VialBacteriostatic Water for Peptides: A Diluent Guide
Bacteriostatic water — usually written "BAC water" — is sterile water with 0.9% benzyl alcohol added as a preservative. It is the standard diluent for multi-use vials. This guide explains why, how it differs from sterile water, and how the diluent volume drives your entire reconstitution math.
Why multi-use vials use bacteriostatic water
Once you pierce a vial's stopper, the contents are exposed to whatever comes through the needle. Sterile water has no preservative, so repeated access introduces contamination risk. The 0.9% benzyl alcohol in bacteriostatic water is a preservative that keeps the contents suitable for multiple draws from the same vial over days to weeks. Single-use injections, by contrast, are a different situation — the right diluent always depends on the product and its instructions.
Bacteriostatic water vs sterile water
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Typical use | Multi-use vials, several draws over days/weeks | Single-use dilution immediately before use |
| After opening | Follow supplier labeling; commonly used within its stated period | Use immediately, discard remainder |
Neither is universally "correct" — some products specify sterile water, some specify bacteriostatic water, and a few specify neither. The product's own instructions win every time.
How much diluent to add — and why it matters so much
The diluent volume sets the concentration:
concentration = vial strength ÷ diluent volume
A 5 mg vial with 1 mL of diluent is 5 mg/mL; the same vial with 2 mL is 2.5 mg/mL. Your target dose then divides by concentration to give the draw volume — so a different diluent volume means a different number of units on the syringe for the identical dose. This is the single most common source of reconstitution errors.
How much to add is determined by your product's instructions or your pharmacy — not by a general rule. Once you know the volume, run the numbers in the free reconstitution calculator.
Practical notes
- Refrigeration and storage: follow your supplier's labeling for both the diluent and the reconstituted vial.
- Most multi-use vials follow a 28-day in-use window after first puncture — write the date on the vial.
- Cloudiness, particles or discoloration after mixing are a stop signal: don't use the vial, and check with your pharmacist.
- Order a replacement before the current vial runs out — a reorder reminder is built into the VialDose app.
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