bac water 30 mL Bac water 10 mL Hospira Where to buy
Frequently asked questions
Is bac water the same as bacteriostatic water?
Yes. "Bac water" is just the short name for bacteriostatic water for injection, USP.1
What is bacteriostatic water made of?
Water for Injection, USP with 0.9% (9 mg/mL) benzyl alcohol as a preservative.1
Why can bac water be used more than once?
The benzyl alcohol holds back bacterial growth, so the multi-dose vial can be entered repeatedly with aseptic technique.1,2
Does bac water need to be refrigerated?
No. The label specifies controlled room temperature, 20 to 25 °C, with no refrigeration instruction.3
How is bacteriostatic water different from sterile water?
Sterile water has no preservative and is usually single-dose; bac water's preservative allows repeated withdrawals.1
Related guides
- Bacteriostatic Water for Injection, USP — Description. DailyMed, U.S. National Library of Medicine. View label.
- Bacteriostatic Water for Injection, USP — Uses, Directions, Warnings & Precautions. DailyMed. View label.
- Bacteriostatic Water for Injection, USP — Storage, 20–25 °C (Hospira, Inc.). DailyMed. View document.
- Bacteriostatic Water for Injection — Side effects, warnings, hemolysis and neonate cautions. RxList (FDA drug information). View monograph.
- Beyond-use dating for preserved multiple-dose containers (28-day in-use window). USP General Chapter <797> Pharmaceutical Compounding — Sterile Preparations. (Standard pharmacy practice; the manufacturer label sets no in-use discard date.) USP <797>.