Bacteriostatic water

Bacteriostatic water (BAC water) is sterile water for injection to which 0.9% benzyl alcohol has been added as a preservative. The benzyl alcohol inhibits bacterial growth without killing bacteria outright (bacteriostatic rather than bactericidal), maintaining the sterility of the diluent and — after reconstitution — the resulting peptide solution across multiple withdrawals from the same vial over a typical 28-day use period.

Why it matters in peptide research

When lyophilised peptides are reconstituted for subcutaneous or intramuscular injection, the diluent must be sterile to prevent contamination-related infections, and it must be compatible with the peptide's chemistry and the administration route. Bacteriostatic water is the standard choice for most research peptides because its benzyl alcohol content suppresses microbial growth between uses — critical for multi-draw vials where a fresh needle enters the rubber stopper repeatedly over weeks. Plain sterile water (water for injection, WFI) contains no preservative and should only be used for single-draw applications because each penetration of the stopper introduces contamination risk.

The 0.9% benzyl alcohol concentration is below the threshold for significant cytotoxicity in subcutaneous tissue at typical research peptide injection volumes (usually 0.1–0.5 mL). However, benzyl alcohol is not safe for intrathecal (spinal), epidural, or neonatal intravenous use — contexts that are irrelevant to standard subcutaneous peptide administration but important to note for completeness. Large volumes of benzyl alcohol-containing solutions can cause toxicity; this is not a concern at typical research peptide doses.

BAC water should be stored at room temperature or refrigerated, protected from light, and discarded 28 days after first use per standard pharmaceutical convention. It should not be confused with normal saline (0.9% sodium chloride), which is isotonic but contains no bacteriostatic preservative and is more suited for IV use than multi-draw peptide vials.

Peptides / stacks that act on this

Bacteriostatic water is the reconstitution diluent for virtually all lyophilised research peptides, including CJC-1295, BPC-157, TB-500, GHK-Cu, Epitalon, and others documented across this site.

Common misconceptions

"Sterile water" and "bacteriostatic water" are not interchangeable. Sterile water is free of micro-organisms at the point of manufacture but offers no protection against contamination after the vial is first opened. Bacteriostatic water inhibits re-growth of introduced bacteria across the expected multi-draw use period. Using plain sterile water for a multi-draw peptide vial introduces meaningful infection risk with each subsequent use.