Bacteriostatic water vs sterile water
Both are sterile water made for preparing injectable medicines. Bacteriostatic water contains a preservative and comes in multiple-dose vials. Sterile water for injection has no preservative and comes in single-dose containers. Which one to use is set by the medicine's label or your pharmacist.
What each product is
Bacteriostatic Water for Injection, USP is sterile water with 0.9% benzyl alcohol added, which is 9 mg in each mL. Benzyl alcohol is a bacteriostatic preservative: it holds back the growth of bacteria that might enter when the stopper is punctured. Because of the preservative, the product is supplied in multiple-dose vials.
Sterile Water for Injection, USP is sterile water with nothing added. It has no preservative, so it is supplied in single-dose containers, and its labelling says to discard the unused portion.
"Bacteriostatic" means it inhibits bacterial growth. It does not mean the water sterilises whatever is added to it, and it does not make a vial last indefinitely.
Side by side
| Bacteriostatic water for injection | Sterile water for injection | |
|---|---|---|
| Contents | Sterile water + 0.9% (9 mg/mL) benzyl alcohol | Sterile water only |
| Preservative | Yes | No |
| Container | Multiple-dose vial | Single-dose vial or ampoule |
| Use | Diluting or dissolving medicines for injection, where the medicine's label allows it | Diluting or dissolving medicines for injection, where the medicine's label calls for it |
| Label warning to know | Not for use in newborns, because of the benzyl alcohol | No preservative; single use |
| Effect on the maths | None | None |
Things that are not the same product
- Sterile water for irrigation is made for rinsing, not for injection, and is labelled accordingly.
- Sodium chloride 0.9% (normal saline) is salt water, a different diluent. It also comes in plain and bacteriostatic versions.
- Distilled, purified or bottled water is not sterile and is not an injection product.
Some medicines are supplied with their own diluent in the box. In that case the supplied diluent is the one the label refers to.
Which one to use
This site cannot answer that for your medicine. The diluent is part of the product's instructions: some medicines specify sterile water, some allow bacteriostatic water, and some require a particular diluent supplied with them. Use the one named on the label or by the pharmacist who dispensed it. If the instructions do not name one, ask before mixing.
Storage and how long a vial is used
Follow the storage instructions and the dates printed on the diluent and on your medicine's pharmacy label. How long a mixed vial may be kept depends on the medicine, not only on the water, so there is no single figure that applies to all of them. Your pharmacist can tell you what applies to yours. Reading a vial label explains expiry and beyond-use dates.
Why the choice does not change the calculation
Concentration depends on the amount in the vial and the volume of liquid added. The type of water does not enter into it.
A 10 mg vial with 2 mL added is 5 mg/mL whether those 2 mL are bacteriostatic or plain sterile water. The calculator labels the field "bacteriostatic water added" because that is the term most people search for, but the number it needs is simply the mL of diluent that went in.
What does change the result is the volume. How water volume changes concentration covers that, and the bacteriostatic water calculator works out the volume that produces a chosen concentration. To estimate how many vials of water a supply of medicine vials will use, see the vial supply calculator.
Questions
What is the difference between bacteriostatic water and sterile water?
Bacteriostatic water for injection contains 0.9% benzyl alcohol as a preservative and is supplied in multiple-dose vials. Sterile water for injection contains no preservative and is supplied in single-dose containers.
What is in bacteriostatic water?
Sterile water and 0.9% benzyl alcohol, which is 9 mg of benzyl alcohol per mL. The benzyl alcohol is a preservative that inhibits bacterial growth.
Can I use sterile water instead of bacteriostatic water?
That depends on the medicine. The correct diluent is stated on the medicine's label or by the dispensing pharmacist, and you should use the one they specify. The choice does not affect the concentration arithmetic.
Does the type of water change how many units I draw?
No. The units depend on the amount in the vial, the volume of diluent added and the prescribed amount. The same volume of either water gives the same concentration.