Bacteriostatic water vs saline: the difference
Bacteriostatic water vs saline is a common mix-up, partly because both come in similar vials and both are clear. The difference is what's dissolved in the water: salt in one, a preservative in the other. Here's the full comparison, cited to the label.
What's actually in each one
Saline for injection is water with 0.9% sodium chloride — ordinary salt — which makes it isotonic with the body. Bacteriostatic water has no salt; it's water for injection with 0.9% (9 mg/mL) benzyl alcohol added as a bacteriostatic preservative.1 That single difference in contents is what sets everything else apart.
| Property | Bacteriostatic water | Saline |
|---|---|---|
| Dissolved contents | Benzyl alcohol 0.9% (preservative)1 | Sodium chloride 0.9% (salt) |
| Tonicity | Must be made approximately isotonic before use2 | Isotonic as supplied |
| Vial use | Multiple-dose (preservative allows re-entry)1 | Single or multi-dose, varies by product |
| Typical role | Diluent for drugs needing repeated dosing | Fluid replacement, flushing, irrigation, dilution |
Why people confuse them
Both are clear liquids in similar vials, both are described as "sterile water" in casual conversation, and pharmacies sometimes stock one but not the other. That's why it's worth reading the label rather than going by the vial's appearance: saline lists sodium chloride, bacteriostatic water lists benzyl alcohol.1
Can you use one instead of the other?
They're not interchangeable. Which one a given drug requires depends entirely on that drug's labeling, so the manufacturer's instructions decide it, not a general rule.2 If a preparation calls for bacteriostatic water, saline is not a substitute, and vice versa.
From the label. Bacteriostatic water is for drug-diluent use only and is not for use in neonates, because of the potential toxicity of benzyl alcohol.2 Always follow a healthcare professional's guidance and the drug manufacturer's instructions.
Frequently asked questions
Is bacteriostatic water the same as saline?
No. Saline contains sodium chloride (salt); bacteriostatic water contains benzyl alcohol (a preservative) and no salt.1
Can I use saline instead of bacteriostatic water?
Not as a substitute. They're different products; follow the labeling for the specific drug.2
How do I tell them apart?
Read the label. Saline lists sodium chloride; bacteriostatic water lists 0.9% benzyl alcohol.1
What about plain sterile water?
Sterile water has neither salt nor preservative. See our bacteriostatic vs sterile water comparison.
What is bac water? Vs sterile water
See also: reconstitution solution vs bacteriostatic water · sterile water vs distilled water.
- Bacteriostatic Water for Injection, USP — Description. DailyMed, U.S. National Library of Medicine. View label.
- Bacteriostatic Water for Injection, USP — Directions, Warnings & Precautions (isotonic-before-use, neonate). DailyMed. View label.