Reference
Bacteriostatic water, sterile water and water for injection
Three clear liquids in three identical vials, and the differences between them are not visible. One of them contains something other than water, and that is the whole distinction.
This page describes what each of these is. It does not say which one belongs with what, or how any of them should be used — that is a question for a registered prescriber, and it is outside what a supplier should be answering.
Water for injection
Water for injection, usually written WFI, is a pharmacopoeial grade rather than a product. It is defined by what it must not contain: limits on conductivity, on total organic carbon, on microbial count, and — the one that distinguishes it from ordinary purified water — on bacterial endotoxin.
Endotoxin is the reason the grade exists. It is fragments of the outer membrane of Gram-negative bacteria, it survives sterilisation comfortably, and killing the bacteria in a batch of water does not remove it. WFI is produced by distillation or by membrane processes specifically designed to leave it behind.
Sterile water for injection
Water for injection, put in a container and sterilised, with nothing added. No preservative, no buffer, no salt.
Because there is no preservative, the sterility only holds while the container is sealed. Once a closure has been pierced, nothing in the vial discourages anything that got in from multiplying, which is why sterile water is packaged and labelled as a single-use item.
Bacteriostatic water
The same water, with benzyl alcohol added at 0.9% — nine parts per thousand by volume. That one ingredient is the entire difference, and it is what the name refers to.
Benzyl alcohol is an aromatic alcohol, a colourless liquid with a faint almond smell, used as a preservative in pharmacy since the early twentieth century. At 0.9% it is present at a concentration that suppresses bacterial growth without being a solvent in any meaningful sense.
Bacteriostatic is not the same word as sterile
This is the distinction the name is pointing at and the one most often collapsed.
- Bactericidal — kills bacteria.
- Bacteriostatic — stops them multiplying. They are still there.
- Sterile — a property of a container at a moment in time: nothing viable in it.
Bacteriostatic water is supplied sterile and stays inhibitory after it is opened. It is not self-sterilising, it does not clean anything introduced into it, and it will not rescue a container that has been contaminated with any quantity of anything. What it does is make a multi-entry container a reasonable proposition instead of an unreasonable one.
None of these is a peptide
Worth stating plainly, because bacteriostatic water is sold in the same catalogue as thirty-four peptides and inherits assumptions from them. Water has no amino acid residues and no peptide bonds. Benzyl alcohol is a seven-carbon molecule, not a chain of anything.
Which means the vocabulary used everywhere else on this site does not apply to it. There is no HPLC purity figure for water in the sense used for a peptide, no net peptide content, no counterion and no molecular weight worth quoting on a label. A certificate for bacteriostatic water is a different kind of document testing different things — typically the benzyl alcohol concentration, sterility, endotoxin and pH.
The three side by side
| What it is | Contains | After opening |
|---|---|---|
| Water for injection | A grade, not a product — water meeting endotoxin, conductivity and TOC limits | Bulk material; not supplied as a finished vial |
| Sterile water for injection | WFI and nothing else | No preservative — single use |
| Bacteriostatic water | WFI plus 0.9% benzyl alcohol | Growth suppressed — multi-entry |
Sterility and endotoxin · What is bacteriostatic water? · Bacteriostatic water in the range
Buy in New Zealand
Prices, sizes and stock: Bacteriostatic Water in New Zealand · Reusable Peptide Pen Kit in New Zealand.
