Bacteriostatic water, almost always 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 - 9 mg in every millilitre.

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

Bac water is bacteriostatic water: 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

The alcohol is benzyl alcohol at 0.9% (9 mg/mL) — not ethanol and not the isopropyl alcohol in a swab.

The preservative inhibits bacterial growth after puncture; it does not sterilise contamination already present.

Different from preservative-free sterile water; protocol compatibility decides which diluent to use.

Does not “preserve” peptides by itself; concentration and stability depend on the compound and conditions.

Opened multi-dose vials commonly follow a labelled in-use window (often discussed as about 28 days), and that window — not the volume left — usually decides when a vial is finished with.

Sizes are a trade-off: 3 ml wastes less, 10 ml and 30 ml cost less per millilitre but take more punctures.

New-U bacteriostatic water is supplied for laboratory reconstitution workflows only.

On this page

What bacteriostatic means

What is in bac water

Which alcohol is in it

Bac water vs other diluents

3 ml, 10 ml or 30 ml

Why labs use it

How much to add

Does it preserve peptides?

Storage & opened-vial life

Handling between uses

Why the 28-day rule

Pharmacy, marketplace or lab supplier

Buying checklist

Common misconceptions

Continue the guide

FAQs

The three names describe the same category of product. Bacteriostatic Water for Injection, USP is the name printed on an approved label; bac water and BWFI are the short forms people actually type. What matters in practice is not which name you searched for but what the specific vial in front of you states: preservative, concentration, container type, storage temperature and in-use period.

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:

Sterilise an already contaminated vial

Protect against every microorganism

Replace appropriate laboratory handling

Guarantee sterility after repeated punctures

Determine the stability of the reconstituted compound

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.

Component Purpose Water for injection Sterile aqueous vehicle Benzyl alcohol Bacteriostatic preservative Sealed multi-dose container Supports repeated withdrawal when handled according to the label

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.

What kind of alcohol is in bac water?

Benzyl alcohol - and only benzyl alcohol. It is an aromatic alcohol, chemically distinct from the two alcohols people usually have in mind, and the substitution people occasionally imagine is not a substitution at all.

Alcohol Where you meet it Role in bac water Benzyl alcohol Preservative in multi-dose injectable products The bacteriostatic agent, at 0.9% (9 mg/mL) Ethanol Drinking alcohol; some tinctures and extracts Not present, and not a substitute Isopropyl alcohol Rubbing alcohol; the swab used on the stopper Not present; used on the outside of the vial only

The 0.9% figure is a weight-in-volume concentration: 0.9 g of benzyl alcohol per 100 mL, which works out at 9 mg in every millilitre. That concentration is high enough to inhibit bacterial reproduction inside the vial and low enough to leave the water usable as a diluent - which is exactly the compromise the product exists to strike.

Benzyl alcohol is also the reason preservative-free water still exists as a separate product. Anything that must exclude the preservative - a protocol where benzyl alcohol interferes with the assay or the compound, or a population the label excludes - needs sterile water rather than bac water.

Label warning worth knowing: approved bacteriostatic water labelling carries an explicit warning that benzyl alcohol has been associated with a fatal toxic syndrome in premature infants and newborns, and states that the product must not be used in neonates. This is one of the clearest illustrations that the preservative is a real pharmacological ingredient, not inert filler. (DailyMed)

Bac water versus the other diluents

Sterile water is the usual comparison, but it is not the only alternative a protocol may specify. Four aqueous options come up regularly, and they are not interchangeable.

Feature Bacteriostatic water Sterile water 0.9% sodium chloride Acetic acid water Preservative Benzyl alcohol 0.9% None None in single-use presentations None Container type Commonly multi-dose Commonly single-use Commonly single-use Commonly single-use Repeated withdrawal Permitted when stated by the manufacturer Generally not intended Generally not intended Generally not intended Tonicity Hypotonic; unbuffered Hypotonic; unbuffered Isotonic Hypotonic; acidic Typical reason to pick it The same container is entered more than once The protocol excludes preservatives The protocol specifies an isotonic vehicle Documentation calls for an acidic vehicle to help a poorly soluble compound dissolve In-use period Follow manufacturer instructions Single-use label Single-use label Single-use label

Bacteriostatic water

Preservative

Benzyl alcohol 0.9%

Container

Commonly multi-dose

Repeated withdrawal

When the manufacturer states it

Pick it when

The same container is entered more than once

Preservative-free sterile water

Preservative

None

Container

Commonly single-use

Repeated withdrawal

Generally not intended

Pick it when

The protocol excludes preservatives

0.9% sodium chloride

Preservative

None in single-use presentations

Container

Commonly single-use

Tonicity

Isotonic

Pick it when

The protocol specifies an isotonic vehicle

Acetic acid water

Preservative

None

Container

Commonly single-use

Tonicity

Hypotonic and acidic

Pick it when

Documentation calls for an acidic vehicle for a poorly soluble compound

The experimental protocol determines the correct diluent. Bacteriostatic water should not automatically replace sterile water, and a shortage of one is not a reason to reach for another - a single-use container does not acquire a multi-dose window by being refrigerated. Read the full comparison in bacteriostatic water vs sterile water.

Bac water sizes: 3 ml, 10 ml and 30 ml

Approved bacteriostatic water is commonly supplied in 10 mL and 30 mL multiple-dose vials; research accessory packs add a 3 mL size. The arithmetic of which one to choose is simple, and almost everyone gets it wrong in the same direction.

Vial size 1 mL reconstitutions 2 mL reconstitutions Punctures over one in-use window 3 ml 3 1 (with 1 mL left) Few - often emptied well inside the window 10 ml 10 5 Moderate 30 ml 30 15 Many - most likely to be discarded part-used

The volume is rarely the binding constraint. The in-use period is. A 30 mL vial is the cheapest per millilitre and the most likely to be thrown away half full, because the labelled window closes long before thirty millilitres have been drawn. Every extra millilitre you buy is also an extra puncture you have to handle correctly.

The size that costs least in practice is the smallest one that covers the work actually planned inside a single window. New-U supplies bacteriostatic water in 3 ml and 10 ml vials, packed ten to a box, so a box can be opened one vial at a time rather than committing a single large container to a long window - see the bacteriostatic water product page for current sizes.

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:

The compound is compatible with an aqueous solution

Benzyl alcohol is compatible with the experiment

Repeated access to the same container is required

The intended concentration can be achieved within the vial capacity

The research documentation permits its use

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:

Compound mass Water volume Final concentration 5 mg 1 mL 5 mg/mL 5 mg 2 mL 2.5 mg/mL 10 mg 1 mL 10 mg/mL 10 mg 2 mL 5 mg/mL 20 mg 2 mL 10 mg/mL

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.

Handling a multi-dose vial between uses

A multi-dose vial is only as clean as the last thing that went through its stopper. Most of what shortens a vial’s usable life happens in the seconds around each entry, not during storage, so the handling routine matters more than where the vial sits in between.

Disinfect the septum before every entry. Wipe the rubber closure with alcohol and let it air-dry. Wet alcohol dragged through the stopper by a needle defeats the point.

New sterile needle and new syringe, every time. CDC guidance for multi-dose vials is a fresh sterile needle and syringe for each entry. Re-entering with something that has already been somewhere else is the single most common way a vial is lost. (CDC)

Never park a needle in the stopper. A needle left in place holds the closure open for the whole interval between uses.

Enter cleanly to avoid coring. Standard technique is bevel up, puncture at an angle of roughly 45–60°, then bring the needle upright as it passes through - one path through the rubber rather than several. Cored fragments both compromise the seal and end up in the drawn liquid.

Date the vial at first puncture. Write the date on the label itself, not on the carton, and calculate the discard date from it.

Store to the label, in its carton. Follow the printed temperature; keep the vial in its packaging so the label stays legible and the closure is protected. Refrigeration is not a universal requirement and does not extend a labelled in-use period.

Inspect before each use, and discard on doubt. Compromised seal, visible particles, discolouration, an unknown storage history or a passed in-use date all end the vial early - regardless of how much liquid is left.

These are handling controls for the water container itself. They say nothing about the stability of whatever has been reconstituted with it, which is compound-specific and tracked separately.

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.

Pharmacy, marketplace or laboratory supplier?

Bac water reaches people through three quite different routes, and the differences are worth understanding before searching for whichever one is nearest.

Pharmacies

Bacteriostatic Water for Injection, USP is a labelled drug product, so a pharmacy is a conventional supply route for it. Whether any given pharmacy stocks it, and whether a prescription is required to dispense it, varies by pharmacy, by country and by jurisdiction - there is no single answer that holds everywhere, and the rules that apply are the ones where you are, not the ones described in an article written elsewhere.

General marketplaces

Listings on large consumer marketplaces appear and disappear, and availability there has been visibly inconsistent - which is why “where has all the bac water gone” is a recurring question rather than a one-off. The practical problem is not that marketplaces are unusable; it is that the storefront tells you nothing about the vial. Judge the product, not the listing: the label must state the preservative and its concentration, a lot or batch reference, an expiry date, storage conditions, and it must arrive sealed.

Laboratory and research suppliers

Research suppliers ship bacteriostatic water as a laboratory accessory, usually alongside the compounds it will be used with. The advantage is coherence rather than price: one shipment, one set of documentation, matched vial sizes, and no separate order arriving a week after the material it was meant to accompany.

Whichever route you use, the verification is identical and takes about ten seconds per vial. Nothing about a supplier’s reputation substitutes for reading the label that arrived.

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.

You can make bac water by adding rubbing alcohol to distilled water.

The preservative is benzyl alcohol, not isopropyl - a different compound at a specified concentration, in sterile non-pyrogenic water, sealed under controlled manufacturing conditions. Improvised preparation reproduces none of that.

The largest vial is always the best value.

It is the cheapest per millilitre, but the labelled in-use window usually closes before a 30 ml vial is emptied - so the discarded remainder is part of the real price.

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.

From $21 / 10-vial pack

In stock · follow the vial label for storage and opened-vial life

10-vial pack · 3ml $21

10-vial pack · 10ml $43

Research use only - all claims made on this site are for testing and research use only. Product-specific accessory documentation is separate from peptide HPLC / mass-spectrometry certificates.

Sources and editorial review

DailyMed: Bacteriostatic Water injection, solution

CDC: Preventing unsafe injection practices / multi-dose vial guidance

CDC: Standard precautions summary

New-U bacteriostatic water product specification

New-U accessory documentation status (/coa/bacteriostatic-water)

Frequently asked questions

What is bac water?

Bac water is the everyday short form of bacteriostatic water: 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. Research use only.

What kind of alcohol is in bac water?

Benzyl alcohol, an aromatic alcohol, at 0.9% - that is 9 mg per millilitre. It is not ethanol (drinking alcohol) and not isopropyl alcohol (rubbing alcohol), and the three are not interchangeable as a diluent.

Is BAC water the same as sterile water?

No. Both begin sterile, but bacteriostatic water contains a preservative. Standard sterile water does not.

Is bac water the same as Bacteriostatic Water for Injection, USP?

Bacteriostatic Water for Injection, USP is the official product name that appears on the approved label; bac water and BWFI are informal short forms of it. The name on the vial you receive, not the shorthand, tells you the formulation, concentration and in-use period.

Can I buy bac water at a pharmacy?

Bacteriostatic Water for Injection, USP is a labelled drug product, so pharmacies are a normal supply route, but stocking and whether a prescription is required vary by pharmacy, country and jurisdiction. Research suppliers ship it as a laboratory accessory instead. Check the rules that apply where you are.

Should I choose 3 ml, 10 ml or 30 ml bac water?

Match the vial to the volume actually consumed inside one in-use period. A larger vial is cheaper per millilitre but takes more punctures and is more likely to be discarded part-used when the labelled window ends; smaller vials waste less but cost more per millilitre.

How many reconstitutions does a 10 ml bac water vial cover?

Ten millilitres divides into ten 1 ml reconstitutions, five at 2 ml or about three at 3 ml. In practice the labelled in-use period usually runs out before the volume does, so plan against the window rather than the capacity.

How much BAC water should be added to a peptide vial?

The required volume depends on the target laboratory concentration, solubility, vial capacity and protocol. Concentration equals compound mass divided by solvent volume.

Does adding more BAC water weaken the compound?

It reduces the concentration per millilitre but does not change the total amount of compound in the vial.

Do I need a new needle each time I draw from the vial?

Yes. CDC guidance for multi-dose vials is a new sterile needle and syringe for every entry, with the rubber septum disinfected and allowed to dry first. A needle must never be left parked in the stopper between uses.

How long does bacteriostatic water last after opening?

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.

Does bacteriostatic water need refrigeration?

Not every product does. Some official labelling specifies controlled room-temperature storage of 20 to 25°C. Always follow the exact label attached to the supplied vial.

What does the benzyl alcohol do?

It inhibits bacterial reproduction. It does not sterilise contamination or replace appropriate laboratory controls.

Can BAC water be used with every research peptide?

No. Compatibility depends on the compound and experimental protocol. Some materials require preservative-free water, saline, a buffer or another solvent system.

What if bacteriostatic water is unavailable?

Substitute on documentation, not on convenience. Preservative-free sterile water, 0.9% sodium chloride and dilute acetic acid water are separate products with different in-use behaviour, and single-use containers do not gain a multi-dose window by being refrigerated.

When should a vial be discarded early?

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.

Shop

All research compounds

Price list

Where to buy peptides

Research