Last updated: May 18, 2026
What bacteriostatic water is
Bacteriostatic water for injection (BWFI) is sterile water containing 0.9% (9 mg/mL) benzyl alcohol as a bacteriostatic preservative. Available in multi-dose vials, typically 30 mL or 20 mL.
The benzyl alcohol prevents microbial growth without killing organisms (bacteriostatic, not bactericidal). This makes multi-dose vials viable for repeated puncture over 28 days after first opening.
Why it’s the standard for peptide reconstitution
Three properties make BAC water the standard solvent:
- Sterility: Manufactured under cGMP standards in sealed vials
- Preservative activity: Allows reconstituted peptide solutions to remain stable for 4-6 weeks under refrigeration
- Compatibility: Compatible with virtually all peptides at physiologic concentrations
BAC water vs other solvents
| Solvent | Sterile | Preservative | Multi-use? |
|---|---|---|---|
| Bacteriostatic water | Yes | 0.9% benzyl alcohol | Yes (28 days) |
| Sterile water for injection | Yes | None | Single-use |
| 0.9% saline (normal saline) | Yes | None (single-use) or methylparaben/propylparaben (multi-dose) | Depends on formulation |
| Dextrose 5% in water (D5W) | Yes | None | Single-use |
How much to use per vial
Common ratios for peptide reconstitution:
- 5 mg vial: 1-3 mL BAC water (concentration 5000-1666 mcg/mL)
- 10 mg vial: 2-5 mL BAC water
- 1 mg vial: 0.5-1 mL BAC water
More BAC water = more dilute = larger volume per dose = easier accurate dosing for small target doses. But more dilute = shorter total useful life (since you’re using more of the vial per dose).
Storage and shelf life
Unopened BAC water vial: Room temperature, 2+ year shelf life from manufacture date.
Opened (first punctured) BAC water vial: 28 days at room temperature, then discard.
Reconstituted peptide in BAC water: Refrigerated 36-46°F (2-8°C), 4-6 weeks for most peptides (some are more stable, some less).
Where to obtain BAC water
Pharmacy-grade bacteriostatic water requires prescription in the US (often available through telehealth services). Research-grade BAC water is sold by peptide vendors. Quality verification (sterility, accurate benzyl alcohol concentration) is important — counterfeit or improperly manufactured BAC water introduces sterility risk.
Safety considerations
Benzyl alcohol is metabolized to benzoic acid and excreted. In adults, the small amounts used in BAC water are considered safe. Important exceptions:
- Neonates: Benzyl alcohol toxicity (“gasping syndrome”) in newborns — BAC water contraindicated in pediatric injection
- Pregnancy/lactation: Benzyl alcohol crosses placenta; use with caution
- Allergy: Rare benzyl alcohol hypersensitivity exists
Can I use saline instead of bacteriostatic water?<br />
Single-use sterile saline works for one-time dose preparation, but lacks the benzyl alcohol preservative needed for multi-week storage of reconstituted peptide.
How is BAC water different from sterile water?<br />
Same sterility, but BAC water contains 0.9% benzyl alcohol as preservative. Sterile water for injection has no preservative and is single-use only.
Can I refill or reuse a BAC water vial?<br />
No — the 28-day shelf life starts when the vial is first punctured. Discard after 28 days regardless of remaining volume.
What if I accidentally use too much BAC water?<br />
Not a safety issue — your peptide will just be more dilute. Recalculate your dose volume based on the new concentration.