BAC Water
Bacteriostatic Water for Injection, USP · benzyl-alcohol-preserved sterile water
BAC water, Bacteriostatic Water for Injection, USP (United States Pharmacopeia), is sterile water plus benzyl alcohol, a preservative that stops bacteria growing. So one vial can be drawn from again and again. People use it to turn freeze-dried peptides back into an injectable liquid.
The short version
BAC water is sterile water with a small amount of benzyl alcohol, a preservative that stops bacteria from growing. It turns freeze-dried (powdered) peptides and medicines back into a liquid.
Because the preservative keeps germs out, the same vial serves many draws over time. Plain sterile water has no preservative, so its bottle is used once and thrown away.
How it works
One ingredient added. Sterile, nonpyrogenic (free of fever-causing contaminants) water plus benzyl alcohol at 0.9% (9 mg/mL) or 1.1% (11 mg/mL), by product. pH 5.7 (range 4.5–7.0). Supplied in a multiple-dose vial for dissolving or diluting drugs for injection.
The preservative is the point. Bacteriostatic means it stops bacteria growing rather than killing on contact. Benzyl alcohol keeps microbes from growing between uses, so one vial serves many draws. Plain Sterile Water for Injection has none, so it is single-dose.
Not saline. BAC water holds no salt, so it is hypotonic (fewer dissolved particles than blood) and must be made approximately isotonic. Normal saline (0.9% sodium chloride) is already isotonic; a bacteriostatic saline exists too, with the same preservative.
Dose calculator
Reconstitution calculator
Calculated for a 1 mL U-100 insulin syringe (100 units/mL).
Units per dose
5 units
Draw this many units on a U-100 syringe
Each unit holds 50 mcg
- Volume per dose
- 0.05 mL
- Doses per vial
- 40
- Concentration
- 5 mg/mL
One vial lasts
- Daily
- 40 days
- Every other day
- 80 days
- 5×/week
- 56 days
Research use only. Not for human consumption. Outputs are reference values based on research literature. Verify all measurements independently.
Side effects and what to watch
Not for newborns. Benzyl alcohol is linked to a fatal 'gasping syndrome' in premature and newborn infants (reported 1982; American Academy of Pediatrics statement 1983). The label warns 'NOT FOR USE IN NEONATES'. Newborns get preservative-free Sterile Water for Injection instead.
Take care if. Not for fluid replacement, epidural or spinal anesthesia. It must be made approximately isotonic, since intravenous water without solute can cause hemolysis. Benzyl alcohol can sensitize some people. In pregnancy, the label says only if clearly needed.
Worth knowing. Animal-derived label estimates put up to 30 mL intravenous (IV) in an adult as free of toxic effects; roughly 9 mL in a 6 kg infant could change blood pressure. The figures scale with the benzyl-alcohol content.
Sources:DailyMedPMID 7133084PMID 6889041
Common questions
What is BAC water?
Bacteriostatic Water for Injection, USP. Sterile water whose only added ingredient is benzyl alcohol, a preservative that stops bacteria growing. It comes in a multiple-dose vial and is used to dissolve or dilute drugs for injection.
How is it different from sterile water for injection?
Sterile Water for Injection has no preservative, so it is single-dose and any leftover is discarded. BAC water's benzyl alcohol stops microbes growing between uses, so one vial can be entered again and again.
How is it different from saline?
BAC water is water alone, with no salt, so it is hypotonic. Normal saline (0.9% sodium chloride) already carries isotonic salt. A bacteriostatic saline also exists, preserved with the same benzyl alcohol.
Is the benzyl alcohol always 0.9%?
No. Depending on the product it is 0.9% (9 mg/mL) or 1.1% (11 mg/mL). The common 30 mL plastic vial is 0.9%.
How long can a vial be used after opening?
The label sets no day limit and only asks for aseptic technique on each draw. The 28 days most people use is the pharmacy convention for opened multiple-dose vials, from USP <797>.
How should I store it?
At controlled room temperature, no fridge needed. Each draw uses aseptic technique (clean, sterile handling).
Deep dive
Molecular identity · References
05ChemistryMolecular identity
- Composition
- Sterile water + benzyl alcohol
- Benzyl alcohol
- 0.9% (9 mg/mL) or 1.1% (11 mg/mL)
- pH
- 5.7 (4.5–7.0)
- Container
- Multiple-dose vial
- Benzyl alcohol identity
- C7H8O · 108.14 g/mol
- Half-life
- Not applicable. Sterile diluent, not a dosed drug (its benzyl alcohol is oxidized to benzoic acid → hippuric acid, renally cleared)Not applicable
06Primary sourcesReferences
- DailyMedBacteriostatic Water for Injection, USP. Hospira/Pfizer (FDA label)
- DailyMedSterile Water for Injection, USP (FDA label)
- DailyMedBacteriostatic 0.9% Sodium Chloride Injection, USP (FDA label)
- PubChem CID 244PubChem CID 244 (benzyl alcohol)
- PubChem CID 962PubChem CID 962 (water)
- PMID 7133084Gershanik et al. 1982 (NEJM, gasping syndrome)
- PMID 6889041AAP 1983 (benzyl alcohol in neonatal units)
- USP <797>USP General Chapter <797> (multiple-dose vial beyond-use dating)
Research use only · Not medical advice · Updated 2026-09-24