Bacteriostatic water, sterile water, and saline are not interchangeable.

They are three different liquids and only one of them is built for a vial you intend to open more than once. Bacteriostatic water contains benzyl alcohol as a preservative, which is what lets you go back into the vial. Sterile water contains nothing, so it has no defence against whatever enters on the second needle. Sodium chloride is salt water, isotonic, and also unpreserved unless the label says bacteriostatic.

If you reconstitute a peptide you plan to dose over several weeks, that difference is the whole decision.

What is actually in each one

Why the preservative is the entire point

Benzyl alcohol is bacteriostatic, which means it stops bacteria multiplying. It is not sterilising and it does not undo contamination that is already there. What it does is give a multi-dose vial a defence against the small introductions that happen every time a needle goes through the stopper.

That is why the Bacteriostatic Water label describes a multiple-dose container from which repeated withdrawals may be made, and why the Sterile Water label does not. Sterile water is sterile in the vial and stays that way exactly until you open it.

A peptide reconstituted with sterile water and then kept for a four week course is a vial with no defence, entered ten or twenty times. That is the scenario the preservative exists for.

Sterile water carries a warning that surprises people

Sterile Water for Injection is hypotonic. Its label carries the warning in capitals: hypotonic and hemolytic, do not inject until made approximately isotonic by addition of appropriate solute. Injected on its own it can rupture red blood cells, and the labels note that hemoglobin induced renal failure has been reported following hemolysis.

In practice this warning is written for volume infusion, not for the fraction of a millilitre left after reconstituting a peptide, where the powder and the injection volume make tonicity a much smaller question. But it is the reason sterile water is labelled for drug diluent use only rather than for injection, and it is worth knowing that the warning is on the bottle rather than discovering it later.

Where saline fits

Sodium chloride 0.9% is already isotonic, so it does not carry the hemolysis warning. What it does not carry is a preservative. Plain saline in a multi-dose role has the same problem as sterile water: nothing is stopping growth between the first withdrawal and the last.

Saline is also not neutral chemistry. Some peptides are less stable in the presence of salt than in water, and a few reconstitution instructions specify one or the other for that reason. Where the manufacturer or the compounding pharmacy has specified a diluent, that instruction beats the general rule in this article.

The one absolute contraindication

There is a second population worth naming: people with a known benzyl alcohol sensitivity. It is uncommon, and it is a real reason someone would be told to use an unpreserved diluent and a single use vial instead.

What this means for how long your vial lasts

The diluent sets the ceiling, and then the peptide sets a lower one. Bacteriostatic water gives you a vial you can keep entering. It does not tell you how long the peptide inside it stays intact, which is a separate question about that specific molecule, its temperature, and its exposure to light.

The Bacteriostatic Water label puts the pH at 5.7, in a range of 4.5 to 7.0. That is mildly acidic, and it is one of the inputs into how a given peptide holds up. It is also the reason the honest answer to "how long does my vial last" is per compound rather than universal.

We hold storage and stability per compound in the app rather than quoting one number that would be wrong for most of them. Where the published stability data does not exist, the app says that instead of estimating.

The short version

Reconstituting something you will dose more than once: bacteriostatic water, unless you have been told otherwise. Single dose, or a benzyl alcohol sensitivity, or a neonate: unpreserved, and use the vial once. Where a manufacturer or pharmacy specified a diluent: use theirs, because they know something about that formulation that a general rule does not.

And whichever you use, the diluent is the smaller half of sterility. Technique is the larger half.