Bacteriostatic water vs sterile water: the difference

Bacteriostatic water is sterile water for injection with 0.9% benzyl alcohol added as a preservative, so the same vial can be punctured more than once; sterile water for injection has no preservative and comes in single-dose containers that are used once, with the rest thrown away. Which one you use isn’t a preference. It’s set by your medicine’s label or your pharmacist, and it changes how long a mixed vial can be kept. To see what a volume of either one means in syringe units, use the bac water calculator.

Side by side

Sources: Hospira’s labels for Bacteriostatic Water for Injection and Sterile Water for Injection.

What the benzyl alcohol does

Benzyl alcohol is a bacteriostatic preservative: it holds back the growth of bacteria that might get in when a needle goes through the stopper. It doesn’t sterilize anything. The label still says to “use aseptic technique for single or multiple entry”, which means a fresh alcohol swab on the stopper and a new sterile needle every time.

How much benzyl alcohol is in bacteriostatic water? 0.9% means 9 mg of benzyl alcohol in every mL. Some glass vials contain 1.1% (11 mg/mL). The label says it’s “not for use in neonates” because benzyl alcohol can be toxic to newborns. For drugs prepared for newborns, only preservative-free sterile water is used.

When sterile water is the right choice

Use sterile water only when the label or your pharmacist says to. That happens when:

• the product is meant to be mixed and used straight away, like the single-dose Serostim vials;
• the drug may not be compatible with benzyl alcohol. Bacteriostatic water’s own label warns that some drugs “may be incompatible ... in a vehicle containing benzyl alcohol” and says to consult a pharmacist;
• the medicine is for a newborn.

If you mix with sterile water, treat the vial as single-use unless your label says otherwise.

Does the type of water change the math?

No. The concentration depends only on the mg in the vial and the mL you add.

For example (numbers for illustration only, not a dose): a 5 mg vial plus 2 mL of either liquid is 5 ÷ 2 = 2.5 mg/mL. Then 10 units = 0.1 mL, which holds 0.1 × 2.5 = 0.25 mg. What changes is how long you can keep using that vial. The bac water calculator and the reconstitution calculator work the same way for either.

How long bac water lasts once opened

Once a multiple-dose vial has been punctured, the CDC, citing USP General Chapter 797, says it should be dated and discarded within 28 days unless the manufacturer specifies a different date (CDC). The FDA-approved instructions for Egrifta WR say the same about its bacteriostatic water: discard the bottle 28 days after first use (Egrifta WR label). Write the date on the vial the first time you use it. An unopened vial is good until the expiry date printed on it, and not after.

Your mixed peptide vial has its own, separate discard date. See how long reconstituted peptides last.

Common mistakes

• Mixing with sterile water, then using the vial for weeks. Without a preservative, the label rules for single-dose use apply.
• Using bac water from a vial opened more than 28 days ago, or past its printed expiry.
• Assuming “bacteriostatic” means sterile forever. It slows bacteria; it doesn’t replace clean technique.
• Swapping in saline. Sodium chloride solutions, including “bacteriostatic saline”, are different products. Use only the diluent your label names.
• Using tap, bottled or distilled water. None of these is made for injection.

Questions people ask