Bacteriostatic water (BAC water)

Bacteriostatic water (often labeled "BAC water") is sterile water for injection that contains a small amount of a bacteriostatic agent, typically benzyl alcohol.

It is supplied as a diluent to reconstitute compatible medications and is not a therapeutic agent by itself.

The bacteriostatic component inhibits growth of certain bacteria in the solution, allowing limited multi-dose use from a single vial under appropriate sterile conditions.

BAC water is used to dissolve or dilute medications that are specifically labeled as compatible with bacteriostatic diluents.

Some products require preservative-free sterile water instead; matching the correct diluent to the product label is essential.

BAC water is not appropriate for all patient populations (for example, neonates may require preservative-free preparations). Product labeling and local policy provide detailed guidance.

BAC water does not have a dose in the therapeutic sense; the volume used is determined by the medication being reconstituted and the instructions in that medication’s labeling.

This page does not provide mixing or administration instructions for any specific drug.

BAC water is packaged in multi-dose vials of varying volumes. It is used in combination with injectable medications that explicitly allow a bacteriostatic diluent.

BAC water is a well-established pharmaceutical excipient; safety and compatibility considerations are addressed in pharmacopeial standards and product-specific labeling rather than in traditional clinical trials.

For deeper dives on this peptide, see:

• Bacteriostatic water (BAC water) benefits and areas of interest
• Bacteriostatic water (BAC water) side effects and safety context
• Bacteriostatic water (BAC water) dosing concepts (educational)
• Bacteriostatic water (BAC water) research and evidence overview

Is bacteriostatic water a peptide, or a treatment for anything? Neither. It is a pharmaceutical diluent, and its FDA-approved labeling says so in one sentence: it "is indicated only for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection, according to instructions of the manufacturer of the drug to be administered" (Bacteriostatic Water for Injection, USP — DailyMed label). It is a prescription product in the United States. It appears in peptide catalogs because peptides are shipped as lyophilized powder and something has to dissolve them, not because it does anything itself.

What does "bacteriostatic" actually mean here? The water contains benzyl alcohol — 0.9% (9 mg/mL) or 1.1% (11 mg/mL) depending on the presentation — added as a bacteriostatic preservative. Bacteriostatic means it inhibits the growth of certain bacteria; it does not sterilize. The practical consequence, and the entire reason the product exists, is that it is supplied in a multiple-dose container "from which repeated withdrawals may be made" (DailyMed label). The preservative is not a substitute for aseptic technique — the same label directs that aseptic technique be used for every entry and withdrawal.

How is it different from sterile water for injection? Sterile Water for Injection, USP "contains no bacteriostat, antimicrobial agent or added buffer and is supplied only in single-dose containers" (Sterile Water for Injection — DailyMed label). That is the whole distinction: preservative and repeat entry versus no preservative and one use. Both share a hazard that surprises people — each label warns that intravenous administration of the water without a solute may cause hemolysis, and that it must be made approximately isotonic before use. Neither is an injectable fluid on its own.

Why does the label say "NOT FOR USE IN NEONATES"? Because of the benzyl alcohol. The warning appears in capitals on the vial and the carton, and the label states that "due to the potential toxicity of benzyl alcohol in neonates, solutions containing benzyl alcohol must not be used in this patient population," directing that only preservative-free Sterile Water for Injection be used when preparing medications for newborns (DailyMed label). The warning traces to a documented episode of fatal benzyl alcohol poisoning in premature infants — the "gasping syndrome" described in the New England Journal of Medicine in 1982 (Gershanik et al., NEJM, 1982). It is one of the clearest examples in pharmacy of an "inactive" ingredient turning out to be anything but.

Can it be used with any injectable medication? No. Some products are specified for preservative-free diluent only, and the label directs users to "consult the manufacturer's instructions for choice of vehicle." Bacteriostatic water is also explicitly contraindicated for fluid replacement and for epidural or spinal anesthesia procedures (DailyMed label). Matching the diluent to the specific drug's label is the whole decision; there is no general-purpose answer.

Does the preservative mean a reconstituted vial keeps indefinitely? No, and the two questions are separate. The preservative is there to protect the diluent vial across repeated withdrawals. Once a drug has been mixed into it, the stability of that solution belongs to the drug, not the water — the label instructs users not to store reconstituted drug solutions unless the manufacturer of the solute directs otherwise, and to mix thoroughly and use promptly (DailyMed label). Storage claims circulating for reconstituted research peptides are not derived from this label.