Bacteriostatic Water
Sterile water containing 0.9 percent benzyl alcohol. The preservative is the entire point: it is what allows a vial to be drawn from more than once without becoming a culture medium.
What it is
Bacteriostatic water is water for injection with benzyl alcohol added at 0.9 percent as a bacteriostatic agent. "Bacteriostatic" means it inhibits bacterial reproduction rather than killing outright, which is sufficient to keep a punctured vial usable across repeated withdrawals.
At a glance
| Composition | Sterile water for injection, 0.9% benzyl alcohol |
|---|---|
| Function | Diluent for lyophilised peptides; permits multiple withdrawals |
| Presentation | Sealed multi-dose vial, commonly 10ml or 30ml |
| Storage before use | Room temperature, protected from light |
| After first puncture | Commonly specified at 28 days |
How it differs from the alternatives
| Diluent | Preservative | Repeat withdrawals | Notes |
|---|---|---|---|
| Bacteriostatic water | 0.9% benzyl alcohol | Yes | The default for multi-use vials |
| Sterile water for injection | None | No | Single use; discard the remainder |
| Sodium chloride 0.9% | None | No | Can cause some peptides to precipitate |
The practical rule: if the vial will be drawn from more than once, it needs a preservative. If it will be used in a single sitting, plain sterile water is adequate.
The 28 day window
Most manufacturers specify 28 days from first puncture. That figure describes the period over which the benzyl alcohol can be relied on to suppress microbial growth, and it is frequently the limiting factor on a reconstituted peptide rather than the peptide's own chemical stability.
The clock starts at first puncture, not at reconstitution and not at opening the carton. Marking the vial with that date is the simplest way to avoid guessing later.
When not to use it
A small number of peptides are documented as sensitive to benzyl alcohol. Where a supplier specifies plain sterile water, follow that. The specification for the material in hand always overrides the general rule.
Benzyl alcohol is also the reason bacteriostatic water is not used in some contexts where preservative-free diluents are required. Again, the batch documentation governs.
Handling
Store sealed vials at room temperature away from light. Swab the stopper before each puncture and let it dry. Draw only what is needed, and do not decant into another container, since transferring introduces exactly the contamination risk the preservative exists to manage.
Full procedure in how to mix peptides for injection.
Not required for pens
A pre-filled pen arrives as a made-up solution in a sealed cartridge. There is no reconstitution step and therefore no diluent to choose. See pens compared with vials.