Sources last read
Supplies · the mixing water
What is bacteriostatic water?
Bacteriostatic water is sterile water with a preservative added. The preservative is benzyl alcohol, at 0.9% or 1.1%. That one ingredient is the whole difference, and it is what lets a needle go into the same vial more than once.
Source: Hospira label for Bacteriostatic Water for Injection, USP, read 2026-08-13
What matters first
It is a prescription drug in its own right, sold in a 30 mL multiple-dose vial, and it is used to dissolve or dilute another drug. It is never the drug itself.
What does the benzyl alcohol actually do?
It holds bacteria in check inside the vial. That is what “bacteriostatic” means: it stops them multiplying rather than killing them outright. Plain sterile water has nothing in it to do that job, so its vial is built for one use and thrown away.
“a sterile, nonpyrogenic preparation of water for injection containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) of benzyl alcohol added as a bacteriostatic preservative. It is supplied in a multiple-dose container from which repeated withdrawals may be made”
Two phrases in there are doing the work. “Multiple-dose container” is the design intent. “Repeated withdrawals may be made” is the permission. Neither phrase appears on the sterile water label, and that is not an oversight.
Who must not use it
Newborns. The label carries a boxed warning in capitals, and it is short: NOT FOR USE IN NEONATES. Benzyl alcohol has been linked to harm in newborns, and the label sends you to preservative-free sterile water instead.
“Benzyl alcohol, a preservative in Bacteriostatic Water for Injection, USP has been associated with toxicity in neonates. Data are unavailable on the toxicity of other preservatives in this age group. Where water is required for preparing or diluting medications for use in neonates, only preservative-free Sterile Water for Injection should be used.”
There are two more bars on the same label that get quoted far less often:
If you react to benzyl alcohol
What the label says it is for
One job: dissolving or diluting another drug, following that drug’s own instructions. The label points somewhere else for the detail, and where those instructions do not exist, the label has nothing to say.
“This parenteral preparation is indicated only for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection, according to instructions of the manufacturer of the drug to be administered.”
That last clause is the honest limit on this page and on every page like it. Mixing a research peptide is not a use this label covers, because no manufacturer has published the instructions it defers to. Any site that tells you otherwise is filling in a gap the label left open.
Does bacteriostatic water need to be refrigerated?
No. The label asks for 20 to 25°C (68 to 77°F), which USP calls controlled room temperature. A fridge is not part of it, before opening or after. Freezing is not on the label either way.
The vial carries a printed expiry date, and that date is the one the maker stands behind for an unopened vial. Once you have put a needle through the stopper, the label sets no new clock at all. It never gives a day count.
That silence is worth knowing about, because plenty of pages state a figure as though the label gave one. Where a day count does exist, it comes from the drug you dissolved, not from the water. Zepbound’s multi-dose vial is a good example: its own label sets 30 days from first use, or four weekly doses, whichever lands first.
How much bacteriostatic water do you mix in?
Whatever makes the dose land on a mark you can read. There is no single right answer, because the water volume sets the strength and the strength sets where the plunger stops.
Take a 10 mg vial. Put in 2 mL of water and it holds 5.0 mg/mL. On a 100-unit barrel, one mark is then 50 mcg, so a 2.5 mg dose is 50 units — a whole number, easy to see. Change the water and every one of those numbers moves.
That is arithmetic, not advice, and it is why the calculator exists. It takes your vial, your water and your dose, and draws the answer on the barrel you picked.
Buying a vial
A pharmacy fills it against a prescription. Every product in the federal drug listing is filed that way, and every carton is printed Rx only. Brand is not the thing to shop on. Five of the six listed waters share one approval, so they are the same water with different names on the box.
We also read the open shopping shelf people use instead. On 2026-08-12 we went through 100 listings offered under this name. Only 20 used the word at all, and 14 of those were humidifier treatments. The rest were vial storage cases, a wound dressing, an eyewash additive and a surgical lubricant.
The titles at the top of that shelf read “30ML Bottles - 2 Pack”, “Deionized Water” and “Filtered Water Solution, Research Use Only”. None of them names the drug. Its best sellers were alcohol wipes, eye drops, nitrile gloves, bottled water and a septic tank treatment.
Not one of the 6 companies in the drug listing was on that shelf. So the shelf is other goods wearing this search term, and we are not going to point you at it.
Where to buy bacteriostatic water, and what its prescription status means
The rest of this section
- Bacteriostatic Water for Injection, USP — Hospira, Inc. (DailyMed SPL)retrieved 2026-08-13FDA label