BAC Water
Bacteriostatic Water for Injection, USP · benzyl-alcohol-preserved sterile water
BAC water — Bacteriostatic Water for Injection, USP — is the diluent half of the peptide kit: sterile, nonpyrogenic water whose only added ingredient is benzyl alcohol, a bacteriostatic preservative. That preservative is the entire point. It inhibits microbial growth so a single vial can be entered repeatedly over time (a true multiple-dose container), which preservative-free Sterile Water for Injection cannot do. It is a diluent, not a therapy — hypotonic, intended only for diluting or dissolving drugs for injection, and it must be made approximately isotonic before use. The one fact everyone must know: benzyl alcohol is contraindicated in newborns, where it can cause a fatal 'gasping syndrome' — a warning written into FDA labeling after the 1982 FDA action.
The short version
BAC water is sterile water with one thing added: a small amount of benzyl alcohol, a preservative that stops bacteria from growing. It is used to turn freeze-dried (powdered) medicines and peptides back into a liquid.
Why the preservative matters: because it keeps germs from growing, the same little bottle can be used more than once over time. Plain sterile water has no preservative, so its bottle is meant to be used once and then thrown away.
The most important safety point: BAC water must never be used in newborn babies. The benzyl alcohol in it has caused a deadly reaction in infants called 'gasping syndrome,' which is why drug labels warn against it for newborns.
Molecular identity
Specs
- 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
Plain English
Mechanism
Bacteriostatic Water for Injection, USP is sterile, nonpyrogenic water for injection with one added ingredient: benzyl alcohol, a bacteriostatic preservative, present at 0.9% (9 mg/mL) or 1.1% (11 mg/mL) depending on the product. Its pH is 5.7 (range 4.5–7.0). It is supplied in a multiple-dose vial and is indicated only for diluting or dissolving drugs for injection — not as a therapy in its own right.
The preservative is the whole point. 'Bacteriostatic' means it inhibits bacterial growth rather than sterilizing on contact; the benzyl alcohol keeps the vial's contents from supporting microbial growth between uses, which is what lets the same vial be entered repeatedly over time. Preservative-free Sterile Water for Injection (SWFI) has no such protection, so it is single-dose only and the unused portion is discarded.
BAC water also differs from saline. It contains no solute, so it is hypotonic and must be made approximately isotonic before use — intravenous administration without a solute can cause hemolysis; 0.9% sodium chloride ('normal saline') already carries isotonic salt. There is also a bacteriostatic 0.9% sodium chloride that, like BAC water, is benzyl-alcohol-preserved for multiple-dose use. The short version: BAC water vs SWFI is preservative present vs absent (multi-dose vs single-dose); BAC water vs saline is water alone vs water-plus-isotonic-salt.
Reconstitution math
Reconstitution calculator
Reconstitution calculator
Calculated for a 1 mL U-100 insulin syringe (100 units/mL).
Units per dose
5
Draw to this mark on a U-100 syringe
- 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.
From the studies
Side effects from research
The defining safety fact is benzyl alcohol and newborns. BAC water carries a prominent headline label warning — 'WARNING: NOT FOR USE IN NEONATES' — and is contraindicated in this population because benzyl alcohol has been associated with a fatal 'gasping syndrome' in premature and newborn infants: progressive metabolic acidosis, gasping respirations, neurologic deterioration, and cardiovascular collapse. The contraindication was written into FDA labeling after a 1982 New England Journal of Medicine report, the 1982 FDA action, and the American Academy of Pediatrics' 1983 statement. Where water is needed to prepare or dilute medications for neonates, preservative-free Sterile Water for Injection is used instead.
Beyond neonates: the label states BAC water must be made approximately isotonic before use — intravenous administration without a solute can cause hemolysis — and that benzyl-alcohol-preserved preparations should not be used for fluid replacement or in epidural or spinal anesthesia. Animal-derived label data estimate that up to 30 mL may be given intravenously to an adult without toxic effects, while roughly 9 mL in a 6 kg infant could produce blood-pressure changes — figures that scale with the benzyl-alcohol content. Benzyl alcohol is also a recognized preservative sensitizer in susceptible individuals. In pregnancy, labeling advises use only if clearly needed.
Sources:DailyMedPMID 7133084PMID 6889041
Quick answers
Frequently asked
What is BAC water?
Bacteriostatic Water for Injection, USP — sterile water whose only added ingredient is benzyl alcohol, a bacteriostatic preservative. It is a USP-defined diluent used to dissolve or dilute drugs for injection, supplied in a multiple-dose vial.
How is it different from sterile water for injection?
Sterile Water for Injection (SWFI) has no preservative, so it is single-dose only and any unused portion is discarded. BAC water's benzyl alcohol inhibits microbial growth, which is what allows a single vial to be entered repeatedly over time.
Why does it contain benzyl alcohol?
Benzyl alcohol is a bacteriostatic preservative — it keeps the vial's contents from supporting bacterial growth between uses. That is the entire reason BAC water can be a multiple-dose container rather than single-use.
Can BAC water be used in newborns?
No. It carries a prominent headline label warning — 'WARNING: NOT FOR USE IN NEONATES' — and is contraindicated in newborns because benzyl alcohol has been linked to a fatal 'gasping syndrome' in infants. Preservative-free Sterile Water for Injection is used for neonates instead.
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 FDA label does not set a fixed in-use window; it says only that repeated withdrawals may be made using aseptic technique. The widely-repeated '28 days' comes from the general pharmacy beyond-use-date convention for opened multiple-dose vials (USP <797>), not from the product label.
Primary sources
References
- 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 2026-05-29