What is Bacteriostatic Water?

Composition

Bacteriostatic water (BAC water) is sterile water for injection containing 0.9% benzyl alcohol (9 mg/ml) as a preservative. The water itself is purified and sterile — equivalent to plain sterile water for injection in all respects except for the addition of the preservative agent.

The benzyl alcohol inhibits the growth of most common bacteria, making it appropriate to draw multiple doses from the same vial over an extended period without the contamination risk that comes from repeated vial puncture. This is the defining property that makes it the preferred diluent for peptide research applications.

Why Not Plain Sterile Water?

Plain sterile water is appropriate only for single-dose reconstitution. Once a vial has been punctured without a bacteriostatic agent present, the seal is broken and subsequent draws introduce contamination risk. For the multi-dose vials used in most peptide research contexts, BAC water is the appropriate diluent.

How Benzyl Alcohol Works

Benzyl alcohol is a bacteriostatic agent — it inhibits bacterial growth rather than sterilising. It works by disrupting bacterial cell membranes and inhibiting metabolic activity. At 0.9% concentration, it is effective against most common environmental bacteria while being well-tolerated at the injection volumes used in typical peptide research contexts.

The important distinction between bacteriostatic and bactericidal is that BAC water does not destroy bacteria already present — it prevents proliferation. This is why starting with a sterile preparation is still essential; the benzyl alcohol suppresses contamination introduced via repeated needle puncture, not contamination introduced before preparation.

How Much BAC Water to Use

The volume of BAC water added determines the concentration of your reconstituted peptide. There is no single correct amount — the appropriate volume is whichever results in a concentration that maps cleanly onto your syringe's graduation scale.

A common approach for a 5 mg vial is to add 2 ml of BAC water. This yields a concentration of 2,500 mcg/ml, so a commonly reported dose of 250 mcg corresponds to 0.10 ml — which is exactly 10 units on a U-100 syringe. Clean numbers reduce calculation errors.

Refer to the reconstitution guide for a full concentration reference table covering common vial sizes and BAC water volumes.

Shelf Life and Storage

• Unopened BAC water vial: stable at room temperature until manufacturer expiry date — typically 2–3 years from production
• Opened BAC water vial: store at 2–8°C (refrigerator); use within 28–30 days of first puncture
• Reconstituted peptide with BAC water: approximately 28–60 days refrigerated, depending on the specific peptide — consult individual peptide profile pages for compound-specific guidance

Peptide stability after reconstitution is affected by temperature fluctuation, light exposure, and the number of freeze-thaw cycles. Consistent refrigeration at 2–8°C is the standard approach for preserving reconstituted solutions.

Where to Source Bacteriostatic Water

BAC water is available from pharmaceutical suppliers, compounding pharmacies, and research supply companies. It is typically sold in 30 ml multi-dose vials. It is classified as a prescription item in some jurisdictions.

Do not substitute with contact lens saline solutions, homemade preparations, distilled water, or tap water. None of these are sterile to the standard required for injection preparations, and their use introduces contamination risk.

Common Mistakes

• Using plain sterile water for multi-dose vials — no bacteriostatic protection exists after the first puncture
• Using expired BAC water — the preservative efficacy of benzyl alcohol degrades over time
• Storing an opened BAC water vial at room temperature beyond 28 days
• Confusing bacteriostatic water with bacteriostatic saline — the saline formulation contains NaCl which may affect the solubility of certain peptides
• Using BAC water that has visible particulates, cloudiness, or colour change — discard any vial showing these signs