Bacteriostatic water vs sterile water
Written and maintained by the AVERON Research Desk · Last reviewed September 2026
Two types of water are used to dissolve lyophilized powders in the laboratory: bacteriostatic water and sterile water. They look identical, but they are made for different situations, and the difference comes down to one ingredient.
Sterile water
Sterile water has been purified and sterilized, and contains nothing else. It has no preservative. Once a vial of sterile water has been punctured, nothing stops bacteria that enter it from growing. For that reason, sterile water is intended for single use: the vial is opened once and any unused portion is discarded.
Bacteriostatic water
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. Benzyl alcohol is a preservative. It does not sterilize the water or kill every microorganism instantly; the word "bacteriostatic" means it inhibits bacterial growth. That allows a vial to be punctured several times over a period of days or weeks with a much lower risk of contamination building up.
Side by side
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Contents | Sterile water + 0.9% benzyl alcohol | Sterile water only |
| Preservative | Yes | No |
| Intended use | Multiple-dose vials | Single use |
| After opening | Can be punctured repeatedly | Discard the unused portion |
| Typical role with peptides | Default choice for multi-use vials | Used where a preservative is unwanted |
Why bacteriostatic water is usually used for peptides
A reconstituted peptide vial is normally used over a period of time, with a needle passing through the stopper each time an amount is drawn out. Every puncture is a chance for bacteria to enter. The preservative in bacteriostatic water limits the growth of anything that does. With sterile water, that protection does not exist.
Pharmacy standards generally set a limit of 28 days for multiple-dose containers after they are first opened, unless a manufacturer specifies otherwise. That is a common reference point for how long an opened vial of bacteriostatic water should be kept.
When sterile water is used instead
Some compounds are better dissolved without benzyl alcohol, and some protocols in the laboratory call for a preservative-free solution. In those cases sterile water is used, and the resulting solution is treated as single-use or kept for a much shorter time.
Limits of the preservative
Bacteriostatic water reduces risk; it does not remove it. It will not make up for poor handling, a contaminated needle or a vial left out of the fridge. Clean technique still matters every time. Benzyl alcohol is also not suitable for every situation; product labels for bacteriostatic water carry warnings about its use in newborns.
At AVERON
AVERON supplies bacteriostatic water in 3 mL and 10 mL vials, and includes a vial with every peptide order. The volume we suggest for each compound is shown on its product page.