What is bacteriostatic water?

Bacteriostatic water for injection (often abbreviated BAC water or BW) is sterile water that contains 0.9% benzyl alcohol as a preservative. The benzyl alcohol is bacteriostatic — it prevents bacterial multiplication in the vial after it has been punctured — which is what allows a multi-dose peptide vial to remain safe to access repeatedly over several weeks after reconstitution.

The word 'bacteriostatic' matters. It does not mean the water is sterilising or that it kills bacteria on contact. It means it prevents proliferation. If a contaminated needle is used to reconstitute a vial with bacteriostatic water, the initial inoculum of bacteria is still present in the reconstituted vial; the preservative merely prevents that inoculum from growing to problematic levels. Adequate aseptic technique prevents the inoculum from entering in the first place — the preservative is a second-line defence.

Benzyl alcohol is generally well-tolerated by injection but has some contraindications. It should not be used in neonates or in patients with known benzyl alcohol allergy. For intranasal peptides, preservative-free sterile saline is often preferred because nasal mucosa can be more sensitive. Otherwise, bacteriostatic water is the standard multi-dose vial diluent used across research-supply peptide practice.