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Math helper, not medical advice. Confirm every dose with your prescriber or pharmacist.

Bacteriostatic water guide

Bac water: what it is and how long it lasts

Bacteriostatic water is the liquid most often used to mix powder vials. Here is what is in it, how it differs from sterile water, how to store it and when an opened vial should be thrown away, based on the product label and CDC guidance.

  • 0.9% benzyl alcohol
  • Room-temperature storage
  • 28 days once opened
  • Label sources
Open the mixing calculator
01

What is bacteriostatic water?

Bacteriostatic Water for Injection, USP is sterile water with a preservative, benzyl alcohol, added to stop bacteria from growing. The Hospira label lists 0.9% benzyl alcohol (9 mg/mL; 1.1% in one glass-vial fill). Because of the preservative, the vial is a multiple-dose container: it can be punctured more than once.

The label limits its use to diluting or dissolving drugs for injection into a vein, muscle or under the skin. It is not for fluid replacement, and it is not for epidural or spinal use.

Sources (checked 30 September 2026)

02

Bacteriostatic water vs sterile water

The two are easy to confuse, and they are not interchangeable in every case.

  • Bacteriostatic water: contains benzyl alcohol, multiple-dose vial.
  • Sterile Water for Injection: no preservative, antimicrobial or buffer; single-dose containers only, so leftover water is discarded.
  • Some products must be mixed with one and not the other. The product’s own instructions say which.

Sources (checked 30 September 2026)

03

How long is bac water good for after opening?

CDC guidance for multi-dose vials: once a vial is opened (punctured by a needle), date it and discard it within 28 days, unless the manufacturer states a different date for that opened vial. It should never be used past the expiry date printed on it.

An unopened vial is good until the expiry date on the label when stored as directed.

Sources (checked 30 September 2026)

04

Does bac water need to be refrigerated?

The Hospira label says to store it at 20 to 25 °C (68 to 77 °F), which is room temperature. A vial that has been mixed with a peptide is a different product: store it the way that product’s label or your pharmacist says, which may differ from the water on its own.

05

Who should not use it

The label carries a boxed warning: not for use in newborns, because benzyl alcohol can be toxic to them. Only preservative-free sterile water is used for neonates.

Worked example

15 units, step by step

The example numbers show how the math works. They are not a dose recommendation.

  1. 1A 10 mg vial mixed with 3 mL of water is 10 ÷ 3 = 3.333 mg/mL.
  2. 2A 500 mcg dose is 0.5 mg. 0.5 ÷ 3.333 = 0.15 mL.
  3. 30.15 mL × 100 = 15 units on a U-100 syringe.
  4. 4The vial holds 10 ÷ 0.5 = 20 full doses.

Mixing calculator

Your vial

The amount printed on the vial.

mL

Bacteriostatic water, in mL.

The dose your prescriber gave you. Check mg or mcg.

Syringe

Used only to show how long a vial lasts.

Draw to

—

units
on U-100

Enter the vial, the mix and the dose to see where to draw.

—mg/mL

Concentration

—mL

Volume to draw

—doses

Doses per vial

—days

Vial lasts (set frequency)

U-100 · 1 mL (100 units)

Drawn to scale
102030405060708090100

About this page

Who made it, and how

Made by FangFang, an independent app studio run by Fang Fang in Auckland, New Zealand.

We draft with AI assistance. Facts are checked against the primary sources linked on the page (US prescribing information and product labels on DailyMed, CDC and NLM pages), each with the date it was checked. Every number comes from the calculator’s own formula, which is covered by automated tests, including the worked example above. A person reviews the page before it is published. No doctor or pharmacist has reviewed it, which is why it sticks to arithmetic and quoted labels.

Last reviewed

FAQ

Questions

What is bac water used for?

Bacteriostatic water is used to dilute or dissolve medicines that are given by injection into a vein, muscle or under the skin. It is not meant to be injected on its own as fluid replacement.

How long is bacteriostatic water good for?

Unopened, until the expiry date on the vial. Once opened, CDC guidance is to discard a multi-dose vial within 28 days unless the manufacturer states another date.

Does bac water need refrigerating?

Not according to the Hospira label, which says to store it at 20 to 25 °C (68 to 77 °F). Storage for a vial after it is mixed with a medicine depends on that medicine’s instructions.

Is bacteriostatic water the same as sterile water?

No. Bacteriostatic water contains benzyl alcohol as a preservative and comes in multiple-dose vials. Sterile Water for Injection has no preservative and is single-dose only.

How much bac water should I add to a vial?

Use the volume in the product instructions or from your pharmacist. The calculator on this page shows what a given volume means for the concentration and the syringe units.

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