Bacteriostatic Water for Peptides: The Complete UK Reconstitution Guide

If you are working with peptides such as tirzepatide, retatrutide, semaglutide or BPC-157 in the UK, bacteriostatic water is the single most important consumable in your kit. It is the solvent used to turn freeze-dried (lyophilised) peptide powder into a stable solution, and the small amount of benzyl alcohol it contains keeps that solution usable for several weeks instead of a single day.

What is bacteriostatic water?

Bacteriostatic water (often abbreviated to BAC water) is sterile water that has been pre-mixed with 0.9% benzyl alcohol. The benzyl alcohol is a mild bacteriostatic preservative — it inhibits the growth of common bacteria after the vial has been pierced. Plain sterile water contains no preservative, so once it is opened it must be used immediately or discarded.

Why bacteriostatic water matters for peptides

Most modern peptides are supplied as lyophilised powder because they are unstable in solution. Once you add solvent the clock starts ticking. With sterile water that clock is roughly 24 hours; with bacteriostatic water you typically get 28 days of reliable, refrigerated shelf life. For multi-week dosing protocols this difference is decisive.

How to reconstitute a peptide vial

1. Allow the lyophilised vial to reach room temperature.
2. Wipe the rubber stoppers of both vials with an alcohol swab.
3. Draw your target volume of bacteriostatic water into a sterile syringe.
4. Insert the needle into the peptide vial at an angle and let the water run slowly down the inside of the glass. Do not blast the powder.
5. Gently swirl — never shake — until the powder is fully dissolved.
6. Label the vial with the reconstitution date and store in the fridge between 2–8°C.

Need to work out the volume? Use our peptide calculator to convert milligrams to insulin syringe units in seconds.

Storage after reconstitution

Refrigerate at 2–8°C. Avoid the door (temperature swings) and direct contact with the back wall (freezing risk). A solution that has been frozen and thawed should be discarded — peptide chains do not survive the ice crystals intact.

Common mistakes to avoid

• Shaking the vial. Peptides denature under mechanical stress. Always swirl.
• Using tap or bottled water. Only pharmaceutical-grade bacteriostatic water is suitable.
• Skipping the alcohol swab. The rubber stopper is the most common contamination point.
• Re-using needles. Use a fresh sterile syringe for every draw.

Frequently asked questions

Is bacteriostatic water legal in the UK?

Yes. Bacteriostatic water is freely available in the UK for laboratory and reconstitution purposes. Next Day Pharma does not sell bacteriostatic water — our products arrive as pre-filled pens.

How long does reconstituted peptide last?

Refrigerated at 2–8°C, most peptides reconstituted with bacteriostatic water remain stable for 28 days. Some shorter half-life peptides should be used within 14 days.

Can I use saline instead?

Sterile 0.9% saline can be used for some peptides but does not contain a preservative, so shelf life is dramatically shorter. Bacteriostatic water is the standard solvent.

What size bacteriostatic water vial should I buy?

A 10 ml or 30 ml multi-dose vial is the most cost-effective for ongoing reconstitution. See our BAC water product page.