The quick comparison

Water What it is Preservative? Best for
Bacteriostatic water Sterile water with 0.9% benzyl alcohol Yes Multi-dose vials - the standard for peptides
Sterile water for injection Sterile water, nothing added No Single use - mix and use immediately
Saline (0.9% sodium chloride) Sterile salt solution Plain: no Mostly IV dilution; not the usual peptide diluent
A vial of bacteriostatic water with an insulin syringe - the standard diluent for reconstituting peptides
A vial of bacteriostatic water with an insulin syringe - the standard diluent for reconstituting peptides.

Bacteriostatic water - the default

Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. That preservative is the whole point: once you puncture a vial's rubber stopper you create a possible route for bacteria, and the benzyl alcohol keeps the solution usable across repeated draws - around 28 days refrigerated. Because most peptides are dosed from one vial over days or weeks, bacteriostatic water suits them. It is what we stock and what the reconstitution guides on this site assume.

Sterile water - single use only

Sterile water for injection is just that: sterile water with nothing added. With no preservative, once it is opened or a vial is mixed with it, it must be used immediately and the rest discarded - there is nothing to stop bacterial growth on a second puncture. It has one clear place: when benzyl alcohol must be avoided. Benzyl alcohol is not suitable for newborns, and a small number of people are sensitive to it. In those cases sterile water is the alternative, but the vial is then single-dose.

Saline - not the usual choice

Saline is 0.9% sodium chloride - a sterile salt solution used widely for intravenous fluids and dilution. Plain saline has no preservative, so like sterile water it is single-use once opened. A preserved version (bacteriostatic saline) exists, but for reconstituting peptides bacteriostatic water remains the standard, and that is what most peptide instructions specify. Unless your prescriber or supplier tells you otherwise, reach for bacteriostatic water.

Bottom line

For a multi-dose peptide vial, use bacteriostatic water. Choose sterile water only if benzyl alcohol has to be avoided, and then use the vial as a single dose. Saline is not the usual peptide diluent. And never substitute any non-sterile water.

Supplies

Browse the full peptide supplies range.

Frequently asked questions

Can I use sterile water instead of bacteriostatic water?

You can, but only as a single dose - sterile water has no preservative, so a vial mixed with it must be used immediately and the remainder discarded. For a multi-dose vial, bacteriostatic water is the right choice.

Tip: a vial storage case keeps your vials upright, cushioned and organised - in the fridge or while travelling.

Is saline OK for reconstituting peptides?

It is not the usual choice. Plain saline has no preservative (single-use), and peptide instructions almost always specify bacteriostatic water. Use bacteriostatic water unless your prescriber tells you otherwise.

Why can't I just use tap or distilled water?

Because they are not sterile. Injecting a solution mixed with non-sterile water risks introducing bacteria. Always use a purpose-made sterile diluent.

Who should not use bacteriostatic water?

It is not suitable for newborns, and a small number of people are sensitive to benzyl alcohol. In those cases sterile water is used instead, as a single dose. If you have a known benzyl alcohol sensitivity, check with your prescriber.

This guide compares injection supplies and is not medical advice. Follow your prescriber's or supplier's instructions for your specific medication.