Bacteriostatic water (BAC water) is water for injection containing 0.9% benzyl alcohol, a preservative that inhibits bacterial growth — hence "bacteriostatic" (growth-inhibiting, not bactericidal). It is the standard diluent for reconstituting lyophilised (freeze-dried) research peptides. Understanding why BAC water is used, how it differs from sterile water, and the correct reconstitution procedure is fundamental to safe peptide research. Using the wrong diluent or poor technique is one of the most common errors in peptide research.
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Everything about bacteriostatic water for peptide reconstitution: what it is, why it's used over sterile water, how much to add, storage rules, and common mistakes.
BAC Water vs Sterile Water: Key Differences
Sterile water for injection (SWFI) is pyrogen-free water in a sealed vial — sterile at manufacture but without any antimicrobial preservative. Once the vial is punctured, sterile water is at risk of bacterial contamination with each subsequent needle insertion. It is appropriate for single-use reconstitutions — use the entire vial at once and discard. Multi-dose use of SWFI is not recommended.
Bacteriostatic water contains 0.9% benzyl alcohol, which inhibits bacterial proliferation even after the vial has been opened and punctured multiple times. This allows safe multi-dose use of a reconstituted peptide vial over weeks. For research peptides that are reconstituted and then used in small aliquots over 4–6 weeks, BAC water is the appropriate choice. Benzyl alcohol is safe at the concentrations present in BAC water for injection use in adults, though it is contraindicated in neonates (benzyl alcohol toxicity risk in premature infants).
How Much BAC Water to Add
The volume of BAC water to add per vial is not a fixed rule — it depends on how you want to dose from the vial. The reconstitution calculation: (peptide amount in mg) ÷ (desired concentration in mg/mL) = volume of BAC water in mL.
Example: 5 mg peptide vial, desired concentration 1 mg/mL → add 5 mL BAC water. Desired 0.5 mg/mL → add 10 mL. For most peptides dosed in micrograms (mcg), working with 1 mg/mL or 2 mg/mL concentrations simplifies dose calculation. A 5 mg vial at 1 mg/mL (add 5 mL = 50 units on a 50-unit insulin syringe) gives 1 mg per 10 units — making 250 mcg = 2.5 units, 500 mcg = 5 units. Calculate your target concentration before reconstituting to avoid under- or over-diluting.
How Do You Reconstitute Bacteriostatic Water?
Proper reconstitution prevents peptide degradation and maintains sterility. Step by step: (1) Allow the lyophilised peptide vial and BAC water vial to reach room temperature. (2) Swab both vial tops with a fresh alcohol swab and allow to dry. (3) Draw up the calculated BAC water volume into your syringe. (4) Insert the needle into the peptide vial and aim the BAC water stream at the glass wall of the vial — do not inject directly onto the lyophilised powder as this can shear peptide structure. (5) Allow BAC water to run down the vial wall onto the powder slowly. (6) Do not shake the vial. Gently swirl or roll between palms if needed. Allow several minutes for complete dissolution. (7) The solution should be clear and colourless (or very slightly yellow for some peptides). Do not use if cloudy, particulate, or deeply coloured.
Storage After Reconstitution
Reconstituted peptide in BAC water should be stored in the refrigerator (2–8°C). Do not freeze reconstituted solutions — freeze-thaw cycles can cause peptide aggregation and activity loss. Protect from light (most peptide compounds are photosensitive to varying degrees — keeping in the box or wrapped in foil is prudent). BAC water's benzyl alcohol provides multi-week antimicrobial protection in the refrigerator; most reconstituted peptides should be used within 4–6 weeks for optimal potency.
For long-term storage of unused peptide, leave in lyophilised form (unreconstituted) and store in the freezer. Reconstitute only what you plan to use within 4–6 weeks.
BAC Water Reconstitution Quick Reference
| Vial Size | Dose | Route | Frequency | Notes |
|---|---|---|---|---|
| 2 mg | 2 mL BAC water | 1 mg/mL | 10 units on syringe = 0.1 mg (100 mcg) | Standard for high-dose peptides |
| 5 mg | 5 mL BAC water | 1 mg/mL | 10 units = 100 mcg; 2.5 units = 250 mcg | Common BPC-157/TB-500 vial size |
| 5 mg | 2.5 mL BAC water | 2 mg/mL | 10 units = 200 mcg | Smaller volume per dose |
| 10 mg | 10 mL BAC water | 1 mg/mL | 10 units = 100 mcg | Good for peptides used long-term |
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Frequently Asked Questions
Sterile 0.9% sodium chloride (normal saline) for injection can be used for single-dose reconstitutions but is not recommended for multi-dose use as it contains no preservative. BAC water is preferable for any peptide vial you plan to use over multiple injections. Normal saline for wound irrigation (not for injection, without benzyl alcohol) should never be used.
BAC water in a sealed vial is stable indefinitely at room temperature within its expiry date. Once opened and punctured, it remains suitable for injection use for approximately 30 days at room temperature or longer refrigerated — the benzyl alcohol preservative maintains antimicrobial activity. Inspect for particulates or cloudiness before use.
Vigorous shaking can cause peptide denaturation — disrupting the three-dimensional structure of the peptide chain, reducing biological activity. Some peptides are more sensitive to mechanical stress than others. Gentle swirling or rolling between palms is sufficient for dissolution. If the powder is not dissolving, allow more time rather than shaking.
Using a 0.3 mL or 0.5 mL insulin syringe (100 units total) with appropriate reconstitution concentration allows measurement of doses as small as 50–100 mcg per division. Calculate your target concentration before reconstituting so that your target dose falls in a readable syringe range (e.g., 250 mcg at 1 mg/mL = 25 units = easy to read on an insulin syringe).