Bacteriostatic water: what it is used for, where to buy it and at what price
Bacteriostatic water (“BAC water” in English) is the reference solvent for reconstituting a lyophilized peptide in the laboratory. Here is what it contains, what distinguishes it from sterile water, whether it can be found in pharmacies, how to store it, and how much it costs.
Bacteriostatic water 3 ml – €5Reconstitution Pack – €14.90Peptide packs + kit
What is bacteriostatic water?
This is sterile water for injection to which 0.9% benzyl alcohol has been added. This alcohol is a bacteriostatic agent: it does not sterilize, but it prevents bacteria from multiplying if they enter the vial during a sample collection. This is what allows the same vial to be used multiple times, whereas sterile water without preservatives must be used only once.
It is found under the names bacteriostatic water, BAC water or BWFI ( Bacteriostatic Water for Injection).
Bacteriostatic water, sterile water, physiological saline: the differences
| Liquid | Composition | After opening | To reconstitute a peptide |
|---|---|---|---|
| Bacteriostatic water | Sterile water + 0.9% benzyl alcohol | Several samples, approximately 28 days | Reference solvent |
| Water for injection (WFI) | Sterile water, preservative-free | Single use only, discard after opening | It's possible, but the solution needs to be used quickly. |
| Physiological saline | Sterile water + 0.9% sodium chloride | Single use | Contains salt: avoid without compatibility data |
| Distilled, demineralized or tap water | Non-sterile | — | Avoid: risk of contamination |
To perform the calculation using your bottle, and convert it to syringe graduations:reconstitution calculator.
Alternatives to bacteriostatic water, and when to use them
- Dilute acetic acid (approximately 0.6%, or 0.1 M), sterileThis is for peptides that dissolve poorly in neutral water or precipitate at neutral pH, often basic peptides. It slightly lowers the pH and improves their solubility.
- PBS (phosphate saline buffer): used in certain cell culture protocols. Preservative-free, the solution must be used quickly.
- low concentration DMSO(generally 10% or less in sterile water): reserved for highly hydrophobic peptides, rarely necessary. It is itself biologically active and can influence the results of an experiment.
For the vast majority of research peptides (retatrutide, GHK-Cu, NAD+, tesamorelin…), bacteriostatic water remains the reference solvent.
Is bacteriostatic water available in pharmacies?
Rarely. In France, pharmacies mainly offer water for injection (preservative-free ampoules or vials, for single use) and physiological saline solution. Bacteriostatic water with added benzyl alcohol is not a common product there: if you are looking for it, ask your pharmacist, but it is most often purchased online from suppliers of laboratory equipment and research peptides.
What to check before buying: the label0.9% benzyl alcohola bottlesealedwith a pierceable cap (septum), a legible expiry date and a volume suitable for your use.
What is the use of bacteriostatic water in the laboratory?
The research peptides are delivered as lyophilized powder, which is more stable for transport and storage. To handle them, they are reconstituted. A known volume of bacteriostatic water is added to the vial, which fixes the concentration of the solution (for example, 2 ml in a 10 mg vial gives 5 mg/ml).
Best practices: Disinfect the stopper of both bottles with a swab, slowly pour water against the side of the bottle, then gently swirl until completely dissolved, without shaking. Our retatrutide and retatrutide guide pages detail the storage of the reconstituted peptide.
Water volume and concentration obtained
The volume of bacteriostatic water added does not change the amount of peptide in the bottle: it only fixes the concentration of the solution, according to the formula concentration (mg/ml) = mass of peptide (mg) ÷ volume of water (ml).
| Peptide in the bottle | Water added | Concentration |
|---|---|---|
| 5 mg | 1 ml | 5 mg/ml |
| 5 mg | 2 ml | 2.5 mg/ml |
| 10 mg | 2 ml | 5 mg/ml |
| 10 mg | 3 ml | 3.33 mg/ml |
| 20 mg | 2 ml | 10 mg/ml |
| 20 mg | 3 ml | 6.67 mg/ml |
For the rest of the reconstitution (stability, preservation of the peptide in solution), see our guide to reconstituting a lyophilized peptide.
How to store bacteriostatic water?
- Bottle closed: at room temperature, away from light, until the expiry date.
- Opened bottle: generally28 daysAfter the first sample is taken, note the opening date on the bottle.
- Do not freeze, do not use cloudy water or water that contains particles.
Which volume should I choose: 3 ml or 10 ml?
A 3 ml bottle is generally sufficient to reconstitute a peptide vial, with some room for handling. A 10 ml bottle can be used for several closely spaced reconstitutions, but the opened water will keep for about 28 days; after that, the remainder is wasted. For one or two peptide vials per month, the smaller size prevents waste.
Price of bacteriostatic water
Among online peptide sellers, a single bottle of bacteriostatic water costsapproximately €10 to €13(3 to 10 ml), excluding shipping costs (as of October 24, 2026). When ordered alone, it therefore often incurs shipping costs almost as high as its price.At MyPeptide, the 3ml bottle costs €5 : bacteriostatic water 3 ml, to be added to a peptide order for a single delivery package.
1 bottle of 3 ml bacteriostatic water (0.9% benzyl alcohol), 1 reconstitution syringe, 10 graduated 31G 6 mm syringes, 2 pairs of nitrile gloves, and disinfectant swabs. Add it to a peptide order: a single €10 shipping fee for the entire order, shipped from the European Union.
See the Reconstitution PackRetatrutide 10 mg + kit (Pack R10) – €69.90
For a peptide and its material in one go, the packs already include bacteriostatic water: Pack R10 (€69.90), Pack R20 (€109.90), GLOW1 and GLOW2.
Frequently Asked Questions
Where can I buy bacteriostatic water?
In France, it is mainly purchased online from suppliers of laboratory equipment and research peptides; pharmacies tend to offer preservative-free water for injection. At MyPeptide, it is sold individually (3 ml bottle for €5) or in the Reconstitution Pack (3 ml bottle, syringes, gloves, and swabs) for €14.90, shipped from the European Union.
What is the difference between bacteriostatic water and sterile water?
Both are sterile. The bacteriostatic water also contains 0.9% benzyl alcohol, which inhibits bacterial growth: the bottle can be punctured several times and can be stored for approximately 28 days after opening. The sterile water for injection, which is preservative-free, is for single use only.
How long does bacteriostatic water keep after opening?
Generally, it should be stored for 28 days after the first sample is taken, away from light. Once sealed, it can be kept until the date indicated on the bottle, at room temperature.
Can bacteriostatic water be replaced with tap water or distilled water?
No. Neither tap water nor distilled or demineralized water is sterile: they can contaminate the solution. To reconstitute a lyophilized peptide, sterile water, preferably bacteriostatic, is used.
How much bacteriostatic water is needed for one vial of peptide?
The volume of water does not change the amount of peptide, only its concentration: 1 ml in a 10 mg vial gives 10 mg/ml, 2 ml gives 5 mg/ml. A 3 ml vial is generally sufficient to reconstitute a vial of peptide, and limits waste since the opened water can be kept for approximately 28 days.
Material intended for laboratory and research use only.