Quick answer
Bacteriostatic water is sterile water that contains 0.9% (9 mg/mL) benzyl alcohol as an antimicrobial preservative. Laboratories use it to reconstitute lyophilized research peptides when a vial will be entered more than once, because the preservative suppresses the growth of microorganisms introduced by repeated needle punctures. The volume you add sets the concentration: concentration (mg/mL) = peptide mass (mg) ÷ diluent volume (mL). A 10 mg vial with 2 mL of diluent gives 5 mg/mL. Once a vial of bacteriostatic water has been punctured, the widely adopted laboratory convention is to discard it after 28 days.
The arithmetic for converting a vial size and diluent volume into mg/mL and syringe units is worked through in peptide reconstitution math.
- Composition: sterile water plus 0.9% benzyl alcohol, pH about 5.7 (range 4.5–7.0)
- Best for: multi-use peptide stock solutions held at 2–8 °C
- Not suitable for: live-cell culture work, where benzyl alcohol can be cytotoxic
- Fast math: use the V8 reconstitution calculator or the tables below
Almost every research peptide ships as a freeze-dried cake or powder, and almost every protocol that uses one begins with the same question: what do I dissolve it in? For bench work that draws from a single vial over several days or weeks, the answer is usually bacteriostatic water. This guide explains what the product is, how the preservative works, how it compares with plain sterile water and saline, and gives worked concentration tables for common vial sizes. For laboratory research use only; not for human consumption.
What is bacteriostatic water?
Bacteriostatic water (often shortened to BAC water) is a sterile, non-pyrogenic preparation of highly purified water to which a small, fixed amount of benzyl alcohol has been added. The pharmacopeial formulation is simple, and it has not changed in decades.
| Component | Amount | Function |
|---|---|---|
| Highly purified, sterile, non-pyrogenic water | About 99.1% | Solvent |
| Benzyl alcohol | 0.9% w/v (9 mg per mL) | Antimicrobial preservative |
| pH | About 5.7 (labelled range 4.5–7.0) | Mildly acidic, unbuffered |
| Buffers, salts, tonicity agents | None | — |
Because it carries no salts and no buffer, bacteriostatic water contributes almost nothing to the final solution except water and preservative. That matters for analytical work: the peptide's own counter-ions (commonly acetate or trifluoroacetate left over from purification) and any bulking excipient in the vial will set the final pH far more than the diluent does.
What "bacteriostatic" actually means
The word describes a mode of action, not a guarantee of sterility. A bacteriostatic agent holds microbial populations in check by stopping replication; a bactericidal agent kills outright. At 0.9%, benzyl alcohol disrupts microbial membranes enough to stop the small number of organisms that might enter through a punctured stopper from multiplying into a meaningful contamination. It does not sterilize a grossly contaminated solution, and it does not replace aseptic technique.
The performance standard behind that claim is USP General Chapter <51>, Antimicrobial Effectiveness Testing. In that test a preserved product is deliberately challenged with five reference organisms (Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans and Aspergillus brasiliensis) and sampled over 28 days. For this product category, bacterial counts must fall by at least 1 log by day 7 and 3 logs by day 14, with no rebound at day 28, while yeast and mold must show no increase. The 28-day duration of that challenge test is the origin of the familiar 28-day in-use period.
Why labs use bacteriostatic water for peptides
The multi-use problem
A lyophilized peptide vial rarely gets used in one sitting. A typical bench workflow reconstitutes the vial once and then withdraws small volumes across many sessions. Every withdrawal pushes a needle through the stopper, and every puncture is an opportunity for airborne or surface organisms to reach the liquid. Peptide solutions are nutrient-poor, but they are not hostile: without a preservative, a few colony-forming units can become a turbid vial within days at refrigerator temperature.
Benzyl alcohol addresses exactly this risk. It allows a stock solution to be entered repeatedly over a defined window while microbial growth stays suppressed. Unpreserved diluents offer no such margin and should be considered single-use.
In-use window by diluent
| Diluent | Preservative | Typical in-use window at 2–8 °C | Repeated vial entry? |
|---|---|---|---|
| Bacteriostatic water (0.9% benzyl alcohol) | Yes | Up to 28 days (peptide-dependent) | Yes |
| Sterile water (unpreserved) | No | Same day; discard remainder | No |
| Sterile 0.9% sodium chloride (unpreserved) | No | Same day; discard remainder | No |
| Bacteriostatic 0.9% sodium chloride | Yes | Up to 28 days | Yes, but salt can reduce solubility of some peptides |
| Distilled, deionized or tap water | No | Not appropriate | No — not sterile, endotoxin uncontrolled |
Two caveats keep this table honest. First, the 28-day figure is a microbiological limit on the container, not a promise about the chemical stability of any specific peptide; some sequences oxidize, deamidate or aggregate faster than that. Second, the window only applies when the vial is stored cold and entered with a fresh sterile needle each time.
Peptide compatibility
Most short and mid-length synthetic peptides dissolve readily in bacteriostatic water and tolerate 0.9% benzyl alcohol well. Exceptions exist. Benzyl alcohol is mildly hydrophobic and has been shown to promote aggregation in some larger proteins during storage, so high-molecular-weight or aggregation-prone material deserves a quick literature check before a long hold. Strongly hydrophobic sequences may need a small amount of dilute acetic acid or another co-solvent to wet the powder first. And the preservative that protects the vial is toxic to cultured cells at working concentrations, so cell-based assays normally use sterile water, PBS or culture medium instead, prepared fresh.
Bacteriostatic water vs sterile water
| Feature | Bacteriostatic water | Sterile water (unpreserved) |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Container type | Multi-use vial | Single-use vial or ampoule |
| Microbial growth after puncture | Suppressed | Unchecked |
| In-use period after first entry | 28 days by convention | Discard after one session |
| Typical pH | About 5.7 | About 5.5 (5.0–7.0) |
| Suitable for cell culture | No | Yes |
| Suitable for multi-week peptide stock | Yes | No |
| Interference in UV assays near 254–260 nm | Possible (aromatic ring absorbs) | None |
The practical rule: if the vial will be used once and discarded, either diluent works and sterile water is the cleaner analytical choice. If the vial will be entered again tomorrow, next week or next month, use a preserved diluent.
Bacteriostatic water vs normal saline
Saline (0.9% sodium chloride) is isotonic, which is why it is common in in-vivo model work. As a peptide diluent it has two drawbacks. Unpreserved saline shares the single-use limitation of sterile water. And dissolved salt raises ionic strength, which can lower the solubility of some peptides, encourage gel formation in lipidated sequences, and complicate downstream mass spectrometry. Bacteriostatic saline exists and is preserved, but unless a protocol specifically calls for an isotonic stock, plain bacteriostatic water is the simpler default.
How much bacteriostatic water to add: worked math
There is no single "correct" volume. The diluent volume is a choice that sets the concentration, and the best choice is the one that makes later pipetting or syringe measurement easy and accurate.
- Concentration (mg/mL) = peptide mass in vial (mg) ÷ diluent volume (mL)
- Diluent volume (mL) = peptide mass (mg) ÷ target concentration (mg/mL)
- Volume to withdraw (mL) = mass needed for the aliquot (mg) ÷ concentration (mg/mL)
Concentration by vial size and diluent volume
| Peptide in vial | 1 mL added | 2 mL added | 3 mL added | 5 mL added |
|---|---|---|---|---|
| 2 mg | 2 mg/mL | 1 mg/mL | 0.67 mg/mL | 0.4 mg/mL |
| 5 mg | 5 mg/mL | 2.5 mg/mL | 1.67 mg/mL | 1 mg/mL |
| 10 mg | 10 mg/mL | 5 mg/mL | 3.33 mg/mL | 2 mg/mL |
| 15 mg | 15 mg/mL | 7.5 mg/mL | 5 mg/mL | 3 mg/mL |
| 20 mg | 20 mg/mL | 10 mg/mL | 6.67 mg/mL | 4 mg/mL |
| 30 mg | 30 mg/mL | 15 mg/mL | 10 mg/mL | 6 mg/mL |
Reading the result on a 1 mL (U-100) lab syringe
Many benches measure small volumes with 1 mL syringes graduated in 100 units, where 1 unit equals 0.01 mL (10 µL). The table converts concentration into the peptide mass contained in each graduation.
| Concentration | Mass per 1 unit (0.01 mL) | Mass per 10 units (0.1 mL) | Mass per 50 units (0.5 mL) |
|---|---|---|---|
| 1 mg/mL | 10 µg | 100 µg | 500 µg |
| 2 mg/mL | 20 µg | 200 µg | 1 mg |
| 2.5 mg/mL | 25 µg | 250 µg | 1.25 mg |
| 5 mg/mL | 50 µg | 500 µg | 2.5 mg |
| 10 mg/mL | 100 µg | 1 mg | 5 mg |
Worked example. A 10 mg vial is reconstituted with 2 mL of bacteriostatic water. Concentration = 10 ÷ 2 = 5 mg/mL. An assay needs 250 µg (0.25 mg) per well. Volume = 0.25 ÷ 5 = 0.05 mL, which is 50 µL or 5 units on a U-100 scale. The vial holds 2 mL ÷ 0.05 mL = 40 such aliquots, before allowing for dead volume in the needle hub.
If you would rather not do this by hand, the reconstitution calculator performs the same arithmetic for any vial size. Peptide-specific walkthroughs are available for retatrutide, tirzepatide and BPC-157.
How to reconstitute a peptide with bacteriostatic water
Equipment
- Lyophilized peptide vial and its batch certificate of analysis
- Bacteriostatic water, within its expiry date and inside its 28-day in-use window
- Sterile syringe and needle (or calibrated pipette with sterile tips for open-vial work in a hood)
- 70% isopropyl alcohol swabs, nitrile gloves, a clean work surface or laminar-flow hood
- Labels and a marker; a sharps container
Step-by-step protocol
- Equilibrate. Let the peptide vial and the diluent stand at room temperature for 15–20 minutes. Opening a cold vial pulls condensation onto a hygroscopic powder.
- Decide the volume first. Pick a target concentration from the tables above and write it down before touching a syringe.
- Disinfect both stoppers. Swab each rubber stopper with 70% isopropyl alcohol and let it air-dry for about 30 seconds.
- Draw the diluent. Withdraw the calculated volume of bacteriostatic water with a sterile syringe.
- Add it slowly down the glass. Angle the needle so the liquid runs down the inside wall of the peptide vial rather than striking the cake. Many vials are sealed under partial vacuum and will pull the liquid in; let that happen gradually.
- Swirl, do not shake. Roll or gently swirl the vial. Vigorous shaking and vortexing introduce air–liquid interface stress and foam, both of which promote aggregation.
- Wait and inspect. Most peptides clear within one to five minutes. The solution should be transparent and free of particles.
- Label and refrigerate. Record the peptide, lot, concentration in mg/mL, diluent, date and initials, then store at 2–8 °C away from light.
A fuller walkthrough with troubleshooting is in how to reconstitute a lyophilized peptide and how to mix peptides with bacteriostatic water.
Storage and handling of bacteriostatic water
Unopened vials
Sealed bacteriostatic water is labelled for controlled room temperature, 20–25 °C (68–77 °F), with brief excursions between 15 and 30 °C permitted under USP storage definitions. Keep it in its carton, out of direct sunlight, and use it before the printed expiry date. It does not need refrigeration while sealed.
Opened vials
From the first puncture, write the date on the label. Sterile-compounding standards (USP <797>) and infection-control guidance for multiple-entry containers converge on the same limit: discard 28 days after first entry unless the manufacturer states otherwise. Storing an opened vial at room temperature is consistent with its labelling; many labs refrigerate opened vials anyway to keep all in-use reagents in one controlled location.
Storage reference table
| Item | Temperature | Light | Use period | Freeze? |
|---|---|---|---|---|
| Unopened bacteriostatic water | 20–25 °C | Keep in carton | To printed expiry | No |
| Opened bacteriostatic water | 20–25 °C or 2–8 °C | Protect from light | 28 days from first puncture | No |
| Lyophilized peptide, sealed | −20 °C long term; 2–8 °C short term | Dark, dry | Per COA / retest date | Yes (preferred) |
| Peptide reconstituted in bacteriostatic water | 2–8 °C | Dark | Up to 28 days, peptide-dependent | Avoid repeated freeze–thaw |
More detail on expiry dates, refrigeration and freezing is in our companion article, does bacteriostatic water expire?, and general peptide guidance is in storing research peptides.
What to check when sourcing bacteriostatic water
- Stated preservative level. The label should read 0.9% benzyl alcohol (9 mg/mL). Other concentrations are not the standard product.
- Water grade and sterility. Look for pharmacopeial-grade sterile water, terminal sterilization or sterile filtration, and an endotoxin specification.
- Container and closure. A multi-use vial with a self-sealing stopper and an intact flip-off cap. Ten-millilitre and thirty-millilitre fills are the most common; smaller vials are easier to finish inside 28 days.
- Lot number and expiry date. Both should be printed on the vial, not only on the outer packaging.
- Documentation. A lot-specific certificate of analysis on request.
V8 Peptides supplies bacteriostatic water in 10 mL multi-use vials for laboratory reconstitution work.
How to read a bacteriostatic water COA
| COA line | What it tells you | What to expect |
|---|---|---|
| Benzyl alcohol assay | Preservative content | Close to 0.9% w/v (pharmacopeial limits apply) |
| pH | Acidity of the unbuffered solution | 4.5–7.0 |
| Sterility (USP <71>) | Absence of viable organisms at release | Pass / no growth |
| Bacterial endotoxins (USP <85>) | Pyrogen burden | Below the stated EU/mL limit |
| Particulate matter (USP <788>) | Sub-visible particles | Within limits |
| Appearance | Visual check | Clear, colorless, particle-free |
| Lot number and expiry | Traceability | Must match the vial in hand |
The same reading skills carry over to peptide documentation; see how to read a COA and our batch COA library.
Quick reference summary
| Question | Short answer |
|---|---|
| What is it? | Sterile water with 0.9% benzyl alcohol |
| Why use it for peptides? | Suppresses microbial growth in vials entered repeatedly |
| How much do I add? | Peptide mg ÷ target mg/mL = mL of diluent |
| How long after opening? | 28 days from first puncture |
| Where do I store it? | Sealed: room temperature. Reconstituted peptide: 2–8 °C |
| When not to use it? | Cell culture, single-use analytical preps, benzyl-alcohol-sensitive assays |
References
- United States Pharmacopeia. General Chapter <51> Antimicrobial Effectiveness Testing. USP–NF.
- United States Pharmacopeia. General Chapter <797> Pharmaceutical Compounding—Sterile Preparations. USP–NF.
- United States Pharmacopeia. Monograph: Bacteriostatic Water for Injection. USP–NF.
- United States Pharmacopeia. General Chapter <659> Packaging and Storage Requirements. USP–NF.
- Hospira, Inc. (Pfizer). Bacteriostatic Water for Injection, USP — package insert. DailyMed, U.S. National Library of Medicine.
- Centers for Disease Control and Prevention. Injection Safety: Questions about Multi-dose Vials.
- Meyer BK, Ni A, Hu B, Shi L. Antimicrobial preservative use in parenteral products: past and present. J Pharm Sci. 2007;96(12):3155–3167.
- Roy S, Jung R, Kerwin BA, Randolph TW, Carpenter JF. Effects of benzyl alcohol on aggregation of recombinant human interleukin-1-receptor antagonist in reconstituted lyophilized formulations. J Pharm Sci. 2005;94(2):382–396.
- Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharm Res. 2010;27(4):544–575.
Frequently asked questions
What is bacteriostatic water used for in peptide research?
It is the standard diluent for turning a lyophilized peptide into a liquid stock solution when the vial will be entered more than once. The 0.9% benzyl alcohol preservative suppresses microbial growth between uses.
How much bacteriostatic water do I add to a peptide vial?
Divide the peptide mass by the concentration you want. A 5 mg vial with 2 mL gives 2.5 mg/mL; a 10 mg vial with 2 mL gives 5 mg/mL; a 30 mg vial with 3 mL gives 10 mg/mL. The calculator handles any combination.
Is bacteriostatic water the same as sterile water?
No. Both are sterile at release, but only bacteriostatic water contains a preservative. Unpreserved sterile water is a single-use diluent and any remainder should be discarded after the session.
Does bacteriostatic water need to be refrigerated?
Sealed vials are labelled for controlled room temperature (20–25 °C). Refrigeration after opening is optional for the diluent itself but required for the peptide solutions you make with it.
How long does bacteriostatic water last after opening?
The standard laboratory limit is 28 days from the first puncture, which mirrors the 28-day duration of the USP <51> preservative challenge test and the multiple-entry container limit in USP <797>.
Can I use tap, distilled or deionized water instead?
No. Those waters are not sterile, are not controlled for endotoxin, and contain no preservative. They introduce contamination and uncontrolled variables into the experiment.
What is the difference between bacteriostatic water and bacteriostatic saline?
Bacteriostatic saline adds 0.9% sodium chloride for isotonicity. The added ionic strength can reduce the solubility of some peptides, so plain bacteriostatic water is the usual default for stock solutions.
Does benzyl alcohol interfere with assays?
It can. Benzyl alcohol absorbs UV light around 254–260 nm, elutes as its own peak in reversed-phase HPLC, and is cytotoxic in cell culture. For those applications, prepare the peptide fresh in unpreserved sterile water or an appropriate buffer.
My reconstituted peptide looks cloudy. Is the bacteriostatic water at fault?
Usually not. Cloudiness more often reflects a peptide that has not finished dissolving, a concentration above its solubility limit, aggregation from shaking, or microbial contamination in an old vial. Check the diluent vial itself against a dark background; if the diluent is clear, the cause lies with the peptide or technique.
Can I reuse a bacteriostatic water vial for several peptides?
Yes, within its 28-day in-use window, provided each entry uses a new sterile needle and syringe and the stopper is swabbed with 70% isopropyl alcohol each time. Never return unused liquid to the vial.