If you’re new to peptide research, one of the first things you’ll encounter is the need to reconstitute your peptides — that is, mixing a freeze-dried powder with a liquid before use. The liquid researchers reach for almost universally is bacteriostatic water, often abbreviated as BAC water.

This guide covers everything a beginner needs to know: what BAC water actually is, how it’s made, why it’s the standard choice, and how to use it correctly in your research workflow.

What Is Bacteriostatic Water?

Bacteriostatic water is sterile water for injection that contains 0.9% benzyl alcohol as a preservative. The term “bacteriostatic” means it inhibits bacterial growth rather than killing bacteria outright. The benzyl alcohol creates an environment where bacteria cannot reproduce, keeping your reconstituted solution viable across multiple uses over several weeks.

Standard sterile water contains no preservative. Once you puncture the vial and introduce it into a peptide, any trace contamination can begin multiplying within hours. For multi-draw vials used over weeks of research, BAC water is the safer and more practical choice.

How Is Bacteriostatic Water Made?

BAC water is manufactured under sterile pharmaceutical conditions:

1. Start with Water for Injection (WFI) — Highly purified water meeting strict pharmaceutical standards for sterility and freedom from pyrogens.
2. Add benzyl alcohol — Added at exactly 0.9% (9 mg per mL).
3. Sterile filtration — Passed through a 0.22-micron filter to remove any remaining microorganisms.
4. Fill and seal under sterile conditions — Filled into sealed glass vials using aseptic technique.

Why Do Researchers Use BAC Water for Peptides?

• Multi-use stability — Draw from the same vial multiple times over weeks without significantly increasing contamination risk.
• Peptide compatibility — Compatible with most research peptides without causing degradation or precipitation.
• Widely available — Available from research suppliers and compounding pharmacies.
• Established protocol — The standard in published peptide research literature.

BAC Water vs. Other Reconstitution Liquids

BAC Water vs. Sterile Water

Sterile water has no preservative — appropriate only for single-use reconstitution. BAC water is the better choice for any multi-use vial.

BAC Water vs. Normal Saline (0.9% NaCl)

Normal saline can cause certain peptides to precipitate at higher concentrations and lacks a bacteriostatic preservative. BAC water is generally preferred.

BAC Water vs. Sodium Chloride for Injection

Some protocols specifically call for sodium chloride as the diluent for peptides that don’t dissolve well in plain water. Follow the specific protocol when indicated. For general-purpose reconstitution, BAC water remains the default.

Practical Use: How to Reconstitute Peptides with BAC Water

What You’ll Need

• Lyophilized peptide vial
• Bacteriostatic water vial
• Insulin syringe (1 mL / 100 IU)
• Alcohol swabs
• A clean surface

Step-by-Step Reconstitution

1. Wipe both vial tops with an alcohol swab and let air dry 10–15 seconds.
2. Draw the appropriate volume of BAC water into your syringe.
3. Insert the needle at an angle, pointing toward the glass wall rather than directly at the powder.
4. Slowly push the BAC water down the inside of the vial wall — never squirt directly onto the powder. High-velocity liquid can damage peptide structure.
5. Do not shake. Gently roll the vial between your palms until fully dissolved.
6. The solution should be clear and colorless. Any cloudiness may indicate a problem.

Calculating Your Concentration

Concentration (mcg/mL) = Total peptide amount (mcg) ÷ Volume of BAC water added (mL)

Example: A 5 mg (5,000 mcg) vial + 2 mL BAC water = 2,500 mcg/mL. Adding 2 mL to a 5 mg vial also gives you 250 mcg per 10 IU (0.1 mL) — a convenient measurement point for many protocols.

Storage After Reconstitution

Store reconstituted peptides in the refrigerator (2–8°C / 36–46°F), away from light. Never freeze a reconstituted solution — freezing can degrade peptide structure. Most peptides reconstituted in BAC water remain stable for 4–6 weeks when refrigerated.

Signs of degradation or contamination: cloudiness, visible particulates, unusual color, or off smell. When in doubt, discard and reconstitute fresh.

Key Takeaways

• Bacteriostatic water is sterile water with 0.9% benzyl alcohol that inhibits bacterial growth.
• It is the standard reconstitution liquid for peptide research — stable across multi-use vials over weeks.
• Always add BAC water gently down the vial wall — never directly onto the powder.
• Store reconstituted peptides refrigerated, not frozen, and use within 4–6 weeks.
• Calculate your concentration before drawing doses for consistent measurements.

Once you understand BAC water and proper reconstitution technique, you’ve cleared the biggest hurdle for peptide research beginners. For a complete walkthrough, see our Peptide Reconstitution Guide.