Bacteriostatic Water
FDA ApprovedBacteriostatic Water for Injection, USP (0.9% benzyl alcohol)
The preserved sterile diluent used to reconstitute lyophilized peptides. Not a compound you dose — it is what everything else gets mixed with.
Dose
1–3 mL per lyophilized vial, chosen to hit the concentration you want
Route
Diluent
Cycle
N/A — diluent
Storage
Unopened vials: store at controlled room temperature, roughly 20–25 °C (68–77 °F). Protect from light and freezing. Use before the printed expiration date.
What is Bacteriostatic Water?
Bacteriostatic Water for Injection (BAC water) is sterile water containing 0.9% benzyl alcohol as a bacteriostatic preservative. That preservative is the whole point: it suppresses the growth of most common bacteria, which is what allows a multi-dose vial to be punctured repeatedly over days or weeks instead of being discarded after a single entry. It is distinct from the other diluents people confuse it with. Sterile Water for Injection (SWFI) has no preservative and is single-use once opened. Normal saline (0.9% sodium chloride) is isotonic and also typically unpreserved unless labeled bacteriostatic. Only bacteriostatic water carries the benzyl alcohol that makes repeat entry reasonable, which is why it is the default diluent for reconstituting lyophilized research peptides. BAC water contributes no activity of its own. It sets the concentration of whatever it is mixed with — the same 5 mg vial reconstituted with 1 mL versus 2 mL yields two very different concentrations per unit on the syringe — so the volume chosen is a math decision, not a potency decision.
Key Benefits
- Allows multi-dose vial entry over days or weeks
- Preserved with 0.9% benzyl alcohol to suppress bacterial growth
- Sets the working concentration of a reconstituted vial
- Widely available, inexpensive, and shelf stable
Mechanism of Action
- Benzyl alcohol disrupts bacterial cell membranes, producing a bacteriostatic (growth-suppressing) rather than bactericidal environment
- Provides no pharmacological activity of its own; acts purely as a carrier and concentration medium
Best For
- Reconstituting lyophilized peptide vials
- Multi-dose vials used across an extended period
Body Systems
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Molecular Information
Molecular weight
18.02 g/mol (water) with 0.9% w/v benzyl alcohol
Type
Sterile preserved diluent
Research Indications
Why the study exists
USP General Chapter <797> directs that multi-dose containers be discarded within 28 days of first puncture unless the manufacturer specifies otherwise. The operative question for BAC water is whether the benzyl alcohol preservative actually falls below an effective concentration in that window.
Study design
Three 30 mL vials of Hospira BAC water were tracked across three conditions: room temperature with monthly access (low-disturbance control), room temperature with 1 mL withdrawn weekly with air replacement, and refrigerated (~4 °C) with the same weekly withdrawal pattern. Benzyl alcohol was measured by liquid chromatography at days 0, 30, 60, 90 and 120, all assayed by Janoshik Analytical, an independent third-party lab, with figures taken from the issued COAs.
Measured benzyl alcohol (mg/mL)
Room temp / monthly use: 0.883, 0.918, 0.894, 0.898, 0.816. Room temp / weekly use: 0.892, 0.895, 0.893, 0.890, 0.817. Refrigerated / weekly use: 0.888, 0.904, 0.911, 0.899, 0.844. Values at day 0, 30, 60, 90 and 120 respectively; method variance estimated at up to 8%.
What the numbers showed
At day 30 — the closest data point to the 28-day mark — all three samples were essentially unchanged from baseline. Through day 90 the three tracked closely with fluctuation inside method variance and no clear downward trend. Between day 90 and day 120 all three dropped meaningfully, yet every sample remained above the 0.5% v/v literature-supported floor for effective preservation in injectable formulations (commercial BAC water is typically formulated at 0.9% v/v).
Conclusion as the authors state it
Hospira BAC water held benzyl alcohol above the preservative effectiveness floor across all three storage and use conditions for 120 days. The data do not support the conclusion that preservative degradation is what drives the USP <797> 28-day window for multi-dose containers.
The caveat that comes with it
The authors are explicit that these results are educational only and are not intended to inform decisions about human use of any product. Sterility-based discard windows exist for reasons beyond preservative chemistry — repeated stopper punctures, coring and handling technique all introduce contamination risk that a benzyl alcohol assay cannot measure.
Research Protocols
| Goal | Dose | Frequency | Route |
|---|---|---|---|
| Standard 5 mg vial, higher concentration | 1 mL BAC water | Once at reconstitution | Yields 5 mg/mL — 500 mcg per 10 units on a U-100 syringe |
| Standard 5 mg vial, easier small doses | 2 mL BAC water | Once at reconstitution | Yields 2.5 mg/mL — 250 mcg per 10 units, finer resolution for micro-dosing |
| 10 mg vial, balanced draw volume | 2 mL BAC water | Once at reconstitution | Yields 5 mg/mL — 500 mcg per 10 units |
| Fragile or short-course compound | 1 mL BAC water | Once at reconstitution | Smaller total volume so the vial is used up well inside the discard window |
Volume is a concentration decision, not a dose decision. Decide the units-per-dose you want on the syringe first, then work backward to the diluent volume.
Peptide Interactions
- Compatible
Lyophilized peptides (general)
The standard reconstitution diluent for essentially all lyophilized research peptides.
- Compatible
Sterile Water for Injection
Interchangeable at the moment of reconstitution, but unpreserved — treat the vial as single-entry.
- Compatible
Bacteriostatic saline
An alternative preserved diluent; occasionally preferred for local tolerability or for compounds specified in saline.
- Avoid
Benzyl alcohol sensitivity
Anyone with a known benzyl alcohol hypersensitivity should use an unpreserved diluent.
- Avoid
Neonatal or infant use
Benzyl alcohol exposure has been associated with fatal gasping syndrome in newborns.
How to Reconstitute
- 1Wipe both the BAC water stopper and the peptide vial stopper with a fresh alcohol swab and let them dry.
- 2Draw your chosen volume of bacteriostatic water into a sterile syringe.
- 3Insert the needle at an angle and let the water run slowly down the inner wall of the peptide vial — never blast it directly onto the powder.
- 4Let the vacuum pull the remaining volume rather than forcing the plunger.
- 5Swirl gently or roll the vial between your fingers until fully dissolved. Do not shake.
- 6Label the vial with the date of reconstitution and the resulting concentration, then refrigerate.
Add the diluent slowly down the inside wall of the vial rather than straight onto the powder cake — a hard stream can shear fragile peptides.
What to Expect
- The powder cake should dissolve within seconds to a couple of minutes with gentle swirling.
- A properly reconstituted vial is clear and colorless with no visible particles.
- Faint bubbling or foam from over-agitation settles out; persistent cloudiness does not and means the vial is compromised.
- The same vial reconstituted with 2 mL instead of 1 mL is half as concentrated — every unit mark on your syringe now delivers half as much.
Side Effects & Safety
| Effect | Frequency | Severity |
|---|---|---|
| Local irritation at the injection site from benzyl alcohol | Uncommon | Mild |
| Hypersensitivity reaction in benzyl alcohol-sensitive individuals | Rare | Moderate |
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Open CalculatorPeptides discussed here are for research purposes only. Nothing on this page is medical advice. Always consult a qualified professional before making health decisions.