Bacteriostatic water explained: what bac water is and how it differs from sterile water

What is bacteriostatic water (bac water)?

Bacteriostatic water, often shortened to “bac water”, is sterile water with a small amount of preservative added. The US label for Hospira’s Bacteriostatic Water for Injection describes a sterile, non-pyrogenic water containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol, “added as a bacteriostatic preservative”, with a pH of 5.7 (range 4.5 to 7.0).

It has no therapeutic effect of its own, but in the UK it is regulated as a medicine. The UK summary of product characteristics (SmPC) for Pfizer’s Bacteriostatic Water for Injections, a prescription-only medicine, lists its therapeutic indications as “None”, and the Hospira label limits it to diluting or dissolving drugs. “Bacteriostatic” means it holds back the growth of bacteria rather than killing them.

The bacteriostatic water, Water for Injections and saline products described in this article are medicines made by other companies and are not sold here. The research-use notice at the top of this page refers to this shop’s own products.

Why the benzyl alcohol is there

Benzyl alcohol is a simple aromatic alcohol with the formula C7H8O. The European Medicines Agency (EMA) describes it as an excipient used for its preservative properties, and notes that the body breaks it down to benzoic acid.

A preservative is needed when a container is opened more than once. Hospira’s label describes its vial as a container “from which repeated withdrawals may be made”. Guidance for US healthcare settings from the Centers for Disease Control and Prevention (CDC) draws the same line: vials that the manufacturer labels for more than one use typically contain an antimicrobial preservative, while vials labelled for a single use typically lack one.

Two limits follow. First, the preservative slows the growth of bacteria; it does not make a contaminated container sterile again. The CDC notes that the preservative has no effect on viruses and does not give complete protection against bacterial contamination. Second, benzyl alcohol is an added ingredient with its own chemistry. The Hospira label warns that some drugs may be incompatible with a vehicle containing benzyl alcohol, and it refers to each drug maker’s own instructions for the choice of diluent.

Bacteriostatic water vs sterile water and saline

Three other liquids are often confused with bac water. Their published labels show how they differ.

Water for Injections is a pharmacopoeial grade of water and the base from which bacteriostatic water is made; the hameln pharma SmPC for Water for Injections BP gives its content as “100% Water for Injections Ph. Eur.”. Bio Products Laboratory’s UK SmPC for its sterilised water vials states “For single use” and lists no excipients. The hameln SmPC makes no single-use statement, but it warns that the water is hypotonic (it holds fewer dissolved particles than blood) and on its own may cause haemolysis, the bursting of red blood cells. Hospira’s label gives a similar warning for its water without a dissolved solute.

Sodium chloride 0.9% is water with 9 mg of salt in every millilitre. B. Braun’s SmPC gives it a theoretical osmolarity of 308 mOsm/l, says it has the same osmolarity as plasma, and states that its containers are “for single use only”. A preserved version, bacteriostatic sodium chloride, is sold in the US.

Put simply, bacteriostatic water carries a preservative and its US vial is labelled for repeated withdrawals, while sterile water has nothing added and labels such as Bio Products Laboratory’s mark it for single use. The labels do not treat them as interchangeable, and Hospira’s label reserves preservative-free sterile water for medicines prepared for newborns.

Container sizes and what a label shows

Hospira’s label lists trays of 25 vials: 30 mL plastic flip-top vials with 0.9% benzyl alcohol, and a 20 mL fill in a 30 mL glass vial with 1.1%. DailyMed, the US National Library of Medicine’s label database, records a marketing end date for the glass-vial version, and only the 30 mL plastic vial appeared in the FDA’s National Drug Code Directory on 22 October 2026.

The seven standalone bacteriostatic water listings on DailyMed, checked the same day, were all packed in 30 mL vials. Five were relabelled Hospira stock, and one, from Fujian Genohope Biotech, is marked as an unapproved drug. None was a 10 mL vial, although Pfizer’s preserved saline comes in 10, 20 and 30 mL vials.

The only bacteriostatic water on the UK’s electronic medicines compendium (eMC) is Pfizer’s, supplied as a 1 ml pre-filled syringe holding 1 ml of solution with 9.45 mg/ml benzyl alcohol. A 10 mL “bacteriostatic water” vial therefore matches none of the products checked for this article. Water for Injections BP from hameln pharma is listed in ampoules of 2 to 20 ml and vials of 1 to 200 ml.

The Hospira vial and tray labels show the details a laboratory would check first:

• the product name and its pharmacopoeia: Bacteriostatic Water for Injection, USP
• the preservative and its strength, such as “0.9% benzyl alcohol added”
• the fill volume, for example 30 mL
• the warning “NOT FOR USE IN NEONATES” and, on the tray, “FOR DRUG DILUENT USE ONLY”
• a lot number and an expiry date
• the distributor’s name, and “Rx only” on the tray

A label without a named maker, a lot number or a stated preservative strength gives a laboratory nothing to trace. The same principle applies to peptide batches and their reports, covered in peptide testing and certificates of analysis.

Storage and the in-use period

Bacteriostatic water is a room-temperature product. Hospira’s label gives storage at 20 to 25 °C, the US controlled room temperature. Pfizer’s UK SmPC says to store below 25 °C and gives an unopened shelf life of three years. Water for Injections BP from hameln pharma is likewise not to be stored above 25 °C.

Once a container has been opened, a second date applies. The Hospira label itself prints no in-use period. The CDC’s guidance for US healthcare settings, citing United States Pharmacopeia chapter 797, says an opened vial labelled for more than one use should be dated and discarded within 28 days unless the manufacturer states another date, and that this date should never run past the manufacturer’s original expiry.

Storage conditions are specific to the item they describe: a diluent’s label says nothing about a peptide, whose conditions are set by its own label. How to store peptide pens covers what pen labels state.

Labelled cautions about benzyl alcohol

The best-known caution concerns newborn babies. In June 1982 the CDC’s Morbidity and Mortality Weekly Report described 16 deaths of premature infants at two US medical centres, thought to have been caused by benzyl alcohol in bacteriostatic saline (9 mg/ml benzyl alcohol) used to flush their catheters. The US Food and Drug Administration recommended that flush solutions containing benzyl alcohol, and diluents preserved with it, not be used for newborns.

The EMA explains why: newborns, especially those born early, cannot yet clear benzyl alcohol efficiently, so it accumulates. The resulting “gasping syndrome” involves metabolic acidosis, and most reported cases were fatal. Children under three may also clear it less efficiently than adults.

Hospira’s US vial reads “NOT FOR USE IN NEONATES”. Pfizer’s UK SmPC is worded differently and sets out:

• a contraindication for hypersensitivity to benzyl alcohol, which may itself cause hypersensitivity reactions
• serious adverse events and deaths in children, including neonates, associated with intravenous benzyl alcohol
• a warning that premature and low-birth-weight babies may be more likely to develop toxicity
• a separate caution covering children under three
• a warning that benzyl alcohol can accumulate and cause metabolic acidosis in pregnancy, in breastfeeding and in liver or kidney impairment

Bacteriostatic water in the UK

Under the Human Medicines Regulations 2012, “a product for parenteral administration”, meaning one given by a route that bypasses the digestive tract, is among the medicinal products that “shall be available only on prescription” (Schedule 1, Part 1, read with regulation 5). All four UK products checked for this article fit that pattern: the eMC lists Pfizer’s Bacteriostatic Water for Injections, hameln pharma’s Water for Injections BP, Bio Products Laboratory’s sterilised water for injections and B. Braun’s sodium chloride 0.9% as prescription-only medicines.

Pfizer’s product holds UK product licence PL 00057/0934, and its SmPC lists no indication and describes it only as the diluent for one named Pfizer prescription medicine. It was the only bacteriostatic water returned by an eMC search on 22 October 2026.

Hospira bacteriostatic water, the product behind most of the DailyMed listings, is a US product labelled “Rx only” for the US market; a US label is not a UK marketing authorisation.

Why bac water comes up alongside research peptides

Many peptides used in laboratories are supplied as lyophilised (freeze-dried) powders, so the labelling of diluents comes up in the same discussions. For background on the compounds themselves, see what peptides are.

Whether any liquid is compatible with a given compound is a chemistry question for its maker: solubility, pH and compatibility with a preservative all play a part. For a medicine, the manufacturer names the diluent in its product information, as Pfizer’s UK bacteriostatic water shows; for a research material, the supplier’s technical data sheet is the reference.

A prefilled pen is different: it is supplied with the peptide already in solution in a sealed cartridge, so there is no powder to dissolve. The products this shop sells are prefilled pens, each labelled with its content per pen, and peptide pens explained compares pens with freeze-dried vials. The shop does not supply bacteriostatic water or any other diluent, and this article does not describe any preparation method.