Bacteriostatic water for peptide reconstitution: when to use it and how

Bacteriostatic water is the reagent most laboratories reach for when a lyophilised research peptide has to be reconstituted into a stock that will be sampled more than once. It is simple, but it is also frequently misunderstood: what the preservative does and does not do, when sterile water or buffer is the better choice, how long an opened vial can be used, and which peptides it should not be paired with. This guide covers all of that and how Alphex bacteriostatic water is supplied. All material is for laboratory and in-vitro research only.

What bacteriostatic water is

Bacteriostatic water is sterile water containing 0.9% w/v benzyl alcohol as a preservative, supplied in a multi-dose vial with a septum that can be punctured repeatedly. The benzyl alcohol is bacteriostatic, meaning it inhibits the growth of bacteria introduced during repeated sampling; it is not bactericidal in the sense of sterilising a contaminated solution, and it does nothing about fungal contamination or endotoxin already present. Its job is to keep a stock solution usable across many withdrawals over days to weeks, which plain sterile water cannot do once the seal is broken.

It is not the same as sterile water for reconstitution (no preservative, single use), saline (0.9% sodium chloride, with or without preservative) or buffered diluents. Each has a place; the sections below explain when to use which.

Specification

• Composition: water with 0.9% w/v benzyl alcohol
• CAS numbers: 7732-18-5 (water); 100-51-6 (benzyl alcohol)
• Form supplied: sterile aqueous solution, 10 mL multi-dose vial with puncturable septum
• Storage: 15-25 C, protect from light; discard 28 days after first puncture
• Synonyms: BAC water, bacteriostatic water for reconstitution

When to use bacteriostatic water, sterile water or buffer

• Bacteriostatic water: reconstituting a peptide stock that will be stored refrigerated and sampled repeatedly over up to a few weeks. This is the common case for bench work with research peptides such as BPC-157, TB-500, GHK-Cu and Ipamorelin.
• Sterile water (no preservative): single-use preparations, and any assay where benzyl alcohol would interfere: cell-culture work at concentrations where the preservative is cytotoxic, enzyme assays sensitive to alcohols, and analytical samples where the benzyl alcohol peak would co-elute or crowd the chromatogram at 220 nm (benzyl alcohol absorbs strongly there).
• Buffer: when pH control matters, which for peptides it usually does. Many peptides are most stable and soluble in a defined buffer, and larger or lipidated peptides such as GLP-3 (RT) often need mildly alkaline conditions to dissolve cleanly. Buffer can be prepared with bacteriostatic water if a preservative is wanted, but check compatibility.

How to reconstitute a peptide with bacteriostatic water

1. Let the sealed peptide vial and the water reach room temperature. Opening a cold vial pulls condensation onto the powder.
2. Swab both septa. Draw the required volume of bacteriostatic water into a sterile syringe.
3. Insert the needle through the peptide vial septum and let the water run slowly down the inside wall of the vial, not onto the powder directly.
4. Let the vial stand. Most short peptides dissolve within a minute or two without agitation; if needed, swirl gently. Do not shake or vortex: foaming denatures peptides and lipidated peptides can aggregate.
5. Check the solution is clear. Cloudiness or particulates indicate aggregation or a solubility problem; do not use a cloudy stock for quantitative work.
6. Label the vial with the peptide, concentration, date and batch number, and refrigerate at 2-8 C in the dark.

Concentration is set by the volume added: 1 mL into a 5 mg vial gives a nominal 5 mg/mL stock, remembering that nominal mass is gross vial content, not net peptide (see what HPLC purity actually tells you). Full handling guidance for each stage is in storage, reconstitution and shelf life.

Shelf life and the 28-day rule

An unopened vial of bacteriostatic water is stable for years at room temperature away from light. Once the septum has been punctured, the convention is to discard the vial 28 days later regardless of how much remains: each puncture risks introducing organisms and, over time, benzyl alcohol can partition into the rubber septum and its concentration falls. Write the first-puncture date on the vial. The peptide stock reconstituted with it has its own, usually shorter, useful life set by the peptide's stability, not by the water.

Compatibility notes

• Benzyl alcohol and cells. At 0.9% the preservative is cytotoxic to many cell lines; if reconstituted stock is diluted heavily into culture medium the final benzyl alcohol concentration is usually negligible, but calculate it rather than assuming.
• Benzyl alcohol and analysis. It absorbs at 220 nm and shows up on reverse-phase HPLC. If you are running purity or stability checks on your own stocks, prepare the analytical sample in water or mobile phase instead, or account for the peak.
• Copper peptides. GHK-Cu dissolves well in bacteriostatic water; avoid chelating buffers rather than the water itself.
• Lipidated peptides. Bacteriostatic water is acceptable for GLP-3 (RT), but add it very slowly and let the vial stand; a buffered vehicle may give better solubility.

Buying bacteriostatic water in the UK

Bacteriostatic water is sold widely and inconsistently: some products are unpreserved sterile water mislabelled, some are non-sterile, and some come without a lot number. Look for a stated 0.9% benzyl alcohol content, a sterile multi-dose presentation, and a lot number, and buy from a supplier that ships from UK stock so the vial arrives intact and in date. Alphex lists it alongside the peptides so that a laboratory can order the matched diluent in the same delivery.

Frequently asked questions

Can I use bacteriostatic water instead of sterile water for everything?

Not for cell-based or preservative-sensitive assays, and not for analytical samples where the benzyl alcohol peak matters. For refrigerated multi-sample peptide stocks it is the right choice.

Does bacteriostatic water sterilise a contaminated solution?

No. It inhibits bacterial growth introduced during use; it does not sterilise, and it does not remove endotoxin.

Why 28 days?

It is the accepted convention for multi-dose preserved diluents once opened, balancing preservative efficacy against cumulative contamination risk. Date the vial at first puncture.

Is it in stock?

Stock status is live on the product page; if sold out, the back-in-stock notification will email you when the next lot arrives.

Alphex bacteriostatic water is supplied as a laboratory reagent for the reconstitution of research peptides, for laboratory and in-vitro use only. It is not authorised for human or veterinary use.

Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.

Published 12 August 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.

Alphex Research Team

Alphex Limited, UK research peptide supplier

The Alphex research team writes and maintains these notes for UK laboratories buying research peptides. Every batch Alphex lists is verified by reverse-phase HPLC with a batch-matched Certificate of Analysis. Content covers laboratory and in-vitro research only.