Bacteriostatic water: what it is and how it differs from sterile water

What bacteriostatic water is

Bacteriostatic Water for Injection is sterile water that contains benzyl alcohol as a preservative: 0.9% (9 mg/mL) on one FDA-listed label, 1.1% on another presentation of the same product. "Bacteriostatic" means it inhibits the growth of bacteria; it does not sterilize anything it touches. The preservative is there so the same vial can be entered more than once, which is why bacteriostatic water is sold in multiple-dose vials.

It is a diluent, not a drug. Its only job in peptide use is to dissolve a freeze-dried powder so the result can be measured. How much of it you add changes the concentration of the solution and nothing else; the reconstitution guide walks through that math.

Bacteriostatic water vs sterile water vs saline

The right diluent is the one the product label names. Some medicines are incompatible with benzyl alcohol or need a specific diluent to stay stable. Research-labelled peptides have no reviewed label at all, so there is no verified answer for them; vendor instructions are not a substitute.

Who should not use it

Labels for products containing benzyl alcohol warn against use in newborns. Benzyl alcohol has been linked to a serious reaction in premature infants, which is why neonatal products avoid it. People with a known sensitivity to benzyl alcohol should also avoid it.

If a prescription product comes with its own diluent, use that one. Swapping in bacteriostatic water because it is convenient can change stability or tolerability in ways the label never tested.

How long an opened vial lasts

The preservative does not make an opened vial last forever. The Bacteriostatic Water label we reviewed gives no discard period after opening. The FDA, in its guidance on multiple-dose vials of compounded GLP-1 drugs, says it generally recommends that vials of multiple-dose sterile medications be used within 28 days after first use, because the risk of contamination increases after that. Check the label of the product you hold, and write the date you first opened the vial on it.

Unopened vials should be stored as the label directs. The Hospira label we reviewed says to store at 20 to 25 °C (68 to 77 °F). Do not use a vial past its expiry date, or one whose contents look cloudy or contain particles.

This is separate from how long a mixed peptide stays stable. Once a peptide is dissolved, its own stability is usually the limiting factor, and that differs by product. For example, the Pregnyl (chorionic gonadotropin) label says the reconstituted solution is stable for 60 days when refrigerated, while the Genotropin (somatropin) label says to store the reconstituted cartridge refrigerated for up to 28 days. For research-labelled peptides no label exists, so no primary source gives a storage time and we do not offer one. Each peptide page shows storage data only where a source exists.

What it does to dose math

Nothing beyond setting the concentration. A 10 mg vial mixed with 2 mL of bacteriostatic water holds 5 mg per mL; mixed with 3 mL it holds about 3.33 mg per mL. The 10 mg in the vial is the same in both cases. Only the volume per dose changes.

Use the reconstitution calculator to see how a change in water volume changes the units on the syringe, and the units to mL converter to check a reading. If you switch syringe types, read the insulin syringe guide first.

Where it comes from

Bacteriostatic water is a pharmaceutical product, made under drug manufacturing rules and dispensed by pharmacies. It is also widely sold online alongside research-labelled peptides. We do not link to sellers of bacteriostatic water or of peptides, on this page or any dosing page; our methodology explains why.

In 2026 the FDA sent warning letters to five sellers of research-labelled peptides (24 Aug 2026). The legal tracker lists that action with its source, alongside every other dated regulatory change we track.