Glossary

Plain-English definitions of the terms used across our products and guides.

Almost every term here comes back to one word. Bacteriostatic describes something that stops bacteria multiplying without necessarily killing them — from bacteria and the Greek stasis, a standstill. A bacteriostatic agent is the substance doing that (in these products, 0.9% benzyl alcohol); bacteriostatic water and bacteriostatic saline are the two diluents that carry it, and the reason they can be labelled multi-dose while their preservative-free counterparts are single-use. The rest of the glossary is the vocabulary that surrounds that fact: what the water is made of, what the label promises, and what to look for when a vial is inspected. The word itself, with the distinction from bactericidal and from sterile, has its own guide.

Bacteriostatic water: Also BAC water, bac water or BWFI. Sterile water containing 0.9% (9 mg/mL) benzyl alcohol, a bacteriostatic preservative that inhibits the growth of most common bacteria. The preservative allows a sealed multi-dose vial to be accessed repeatedly. Use only as specified in the manufacturer product information and under qualified healthcare-professional direction. What is BAC water? →
Sterile water: Purified water that has been filtered to remove microorganisms and sealed under sterile conditions, with no preservative added. Because there is no bacteriostatic agent, a sterile water vial is intended for a single withdrawal and should not be re-entered. Bacteriostatic water vs sterile water →
Bacteriostatic sodium chloride (saline): An isotonic 0.9% sodium chloride (saline) solution with benzyl alcohol added as a bacteriostatic preservative, allowing repeated access to a sealed multi-dose vial. Follow the supplied product's manufacturer information for its intended application. What is bacteriostatic sodium chloride? →
Benzyl alcohol: A colourless aromatic alcohol used as a preservative in certain medical solutions. At 0.9% (9 mg/mL) it acts as a bacteriostatic agent, holding bacterial growth in check after a vial's seal is broken. How the preservative works →
Bacteriostatic: Describes an agent that inhibits the growth and reproduction of bacteria without necessarily killing them — from the Greek stasis, a standstill. In bacteriostatic water and bacteriostatic saline, 0.9% benzyl alcohol performs this role, holding bacterial growth in check between entries into a multi-dose vial. A bacteriostatic preservative does not guarantee sterility after opening and does not replace the supplied handling requirements. What bacteriostatic means, in full →
Bacteriostatic agent: Any substance that holds bacterial growth at a standstill without necessarily killing the organisms present — the preservative in a multi-dose vial, as opposed to a disinfectant or an antibiotic that kills. Benzyl alcohol at 0.9% is the bacteriostatic agent in bacteriostatic water and bacteriostatic saline; other injectables use parabens, phenol or chlorobutanol for the same job. The agent's job is to keep a small number of organisms introduced on a needle from becoming a large one, which is why a bacteriostatic presentation can be labelled multi-dose and a preservative-free one cannot. How the preservative works →
Bactericidal: Describes an agent that kills bacteria outright, rather than merely halting their growth. The distinction matters when reading labels: the preservative in bacteriostatic water is bacteriostatic, not bactericidal, so it suppresses growth between vial entries but cannot make a contaminated solution safe.
Reconstitution: The process of dissolving a freeze-dried (lyophilised) substance in a liquid diluent such as bacteriostatic or sterile water, turning a stated mass into a solution with a measurable concentration — the stated milligrams divided by the millilitres of diluent added. Product selection, preparation and use must follow the supplied manufacturer information under qualified healthcare-professional direction. How reconstitution works →
Lyophilisation (freeze-drying): The process of freezing a solution and removing the ice under vacuum, leaving the dissolved substance as a dry cake or powder. Products are supplied lyophilised because most are far more stable stored dry than in solution; a lyophilised solid must be reconstituted in a compatible diluent before it can be measured by volume. How reconstitution works →
Multi-dose vial: A manufacturer-labelled vial presentation intended for more than one withdrawal when handled as specified in its product information by qualified personnel. No maximum number of withdrawals is printed: what limits the vial is the in-use period and the printed expiry, whichever falls first. How many times can you use one? →
In-use period: The fixed period during which a multi-dose vial may continue to be used, running from the moment its stopper is first entered. It is separate from, and much shorter than, the expiry date printed on the sealed vial; whichever falls first ends the vial's usable life. How long does a vial last? →
Clarity: Freedom from visible haze, cloudiness or suspended particulate. Bacteriostatic water is expected to be a clear, colourless solution, and a departure from that appearance is treated as an inspection failure rather than a cosmetic issue. Inspecting a vial for clarity →
Cloudiness (haze): The visible scattering of light by particles that are not dissolved — undissolved solid, a precipitate, aggregated material or contamination. A true solution is optically clear because dissolved molecules are too small to scatter visible light, so haze is direct evidence that something in the container is out of solution. Where material is expected to be clear and is not, it is not used. What cloudiness indicates →
Precipitation: The coming out of solution of a substance that was previously dissolved, appearing as haze, crystals or a deposit. It is commonly driven by a change in pH, concentration, temperature or ionic strength.
Particulate matter: Undissolved foreign or intrinsic material visible in a solution — flakes, fibres, crystals or fragments. Injectable presentations are released against specified particulate limits, and a vial with visible particulate is discarded rather than filtered or decanted.
Coring: The punching out of a fragment of a vial's rubber stopper by a needle, rather than the stopper parting cleanly, leaving the fragment in the solution. It becomes more likely with repeated entries into the same point on a stopper, which is why a multi-dose vial is inspected before every entry. What to check before use →
Water for injection (WFI): The pharmacopoeial grade of water that injectable presentations are made from: purified to remove dissolved solids and organic matter, then rendered sterile and nonpyrogenic, and released against a compendial monograph. Sterile water for injection is this water alone in a sealed container; bacteriostatic water for injection is this water with 0.9% benzyl alcohol added. What is sterile water for injection? →
Diluent: A liquid used to dilute or dissolve a drug before it is given — the vehicle, not the medicine. Bacteriostatic water, sterile water for injection and sodium chloride 0.9% are all diluents. A diluent has no therapeutic effect of its own and is never injected by itself; which one a medicine takes, and how much, is set by that medicine's product information.
Preservative-free: Describes a presentation with no bacteriostatic agent added — sterile water for injection or preservative-free sodium chloride 0.9%, typically in a single-use ampoule or vial. Because nothing inhibits growth once the container is open, the label treats it as a single withdrawal. It is the presentation the manufacturer information directs to where benzyl alcohol is contraindicated, such as neonates. Bacteriostatic water vs sterile water →
Hypotonic: A solution with a lower effective osmotic concentration than the reference — for injectables, than blood plasma. Water for injection with or without a preservative contains no solute and is hypotonic, which is why the manufacturer information warns that it must not be given intravenously on its own: red cells take up water and rupture (haemolysis). It becomes approximately isotonic only by way of whatever it is used to dissolve.
Isotonic: A solution with the same effective osmotic concentration as a reference solution. Sodium chloride 0.9% is commonly described as isotonic saline.
Sodium chloride 0.9% (normal saline): Nine grams of sodium chloride per litre of water for injection — the concentration that is approximately isotonic with blood, which is why it is called normal or physiological saline. Supplied preservative-free in single-use ampoules and bags, and as bacteriostatic sodium chloride in multi-dose vials with 0.9% benzyl alcohol added. It is a different diluent from bacteriostatic water, and a medicine's product information names one or the other. What is bacteriostatic sodium chloride? →
Nonpyrogenic: Free of fever-inducing substances, principally bacterial endotoxin. It is a distinct release requirement from sterility: a solution can be sterile and still contain pyrogens left behind by organisms killed during processing.
USP monograph: The published United States Pharmacopeia standard a compendial product is manufactured and released against, covering identity, sterility, preservative content, particulate limits, container closure integrity and a controlled pH range. Bacteriostatic Water for Injection USP is released against that specification rather than a manufacturer's private recipe. What USP means on a vial →
Medical grade: A product manufactured and labelled for medical use under the quality requirements applicable to its manufacturer presentation. Aus BAC Supply supplies original, factory-sealed manufacturer products without alteration.

Looking for the products these terms describe? We stock factory-sealed bacteriostatic water (BAC water) and bacteriostatic sodium chloride 0.9% (BAC saline), dispatched from Melbourne.

These products are diluents or solvents, not ready-to-administer medicines. Never inject by themselves or self-administer. Follow the supplied manufacturer information and qualified healthcare-professional direction.