BAC Water
Also known as: bacteriostatic water · bacteriostatic water for injection · BWFI
Bacteriostatic water (BAC water) is sterile water for injection with 0.9% benzyl alcohol added as a preservative, used to dissolve drugs for injection.
Bacteriostatic Water for Injection, USP is sterile, nonpyrogenic water for injection containing benzyl alcohol as a bacteriostatic preservative. The common strength is 0.9% (9 mg/mL); some labels also list a 1.1% version. It comes in multiple-dose containers so that repeated withdrawals can be made, and its approved use is only as a diluent: to dilute or dissolve drugs for intravenous, intramuscular or subcutaneous injection, following the instructions of the drug's manufacturer.
The preservative slows bacterial growth after the container has been punctured. It does not sterilize anything, and it does not make a mixed drug stable. The FDA-approved label carries the warning 'Not for use in neonates', because benzyl alcohol has been associated with toxicity in that age group. Plain Sterile Water for Injection has no preservative, and the label says that only preservative-free sterile water should be used where water is needed to prepare drugs for newborns.
Why it matters
BAC water is used to reconstitute peptides that will be drawn from more than once, because the preservative is meant for repeated withdrawals. Approved peptide products that are given in multiple doses from one vial, such as tesamorelin (EGRIFTA WR), come with BAC water as the supplied diluent.
Two label instructions are easy to miss. First, the BAC water label says not to store reconstituted solutions unless the drug's manufacturer says you can, so how long a mixed vial lasts is set by the drug, not the water. Second, an opened multi-dose container has its own time limit: the CDC, citing USP General Chapter <797>, says an opened multi-dose vial should be dated and discarded within 28 days unless the manufacturer states another date, and the EGRIFTA WR instructions say to discard the BAC water bottle after 28 days of use. The discard date must never be later than the bottle's original expiration date.
How to use it
- Check the label says Bacteriostatic Water for Injection and the expiration date has not passed.
- Write the date of first puncture on the bottle.
- Wipe the stopper with an alcohol swab before each puncture and use a new sterile syringe and needle each time.
- Inspect the water before use; do not use it if it is cloudy, contains particles or the seal was not intact.
- Discard the opened bottle by the earlier of its original expiration date and its in-use limit (28 days after first puncture unless the manufacturer specifies a different limit).
Common mistakes
- Treating BAC water as if it makes a reconstituted peptide last for weeks; stability depends on the peptide.
- Using an opened bottle past 28 days or forgetting to date it.
- Reusing a needle or syringe to draw from the bottle, which can contaminate it.
- Confusing BAC water with sterile water or saline; plain sterile water has no preservative.
- Using BAC water for an infant; the label says not for use in neonates.
Sources
- Prescribing informationBacteriostatic Water for Injection, USP (Hospira) - prescribing information. DailyMed, U.S. National Library of Medicine, 2019.
- Prescribing informationEGRIFTA WR (tesamorelin) for injection - prescribing information and instructions for use. DailyMed, U.S. National Library of Medicine, 2025.
- GuidelinePreventing Unsafe Injection Practices. Centers for Disease Control and Prevention, 2024.
Page updated . How we grade evidence: methodology.
Related Terms
Reconstitution
Reconstitution is dissolving a freeze-dried (lyophilized) powder in a measured volume of liquid to make a solution of known concentration for injection.
Vial Shelf Life
Vial shelf life ends at the earlier of the original expiration date and the applicable limit after opening or mixing.
Concentration (mcg/mL)
Concentration (mcg/mL) is how many micrograms of peptide are in each milliliter of solution; it converts a dose by weight into a volume to draw.
Drawing Dose
Drawing a dose means pulling a measured volume of solution from a vial into a syringe, removing air bubbles, and setting it to the exact mark.
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