RECONSTITUTION, STORAGE & INJECTION
What Is Bacteriostatic Water? — Uses, Safety, and How Much to Add
By G1 Peptides · Medically reviewed by the G1 Peptides clinical team · Updated October 1, 2026 · 2 min read
THE SHORT ANSWER
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. The preservative stops bacteria from growing, which lets a vial be used for more than one dose. It is the standard liquid for mixing freeze-dried peptides. Sterile water has no preservative and is meant for a single use.
What it is
Bacteriostatic water for injection is sterile water with 0.9% benzyl alcohol. "Bacteriostatic" means it stops bacteria from multiplying. It does not sterilise anything that is already contaminated, so clean technique still matters every time you draw from the vial.
Bacteriostatic water vs. sterile water
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Intended use | Multiple doses from one vial | A single use |
| After first puncture | Generally up to 28 days | Discard what is left |
| Used for mixing peptides | Yes, the usual choice | Only when a product specifically calls for it |
Peptide vials are drawn from many times over days or weeks. Without a preservative, each puncture is a chance for bacteria to get in and grow. That is the reason bacteriostatic water is the standard.
How much to add
The amount of water sets the concentration. Take a 5 mg vial and a 250 mcg dose:
| Water added | Concentration | A 250 mcg dose |
|---|---|---|
| 1 mL | 5,000 mcg per mL | 5 units |
| 2 mL | 2,500 mcg per mL | 10 units |
| 3 mL | 1,667 mcg per mL | 15 units |
More water makes small doses easier to measure. Less water keeps large doses within one syringe. Use the volume on your pharmacy label, and let the reconstitution calculator do the arithmetic for any vial.
For the mixing steps themselves, see how to reconstitute peptides.
Handling and storage
- Wipe the stopper with an alcohol swab before every draw and let it dry.
- Use a new sterile syringe and needle each time (CDC).
- Store the vial as its label directs and write the date you first opened it.
- Discard it if the liquid is cloudy or has particles in it.
Do pens need it?
No. A prefilled pen is already mixed. Bacteriostatic water is only needed for freeze-dried vials. See peptide pens vs. vials.
FREQUENTLY ASKED
What is bacteriostatic water used for?
It is used to dissolve or dilute medicines for injection, including freeze-dried peptides. Its preservative allows the same vial to be drawn from more than once.
Is bacteriostatic water the same as sterile water?
No. Both are sterile, but bacteriostatic water contains 0.9% benzyl alcohol as a preservative. Sterile water has none and should be used once and then discarded.
How long does bacteriostatic water last after opening?
A multi-dose vial is generally used for up to 28 days after it is first punctured, unless the manufacturer's label says otherwise. Write the date on the vial when you open it.
How much bacteriostatic water do I add to a peptide vial?
Use the volume your pharmacy or provider specifies, commonly 1 to 3 mL. The volume does not change how much peptide is in the vial. It changes the concentration, and so how many syringe units each dose takes.
SOURCES
This article is educational and is not medical advice. Peptides are prescription medications; whether any of them is appropriate for you is a decision for a licensed provider who knows your history.