Bacteriostatic Water UK: What It Is, Where to Buy It and How Long It Lasts
Bacteriostatic water in the UK explained: what the 0.9% benzyl alcohol does, how it differs from sterile water and saline, why it lasts 28 days after opening, where you can and cannot buy it in the UK, and how laboratories use it to reconstitute lyophilised peptides.
Bacteriostatic water is sterile water with benzyl alcohol, typically 0.9%, added as a preservative. The alcohol stops bacteria multiplying in the vial once the seal has been broken, which is why a bacteriostatic vial can be drawn from repeatedly over 28 days while a vial of plain sterile water is meant to be used once and discarded. In a laboratory it is the standard vehicle for turning a lyophilised (freeze-dried) peptide into a solution that can be aliquoted over several sessions. This guide explains what it is, how it differs from the other waters you will see, why the 28-day rule exists, where you can and cannot buy it in the UK, and how to judge the quality of what you are buying. It describes laboratory use only; nothing here is guidance for administering anything to a person.
What is bacteriostatic water?
Start with the name. "Bacteriostatic" means it holds bacteria static: it prevents them from growing, as opposed to "bactericidal", which would mean killing them outright. The product is purified water that has been sterilised and then had benzyl alcohol added, typically at 0.9% weight per volume, which is 9 milligrams in every millilitre. The benzyl alcohol disrupts bacterial cell membranes and interferes with the processes bacteria use to take up nutrients, so any organism that gets into the vial on a needle tip cannot establish a colony.
It is sold in small glass vials with a rubber stopper designed to be entered repeatedly, typically 10 mL or 30 mL. In the United States it is a licensed pharmaceutical product with a formal monograph in the United States Pharmacopeia, which sets limits for pH (4.5 to 7.0), bacterial endotoxins, particulate matter and antimicrobial effectiveness. In the UK the only licensed bacteriostatic water is a prescription-only diluent supplied with a specific medicine; it is not sold as a general-purpose product. Bacteriostatic water supplied to UK laboratories is a reagent manufactured to pharmacopoeial-type specifications, not a licensed medicine.
Bacteriostatic water vs sterile water vs saline
Three clear liquids, three different jobs, and they are not interchangeable.
The three common reconstitution vehicles compared Bacteriostatic water Sterile water for injection Sterile saline (0.9% NaCl) What it is Sterile water + 0.9% benzyl alcohol Sterile water, nothing added Sterile water + 0.9% sodium chloride Preservative Yes No Usually no After the seal is broken Multiple draws over 28 days Single use; discard the remainder Single use unless a preserved version Typical laboratory role Reconstituting lyophilised peptides for aliquoting over time One-off reconstitution used immediately Isotonic diluent where the assay needs physiological salt Watch out for Benzyl alcohol can interfere with some sensitive cell assays Contamination risk rises with every needle entry Salt can affect solubility of some peptides
The one that causes most confusion is sterile water. It is sterile in the sealed vial, but the moment a needle goes through the stopper the guarantee is gone, because nothing inside the vial stops whatever came in on the needle from growing. That is fine if the whole vial is used in one session. It is not fine for a peptide stock that will be drawn from across a week of experiments, which is exactly the case bacteriostatic water exists for.
Why does bacteriostatic water last 28 days after opening?
The 28-day figure is a widely used convention for preserved multi-use vials, reflecting the period over which the preservative is relied on after the vial is first opened under normal storage. It is not a cliff edge at which the water becomes dangerous, and it is not a promise that the water is sterile for 28 days regardless of how it is handled. It is the window in which the preservative can be relied on to do its job, provided the vial is stored properly and aseptic technique is used for each withdrawal, as for any multi-use reagent vial.
Two practical points follow. Write the date the vial was first opened on it, because "about a month ago" is not a record. And remember that the 28 days applies to the water. A peptide dissolved in it has its own stability, which depends on the compound, the concentration and the temperature, and is usually shorter. Our peptide storage guide covers how long reconstituted solutions last and how to hold them.
Where can you buy bacteriostatic water in the UK?
This is the question most people arrive with, so here is the straight answer. Bacteriostatic water is not a pharmacy stock line in the UK. The one licensed UK product is a prescription-only diluent tied to a specific medicine, not a general line. For laboratory work, laboratory suppliers are the source.
In practice, UK laboratories and researchers buy bacteriostatic water from laboratory and research-chemical suppliers, who import it or have it manufactured to pharmacopoeial-type specification and supply it as a reagent. Vivera Labs sells it in 10 mL vials, dispatched from UK stock, as a companion to the lyophilised peptides in the research catalogue. Overseas marketplace sellers also list it, often in 30 mL vials; those parcels come through customs, take longer, and you cannot see how they were stored in transit, which matters for a product whose entire purpose is to stay clean.
What to check when buying, wherever you buy: the label states the benzyl alcohol content and the volume; the vial is sealed with an intact flip-off cap; the supplier can tell you the specification it is made to; the supplier is a real company you can find and contact; and the price is not suspiciously low. A 10 mL vial from a UK supplier costs a few pounds. If someone is selling a case of 30 mL vials for less than that per vial, ask yourself what you are actually receiving.
Is bacteriostatic water legal in the UK?
Bacteriostatic water is not a controlled drug under the Misuse of Drugs Act 1971. Supplied and used as a laboratory reagent it is not presented as a medicine. Whether any given product falls within medicines law depends on how it is presented and what it is supplied for, which is why this site describes laboratory reconstitution only. Our guide to whether peptides are legal in the UK covers the position for the compounds it is used with.
How laboratories use bacteriostatic water to reconstitute peptides
Lyophilised peptides arrive as a dry powder or cake at the bottom of a sealed vial. To work with them in solution, a measured volume of solvent is introduced through the stopper and the vial is swirled gently until the solid dissolves; peptides are not shaken, because agitation and foaming can damage them. The volume added sets the concentration of the resulting stock: the same 5 mg of peptide in 1 mL gives a stock five times as concentrated as in 5 mL. Our reconstitution guide walks through the laboratory technique, and the reconstitution calculator does the arithmetic from mass and volume to concentration so you can plan aliquots for an assay series.
Bacteriostatic water is the default solvent because most research peptides dissolve readily in water and because the preserved stock can be returned to across a series of experiments. Some peptides, particularly very hydrophobic sequences or those with a strongly positive or negative net charge, dissolve poorly in plain water and need a different approach: a small amount of dilute acetic acid or ammonium bicarbonate, or a buffer at a chosen pH, before dilution. The peptide solubility calculator estimates net charge and hydropathy from a sequence and suggests a starting solvent, with the limitations spelled out. None of this is dosing information; it is bench preparation for in-vitro work, and the concentrations involved are chosen by the experiment, not by anything on this page.
When bacteriostatic water is the wrong choice
Benzyl alcohol is a small aromatic molecule, and in some sensitive systems it is not inert. Certain primary cell cultures and viability assays respond to it at the concentrations that carry over from a diluted stock, and a few enzyme assays are affected by it. In those cases the right vehicle is plain sterile water used once, or a defined buffer such as phosphate-buffered saline, with vehicle-only controls included in the experimental design so that any effect of the solvent is measured rather than assumed. Where the peptide stock will be used up in a single session, sterile water is also simply the cheaper and cleaner option.
The other case is any application outside the laboratory. Bacteriostatic water is sold here as a research reagent. It is not supplied for, and this site does not describe, preparing anything for administration to people or animals.
Storing bacteriostatic water
Unopened vials keep at room temperature, out of direct light, until the printed expiry date. Once entered, keep the vial capped and upright and use aseptic technique for each withdrawal, as for any multi-use reagent vial. Mark the date the vial was first opened on the label and discard the vial 28 days later, whatever is left in it. Discard immediately if the liquid is cloudy, coloured or contains visible particles; it should be perfectly clear. Do not freeze it. Freezing can crack the vial and does nothing useful for a preserved solvent. Keep it separate from reconstituted peptide stocks, which have their own, usually refrigerated, storage requirements. How to judge the quality of bacteriostatic water
Water is water, which is exactly why it is easy to sell a poor product. The things that separate a pharmacopoeial-grade vial from a bottle of tap water with a printed label are invisible to the eye: sterility at the point of sealing, endotoxin content (the bacterial cell-wall fragments that survive sterilisation and can wreck a cell assay), particulate matter, correct benzyl alcohol concentration and a pH in range. A serious supplier can tell you the specification the product is made to and give you the lot number. A supplier who cannot name a specification is selling you a clear liquid and a hope.
Vivera Labs supplies USP-grade bacteriostatic water in sealed 10 mL vials from UK stock; a Safety Data Sheet is available on request and original batch reports will be published against the product as they are received. It sits in the laboratory essentials section of the catalogue with the peptides it is used alongside.
Frequently Asked Questions What is bacteriostatic water? Sterile water with benzyl alcohol, typically 0.9%, added as a preservative. The alcohol stops bacteria multiplying inside the vial after the seal has been broken, so the vial can be drawn from repeatedly for up to 28 days. Laboratories use it to reconstitute lyophilised peptides into stock solutions that will be aliquoted over several sessions.
What is the difference between bacteriostatic water and sterile water? Sterile water has nothing added; it is sterile only until the stopper is first entered and is meant to be used once. Bacteriostatic water contains a preservative that keeps bacterial growth suppressed between uses, which is why it is the standard vehicle for a peptide stock that will be accessed over days or weeks.
Where can I buy bacteriostatic water in the UK? From laboratory and research-chemical suppliers. It is not a pharmacy stock line in the UK; the one licensed UK product is a prescription-only diluent tied to a specific medicine. Vivera Labs supplies 10 mL vials from UK stock as a laboratory reagent.
Is bacteriostatic water legal in the UK? It is not a controlled drug. Supplied and used as a laboratory reagent it is not presented as a medicine. Whether a given product falls within medicines law depends on how it is presented and what it is supplied for, which is why this site describes laboratory reconstitution only.
How long does bacteriostatic water last once opened? The preservative is relied on for 28 days after the vial is first opened, provided the vial is stored capped at room temperature and aseptic technique is used for each withdrawal. Mark the date of first use on the vial and discard it after 28 days. A peptide dissolved in it has its own, usually shorter, stability.
Does bacteriostatic water need to be refrigerated? No. Unopened and opened vials are kept at room temperature, out of direct light. It should not be frozen. Reconstituted peptide stocks made with it usually do need refrigeration, but that is a property of the peptide, not the water.
What size does bacteriostatic water come in? Most commonly 10 mL and 30 mL vials. Overseas marketplace sellers usually list 30 mL; Vivera Labs stocks 10 mL vials, which suit small-batch research reconstitution and keep the amount of open, ageing solvent to a minimum.
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For in-vitro laboratory research use only. Not for human or veterinary use, consumption, or therapeutic application. No medical claims are made.
