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What Is Bacteriostatic Water? A Research Guide to BAC Water
Bacteriostatic water, commonly shortened to BAC water, is sterile water containing a small quantity of bacteriostatic preservative. In New-U’s product specification, that preservative is 0.9% benzyl alcohol.
The preservative inhibits bacterial reproduction after a vial has been punctured. This is what distinguishes bacteriostatic water from preservative-free sterile water and allows appropriately labelled products to be supplied in multi-dose containers.
0.9% benzyl alcohol · multi-dose research vial
Bacteriostatic water is sterile water with a bacteriostatic preservative, commonly 0.9% benzyl alcohol. It supports repeated withdrawal from a multi-dose vial when the label allows. The preservative does not erase contamination or extend use forever. Follow the label and discard within the stated in-use period. Research use only. Not for humans or animals.
Key facts
On this page
Bacteriostatic water is not itself a peptide, active research compound or stabilising agent. It is a laboratory diluent used when the relevant compound documentation or research protocol specifies that it is compatible.
What does “bacteriostatic” mean?
A bacteriostatic substance inhibits the reproduction of bacteria. This differs from a bactericidal substance, which kills susceptible bacteria.
The benzyl alcohol in bacteriostatic water helps limit bacterial growth if small amounts of contamination are introduced during repeated vial access. It does not:
Official bacteriostatic-water labelling describes the product as sterile, nonpyrogenic water containing benzyl alcohol and supplied in a multi-dose container. (DailyMed)
What is in bacteriostatic water?
A typical formulation contains water for injection, benzyl alcohol as the bacteriostatic preservative, and a sealed multi-dose container. Repeated withdrawal is only appropriate when the label allows it.
New-U bacteriostatic water is specified as sterile water containing 0.9% benzyl alcohol. Researchers should verify the concentration and storage instructions printed on the exact product they receive rather than assuming every manufacturer uses an identical formulation. See the bacteriostatic water product page.
Bacteriostatic water versus sterile water
Both products are sterile when manufactured and sealed correctly. Their principal practical difference is the preservative.
Bacteriostatic water
Preservative-free sterile water
The experimental protocol determines the correct diluent. Bacteriostatic water should not automatically replace sterile water. Read the full comparison in bacteriostatic water vs sterile water.
Why is BAC water used with lyophilised research compounds?
Many research compounds are supplied as lyophilised material. Lyophilisation removes water to support transport and storage stability.
Before solution-phase research can begin, the material may need to be dissolved in a compatible solvent. Bacteriostatic water is one possible diluent where:
Some compounds require a different solvent, buffer, pH or preparation method. Bacteriostatic water should therefore be treated as one laboratory option, not a universal solvent.
How much bacteriostatic water should be added?
There is no single water volume that applies to every vial. The volume determines the final concentration:
Adding more water does not change the total compound mass in the vial. It distributes the same mass through a larger liquid volume. The correct volume depends on the experimental protocol, target concentration, compound solubility and physical vial capacity. It should not be selected from a generic dosage rule.
Does BAC water preserve the peptide?
Not necessarily. Benzyl alcohol is included primarily to inhibit bacterial growth. It should not be described as a universal peptide stabiliser.
Once a lyophilised compound is in solution, many factors can affect stability. These include temperature, light, pH, oxidation, container adsorption, agitation, temperature cycling, the compound’s chemistry and the chosen solvent or buffer. Stability is compound-specific.
The opened-water period and the reconstituted-compound stability period are also separate concepts. A water vial may remain within its labelled in-use period while a particular compound has a much shorter validated solution-stability period. Always apply the shorter relevant limit. For compound-specific handling, see the peptide storage guide.
Storage and opened-vial life
Storage requirements depend on the manufacturer’s label. An official Hospira bacteriostatic-water label specifies storage at controlled room temperature of 20 to 25°C. Other products may provide additional instructions after opening. The exact label supplied with the vial should therefore be treated as the controlling instruction. (DailyMed)
For punctured multi-dose vials, CDC guidance applies a default discard period of 28 days unless the manufacturer provides a different opened-vial period. The discard date must not extend beyond the original expiry date. (CDC)
Follow printed expiry and manufacturer storage instructions.
Record the date and apply the manufacturer’s in-use period.
Use the labelled period, or the applicable multi-dose-vial policy.
A practical laboratory record should include manufacturer and product name, batch or lot number, original expiry date, date of first puncture, manufacturer’s in-use storage requirement, and calculated discard date. Do not use a vial if the seal is damaged, the solution contains visible particles, the liquid has become discoloured or storage conditions cannot be confirmed.
Full detail: bacteriostatic water shelf life and storage.
Why the 28-day rule exists
Multi-dose vials are exposed to a new contamination opportunity whenever the closure is entered.
The preservative reduces bacterial proliferation, but it does not provide complete protection from contamination. The 28-day convention is therefore an operational beyond-use limit for punctured multi-dose containers, not a claim that every vial remains sterile for exactly 28 days. (CDC standard precautions)
The manufacturer’s instructions take priority where they specify a shorter or longer period.
What should researchers check before buying BAC water?
1. Clear formulation
State the benzyl alcohol concentration, not only “BAC water”.
2. Defined grade
USP-grade claims should be backed by manufacturing or supplier documentation.
3. Batch ID
Each vial or pack should carry a traceable lot or batch reference.
4. Sealed closure
Cap and outer packaging should arrive intact.
5. Storage instructions
Label should state required temperature and opened-vial period.
6. Expiry date
Do not purchase unlabelled water or products without a defined expiry.
7. Product-specific docs
Peptide HPLC certificates do not verify accessory water quality.
Sourcing detail: where to buy bacteriostatic water.
Common BAC-water misconceptions
The preservative makes contamination impossible.
Benzyl alcohol inhibits bacterial growth but does not provide complete protection from poor handling or repeated contamination.
BAC water is always better than sterile water.
Preservative-free sterile water may be required where benzyl alcohol would interfere with the compound or method.
More water creates more compound.
Total mass is unchanged. Additional water lowers concentration.
Every opened vial must be refrigerated.
Storage must follow the specific manufacturer’s label. Some official products specify controlled room temperature.
BAC water makes every reconstituted peptide stable for 28 days.
The 28-day convention relates to punctured multi-dose-vial handling. Compound stability must be assessed separately.
Continue the BAC Water Guide
USP-grade · 0.9% benzyl alcohol · sealed multi-dose
Bacteriostatic Water
Manufactured or supplied to the stated USP-grade specification with 0.9% benzyl alcohol. Pack sizes and prices update from the live catalogue. Research use only — not for human or veterinary use.
From $21 / 10-vial pack
In stock · follow the vial label for storage and opened-vial life
Research use only. Product-specific accessory documentation is separate from peptide HPLC / mass-spectrometry certificates.
Sources and editorial review
Frequently asked questions
Bacteriostatic water is sterile water containing a bacteriostatic preservative, commonly 0.9% benzyl alcohol. It is generally supplied in a multi-dose vial for repeated withdrawal according to the manufacturer’s instructions.
No. Both begin sterile, but bacteriostatic water contains a preservative. Standard sterile water does not.
It inhibits bacterial reproduction. It does not sterilise contamination or replace appropriate laboratory controls.
The required volume depends on the target laboratory concentration, solubility, vial capacity and protocol. Concentration equals compound mass divided by solvent volume.
It reduces the concentration per millilitre but does not change the total amount of compound in the vial.
Follow the product manufacturer’s instructions. Where no different period is provided, punctured multi-dose vials are commonly discarded within 28 days under CDC guidance.
Not every product does. Some official labelling specifies controlled room-temperature storage. Always follow the exact label attached to the supplied vial.
No. Compatibility depends on the compound and experimental protocol. Some materials require preservative-free water, saline, a buffer or another solvent system.
Discard it if the seal is compromised, particles or discolouration are visible, contamination is suspected, the storage history is unknown or the labelled in-use period has passed.
Research use only. Not for human or veterinary use. New-U Research Compounds products are supplied solely for laboratory, analytical and scientific research by appropriately qualified purchasers. Nothing in this guide is medical advice, dosage guidance or an instruction for administration.
PRECISION. PURITY. PERFORMANCE.
Research peptides at >99% HPLC-verified purity, third-party tested by Janoshik Analytical & Freedom Diagnostics, with Certificates of Analysis published per released batch. Supplied strictly for laboratory research use.
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[image: Sealed New-U bacteriostatic water vial labelled 0.9% benzyl alcohol]
[image: Bacteriostatic Water vial — sterile water with 0.9% benzyl alcohol]