Bacteriostatic water vs sterile water: which one for your peptides?

Bacteriostatic water and sterile water look identical, but they behave differently once a vial or ampoule has been opened. Your choice decides how long a reconstituted research peptide stays usable and how many times a vial can be accessed. This guide explains the difference and when each one fits in the lab. All products mentioned are for laboratory research use only – not for human or veterinary use.

What is bacteriostatic water?

Bacteriostatic water – often shortened to bac water – is sterile water with 0.9% benzyl alcohol added as a preservative. Benzyl alcohol 0.9% inhibits the growth of bacteria that could enter the vial when the rubber stopper is punctured. That is what makes a vial of bacteriostatic water suitable for repeated access.

The preservative inhibits bacterial growth, but it does not replace clean working. Once first punctured, a vial is commonly discarded after 28 days.

  • Composition: sterile water + 0.9% benzyl alcohol
  • Container: multi-use vial with rubber stopper
  • Our size: 10 ml vial, sterile filtered

What is sterile water?

Sterile water is water without microorganisms and without any preservative. Because nothing inhibits bacterial growth once the container is open, sterile water is meant for single use: open, use what you need and discard the rest.

That is why sterile water is usually supplied in small, individually sealed ampoules. Ours come as 2 ml ampoules in a box of 10, so every preparation starts from a freshly opened ampoule.

Bacteriostatic water vs sterile water: the key differences

The water itself is the same; the difference is the preservative and everything that follows from it:

  • Preservative – bacteriostatic water contains 0.9% benzyl alcohol; sterile water contains none
  • Use – bacteriostatic water is multi-use, a vial can be punctured repeatedly; sterile water is single-use
  • Shelf life after opening – bacteriostatic water is commonly discarded 28 days after first puncture; sterile water right after use
  • Packaging – bacteriostatic water in a 10 ml multi-use vial; sterile water in 2 ml ampoules, 10 per box

Which one to use for reconstituting research peptides?

For most lab work, bacteriostatic water for peptides is the standard choice. A reconstituted peptide vial is often accessed several times over days or weeks, and every puncture of the stopper is a chance for contamination. The benzyl alcohol limits that risk between accesses.

Sterile water is the better fit when the whole solution is used at once, or when your protocol calls for a preservative-free solution. In that case, prepare only what you need and do not keep the remainder.

Whichever water you choose, keep reconstituted peptides refrigerated at 2–8 °C. Our guide to storing and reconstituting peptides covers the full procedure step by step.

Handling tips in the lab

A preservative is no substitute for good technique. These habits keep both types of water clean:

  • Clean the rubber stopper with an alcohol pad (70% isopropyl alcohol) before every puncture
  • Use a new sterile syringe each time you draw up liquid – never reuse one
  • Label the vial with the date of first puncture, so you know when the 28 days are up
  • Store unopened bacteriostatic and sterile water at room temperature, away from light
  • Discard an opened ampoule of sterile water after use, even if liquid remains

Quality and shipping

Our bacteriostatic water is sterile filtered and contains 0.9% benzyl alcohol in a 10 ml multi-use vial; our sterile water comes in individually sealed 2 ml ampoules, 10 per box. Looking to buy bacteriostatic water together with alcohol pads and syringes? Your order arrives in 1–2 working days in Belgium, the Netherlands and Germany, and in 2–5 working days in Italy and Türkiye.

All Virexa Labs products are intended exclusively for laboratory research use – not for human or veterinary use, and not for diagnostic or therapeutic purposes.

Water and supplies for reconstitution

Frequently asked questions

What is the difference between bacteriostatic water and sterile water?

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative, so a vial can be punctured repeatedly. Sterile water has no preservative and is single-use.

How long can an opened vial of bac water be used?

A vial of bacteriostatic water is commonly discarded 28 days after first puncture. Label it with the date so you can keep track.

Can I keep leftover sterile water from an ampoule?

No. Sterile water contains no preservative, so an opened ampoule is for single use: use what you need and discard the rest.

Which water is best for reconstituting peptides?

For a vial that will be accessed several times, bacteriostatic water is the usual choice. Use sterile water when the solution is used at once or when a preservative-free solution is needed.

Are these products intended for human use?

No. Our bacteriostatic water, sterile water and accessories are sold strictly for laboratory research and are not intended for human or veterinary use.