Bacteriostatic Water Guide

Everything you need to know about bacteriostatic water (BAC water) for peptide reconstitution, including what it is, how to use it, and how it compares to alternatives.

What is Bacteriostatic Water?

Bacteriostatic water (BAC water) is sterile water that contains 0.9% benzyl alcohol as a bacteriostatic agent. The benzyl alcohol inhibits the growth of most bacteria, making it safe to draw from the same vial multiple times over a period of up to 28 days.

It is the standard diluent for reconstituting lyophilized (freeze-dried) peptides, medications, and other injectable compounds. The United States Pharmacopeia (USP) defines the specifications for pharmaceutical-grade bacteriostatic water.

Composition

- Sterile Water for Injection (USP)
- 0.9% Benzyl Alcohol (preservative)
- pH: 4.5-7.0
- Pyrogen-free

BAC Water vs. Alternatives

Property	BAC Water	Sterile Water	Normal Saline
Preservative	Yes (0.9% benzyl alcohol)	No	No (unless bacteriostatic saline)
Multi-dose use	Yes (up to 28 days)	No (single use)	No (single use)
Best for peptides	Yes -- standard choice	Only for immediate use	Some peptides only
Shelf life (opened)	28 days	24 hours	24 hours
pH range	4.5-7.0	5.0-7.0	4.5-7.0
Tonicity	Slightly hypotonic	Hypotonic	Isotonic (0.9% NaCl)

How to Use Bacteriostatic Water

Swab the BAC water vial top and the peptide vial top with separate alcohol pads. Let dry completely.

Draw air into your syringe equal to the amount of water you want to withdraw. This equalizes pressure in the vial.

Insert the needle into the BAC water vial, push in the air, invert the vial, and slowly draw your desired volume of water.

Insert the needle into the peptide vial. Aim the needle at the inside glass wall, not directly at the powder.

Slowly push the plunger to release the water. Let it run gently down the wall onto the peptide.

Remove the syringe. Gently swirl (never shake) until the peptide is fully dissolved.

Storage Requirements

Unopened

Room temperature (15-30 C). Keep away from direct light. Check expiration date on vial.

Opened (Punctured)

Use within 28 days. Room temperature or refrigerated. Always swab with alcohol before each draw.

Frequently Asked Questions

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative that inhibits bacterial growth. Sterile water contains no preservative. BAC water allows multi-dose use for up to 30 days, while sterile water must be used within 24 hours and discarded.

It is technically possible to add benzyl alcohol to sterile water, but this is not recommended. Pharmaceutical-grade BAC water is manufactured under strict sterile conditions and quality controls that cannot be replicated at home. The risk of contamination is too high.

This depends on the peptide dose and your preferred concentration. Use our calculator to determine the optimal amount. Generally, you want your typical dose to fall between 10-50 units on a U-100 syringe for easy measuring.

Yes. Check the expiration date on the vial. Unopened BAC water typically has a 2-year shelf life. Once opened (punctured), use within 28 days. Never use expired or visibly contaminated water.

Bacteriostatic sodium chloride (0.9% NaCl with benzyl alcohol) can be used for some peptides, but plain BAC water is generally preferred. Some peptides may have altered stability in saline. Check peptide-specific guidelines.

Store at room temperature (15-30 C / 59-86 F) away from direct light. Refrigeration is not required for unopened vials but does not harm the product. After opening, some people prefer to refrigerate, though room temperature is acceptable.

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