Bacteriostatic Water: What It Is and How Laboratories Use It
Bacteriostatic water is one of the most commonly used diluents in a research setting, and also one of the most misunderstood. This guide explains what it is, how it differs from other laboratory waters, and how to store and handle it properly.
What is bacteriostatic water?
Bacteriostatic water is sterile, filtered water containing approximately 0.9% benzyl alcohol as a bacteriostatic preservative. The preservative does not sterilise the solution — it inhibits the growth of bacteria that may be introduced when the closure is punctured.
Because of that preservative, a sealed vial can be entered more than once within its labelled in-use period. That is the practical reason laboratories reach for it when returning lyophilised material to solution: the reconstituted vial has a usable working life rather than being a single-entry item.
Bacteriostatic water vs sterile water vs saline
| Bacteriostatic water | Sterile water | 0.9% sodium chloride | |
|---|---|---|---|
| Preservative | Benzyl alcohol ~0.9% | None | None (unless stated) |
| Entries per vial | Multiple, within labelled period | Single entry | Single entry |
| Composition | Water + preservative | Water only | Water + salt (isotonic) |
| Typical lab use | Reconstituting lyophilised material for repeat sampling | Where a preservative would interfere | Where tonicity matters |
Always defer to the compound documentation: some materials specify a particular diluent because the preservative or the salt content can interfere with stability or analysis.
Storage and shelf life
Before first entry
Store unopened vials at controlled room temperature, away from direct light and extremes of heat. Do not freeze. Check the expiry printed on the vial before use.
After first entry
Manufacturer labelling commonly specifies discard 28 days after first entry, with refrigerated storage in the interim. Write the date of first entry on the vial so the in-use window is unambiguous.
- Inspect the solution before every use — it should be clear, colourless and free of particulates.
- Discard any vial that is cloudy, discoloured, damaged or past its labelled in-use period.
- Keep vials in their original packaging so batch and expiry details stay with the material.
- Never top up or combine vials; each vial has its own in-use clock.
Good reconstitution practice
These are general laboratory handling principles. Volumes, ratios and analytical procedures are determined by the protocol and the compound documentation, not by this page.
- Work on a clean surface and allow both vials to reach room temperature.
- Swab both stoppers with an alcohol wipe and let them dry fully before entry.
- Use a fresh sterile needle and syringe for each entry — never re-enter with a used needle.
- Direct the diluent gently down the inside wall of the vial rather than straight onto the lyophilised cake; agitation can damage delicate material.
- Allow the solid to dissolve without shaking. Swirl slowly if needed, and do not vortex unless the protocol calls for it.
- Label the reconstituted vial with the compound, resulting concentration and the date of reconstitution.
- Store as the compound documentation specifies and record the batch details.
Need to work out the resulting concentration of a reconstituted vial? Use our Laboratory Concentration Calculator — it converts compound mass and total solution volume into mg/mL and mcg/mL.
Frequently asked questions
What does bacteriostatic actually mean?
Bacteriostatic means it inhibits bacterial growth rather than killing bacteria outright. The benzyl alcohol preservative suppresses the multiplication of organisms that may be introduced when a vial is entered, which is what allows a multi-use vial to remain usable across several withdrawals.
Is bacteriostatic water the same as sterile water?
No. Both begin sterile, but only bacteriostatic water contains a preservative. Sterile water for injection has no preservative, so it is intended for a single entry and discard.
How long does bacteriostatic water last once opened?
Manufacturer labelling typically gives 28 days from first entry when stored correctly. Always follow the labelling on the specific vial you hold, and record the date of first entry on the vial.
Does it need to be refrigerated?
Unopened vials are normally stored at controlled room temperature. After first entry, refrigeration is commonly specified. The vial's own labelling is the authority.
Can bacteriostatic water be frozen?
Freezing is not recommended. Freeze-thaw cycles can affect container integrity and preservative distribution.
Why is it used for reconstituting lyophilised material?
Lyophilised (freeze-dried) research material is supplied as a powder and must be returned to solution before it can be measured or analysed. A preserved diluent allows a vial to be re-entered over a working period without the growth risk of an unpreserved one.
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Browse our batch-tested research range, including bacteriostatic water.
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Research use only. This page is provided for laboratory and educational reference. It is not medical advice and does not describe or endorse any use in humans or animals. Bacteriostatic water and research compounds sold by Better Body Lab are supplied strictly for laboratory research and are not for human consumption or administration.