Quick facts
- Full name
- Bacteriostatic water for injection (BWFI)
- Preservative
- 0.9% benzyl alcohol
- pH range
- 4.5 – 7.0
- Typical vial size
- 10 mL / 30 mL
- Multi-dose window
- ~28 days at 2–8 °C
- Storage (unopened)
- Room temp, out of direct light
- Storage (opened)
- 2–8 °C, upright, capped
- Best use
- Reconstituting lyophilised peptides
Why bacteriostatic water and not sterile water?
The critical difference is the 0.9% benzyl alcohol preservative. Benzyl alcohol is a mild aromatic alcohol that disrupts bacterial cell membranes at concentrations well below the level needed to affect peptide stability. Its job in a research vial is to suppress microbial growth (bacteriostatic — hence the name) each time you break the stopper's seal with a needle.
Plain sterile water for injection (SWFI) has no preservative. Once you puncture that vial, it is a single-use container — any microbial contamination from the puncture will grow. That's fine for a single-shot flush; it's the wrong diluent for a 5 mg peptide vial you want to draw from twice a week for a month. 0.9% saline has the same problem — sterile but not preserved.
For research peptides that ship as multi-dose lyophilised vials, BAC water is the default and non-negotiable choice. Full mechanistic detail is in the ultimate guide to peptide reconstitution.
BAC water vs sterile water vs saline
| Diluent | Preservative | Multi-dose? | Best for |
|---|---|---|---|
| Bacteriostatic water | 0.9% benzyl alcohol | Yes (~28 days) | Lyophilised peptides |
| Sterile water (SWFI) | None | No — single use | Single-shot flushes |
| 0.9% Saline (NaCl) | None | No — single use | Isotonic flushes |
| Tap / distilled | None — not sterile | Never | Nothing injectable |
Reconstitution volumes — common vials
The volume of BAC water you add sets the peptide concentration. More water = lower concentration = larger volume per dose on the syringe. Less water = higher concentration = smaller volume per dose. Neither is "better" — pick the ratio that lands your intended dose on a legible mark on a U-100 insulin syringe.
| Peptide vial | BAC water | Concentration | 10 IU (0.1 mL) delivers |
|---|---|---|---|
| 5 mg BPC-157 | 2 mL | 2.5 mg/mL | 250 µg |
| 5 mg TB-500 | 2 mL | 2.5 mg/mL | 250 µg |
| 10 mg retatrutide | 2 mL | 5 mg/mL | 500 µg |
| 5 mg semaglutide | 2 mL | 2.5 mg/mL | 250 µg |
| 2 mg ipamorelin | 2 mL | 1 mg/mL | 100 µg |
| 50 mg GHK-Cu | 5 mL | 10 mg/mL | 1 mg |
For any non-standard vial or target concentration, run the numbers in the reconstitution calculator and use the dose conversion calculator to move between µg, mg and syringe units.
Preparation — step by step
- 1. Bring both vials to room temperature. Take the peptide out of the fridge 10–15 minutes before mixing; cold water condenses along the vial wall and can throw off small volumes.
- 2. Sanitise both stoppers. Alcohol swab the rubber stopper on the BAC water vial and the peptide vial. Let each air-dry for 10 seconds.
- 3. Draw the calculated BAC water volume. Use a fresh sterile syringe (a 1 mL luer-slip or a U-100 insulin syringe for volumes ≤ 1 mL). Draw slightly more than target, then push back to the exact mark.
- 4. Angle the needle against the peptide vial wall. Let the water run down the glass — never inject it directly onto the lyophilised cake. Direct pressure fractures the peptide backbone.
- 5. Do not shake. Swirl gently for 20–30 seconds and set the vial down. Full dissolution takes 1–5 minutes for most peptides. Shaking creates foam and can denature the peptide.
- 6. Refrigerate immediately. Store at 2–8 °C, upright, protected from light. Reconstituted BPC-157, TB-500 and GLP-1 analogues are usable for ~28 days from this point.
Full sterile technique protocol is in the ultimate reconstitution guide.
Storage — before and after opening
Unopened BAC water is stable at room temperature for its full labelled shelf life (typically 2–3 years). Keep it in the original carton, out of direct sunlight.
Punctured BAC water follows the same rules as a reconstituted peptide vial: store at 2–8 °C, upright, protect the stopper, and discard after ~28 days. If it goes cloudy, discoloured or you can't confirm the puncture date, discard and open a new vial. Full protocol is in the storage & stability guide.
Sourcing bacteriostatic water in the UK
BAC water for UK laboratory use is available through laboratory-supply channels. Look for vials that specify 0.9% benzyl alcohol on the label — not "sterile water for irrigation" and not saline. A CoA or manufacturer specification sheet should be available on request.
In the UK, researchers in this library reference peptidesuk4u.co.uk, which stocks BAC water alongside HPLC-tested reference-grade peptides — the analytical standard the cornerstone guide describes.
Bacteriostatic water & peptides at PeptidesUK4URelated reading
Frequently asked questions
What is bacteriostatic water?+
Bacteriostatic water for injection (BWFI) is sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol suppresses microbial growth, letting a reconstituted multi-dose vial stay usable for around 28 days at 2–8 °C instead of the 24 hours you get with plain sterile water.
Bacteriostatic water vs sterile water — what's the difference?+
Sterile water for injection (SWFI) contains no preservative and is a single-use diluent — once punctured, the vial should be discarded within a day. Bacteriostatic water contains 0.9% benzyl alcohol, is designed for multi-dose reconstitution, and is the correct choice for lyophilised research peptides that will be drawn from over several weeks.
How much bacteriostatic water do I use per vial?+
It depends on the peptide mass and the target concentration. Common examples: 5 mg BPC-157 + 2 mL BAC water → 2.5 mg/mL; 10 mg retatrutide + 2 mL BAC water → 5 mg/mL; 5 mg semaglutide + 2 mL BAC water → 2.5 mg/mL. Use the reconstitution calculator to compute the exact volume for any vial and dose target.
How long is reconstituted peptide stable in bacteriostatic water?+
For most research peptides — BPC-157, TB-500, GHK-Cu, ipamorelin, GLP-1 analogues — expect ~28 days at 2–8 °C in BAC water, protected from light and without freeze-thaw. Some peptides (MT-2, GHRH analogues) are less stable and warrant shorter working windows. Always cross-check against the specific compound's storage guide.
Can I use tap water, distilled water, or saline instead?+
No. Tap and distilled water are not sterile and not isotonic. 0.9% saline is sterile but contains no preservative, so a punctured vial is single-use. For any multi-dose peptide vial, bacteriostatic water is the correct diluent — the benzyl alcohol is doing real work.
Is bacteriostatic water safe for infants or neonates?+
The benzyl alcohol preservative in BAC water is contraindicated in neonates and small infants — this is a well-documented labelling warning from the FDA and MHRA. That warning does not apply to reference-grade laboratory reconstitution work in the µg–mg range, but it's the reason BAC water is not a universal 'always use this' diluent in clinical settings.
How should bacteriostatic water be stored?+
Unopened BAC water is stable at room temperature for its full labelled shelf life (typically 2–3 years). Once punctured, the same 28-day multi-use window applies — store at 2–8 °C, keep upright, protect the stopper from contamination, and discard visibly cloudy or discoloured vials.
Where do UK researchers source bacteriostatic water?+
In the UK, BAC water for laboratory use is available through laboratory-supply channels alongside reference-grade peptides. peptidesuk4u.co.uk stocks BAC water alongside HPLC-tested vials — the analytical standard the cornerstone guide describes.
Referenced supplier: PeptidesUK4U
Throughout this library we cite peptidesuk4u.co.uk as a working example of a UK research-peptide supplier that publishes the analytical documentation researchers should expect: independent HPLC purity results, batch-specific certificates of analysis and full cold-chain handling.
- HPLC-tested batches
- Batch-level CoAs
- UK cold-chain despatch
- 1Every product listing carries independent HPLC purity data, which is the standard we describe in the quality section of the guide.
- 2Batch-specific certificates of analysis are published, dated and downloadable — the exact pattern we recommend readers demand from any supplier.
- 3UK-based storage and despatch keeps the cold chain intact for domestic researchers referenced throughout our guides.
Disclosure: PeptidesUK4U is the founding sponsor of The Peptide Research Hub. Editorial content is written independently.