Bacteriostatic Water vs Sterile Water: What Is the Difference?
Both start as sterile water, but bacteriostatic water contains 0.9% benzyl alcohol as a preservative, so a vial can be entered more than once. Sterile water contains no preservative and is packaged in single-use containers; any unused portion is discarded after first entry. Opened multiple-draw vials are typically dated and discarded within 28 days unless the labeling states otherwise. Distilled and deionized water are different again: they are not sold as sterile.
What is the difference between bacteriostatic water and sterile water?
The difference is one ingredient. The labeling for the FDA-labeled prescription bacteriostatic product describes it as a sterile, nonpyrogenic preparation of water with 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol "added as a bacteriostatic preservative" [1]. The labeling for sterile water describes a sterile, nonpyrogenic preparation that "contains no bacteriostat, antimicrobial agent or added buffer" [2]. Everything else that follows, from packaging to how long an opened vial stays in use, comes from that difference.
Both FDA-labeled versions carry an "Rx only" designation on their labels [1, 2]. They are prescription products for clinical settings. Batch bacteriostatic water is a separate product supplied for laboratory and research use only. It is formulated with the same preservative concentration, 0.9% benzyl alcohol in sterile water; see the product specifications for details.
| Property | Bacteriostatic water | Sterile water |
|---|---|---|
| Base | Sterile, nonpyrogenic water [1] | Sterile, nonpyrogenic water [2] |
| Preservative | Benzyl alcohol, 0.9% (9 mg/mL); a 1.1% strength also exists [1] | None: no bacteriostat or antimicrobial agent [2] |
| Container type | Multiple-draw vial; repeated withdrawals may be made [1] | Single-use container only [2] |
| After first entry | Typically dated and discarded within 28 days unless labeling states another date [4] | Do not reuse; discard the unused portion [2] |
| pH (labeled) | 5.7 (range 4.5 to 7.0) [1] | Varies by product; one labeled range is 5.0 to 7.0 [3] |
| Storage (labeled) | 20 to 25°C (68 to 77°F) [1] | 20 to 25°C (68 to 77°F) [3] |
| Tonicity | Hypotonic; labeling says it must be made approximately isotonic before use [1] | Hypotonic; labeled osmolarity is 0 [3] |
What does the benzyl alcohol preservative do?
Benzyl alcohol is included as a bacteriostatic preservative [1]. "Bacteriostatic" means it inhibits the growth of bacteria that may be introduced into the vial; it does not make a contaminated solution sterile again. That is why the labeling still calls for aseptic technique for single or multiple entry and withdrawal from all containers [1]. For a deeper look at the compound itself, see benzyl alcohol in bacteriostatic water.
The preservative is also a reason not to swap the two waters casually. Labeling for both products notes that some materials may be incompatible in a vehicle containing benzyl alcohol [1, 2]. When a protocol or a material's own instructions specify preservative-free sterile water, the preservative is a variable you would be adding, not a neutral upgrade.
Single-use vs multiple-draw vials: why does it matter?
A multiple-draw (multi-use) vial is designed so that "repeated withdrawals may be made" from the same container [1]. A single-use container is meant to be entered once. Sterile water labeling says not to reuse these containers and to discard the unused portion [2]. CDC guidance explains the reasoning: single-use vials "typically lack an antimicrobial preservative" and can become a source of contamination when used inappropriately [4].
In a lab, this changes workflow and waste. If you draw small volumes from one container across several work sessions, a preserved multiple-draw vial is designed for that pattern. If each preparation uses a fresh container and the protocol calls for no additives, unpreserved sterile water in single-use containers fits better. Vial size matters here too; see bacteriostatic water vial sizes.
How long is bacteriostatic water good for after the first puncture?
Two dates apply. The printed expiration date covers the sealed vial. The in-use date starts at first puncture. CDC guidance states that once a multi-use vial is opened (needle-punctured) it should be dated and discarded within 28 days unless the manufacturer states another date for that opened vial, and that this beyond-use date should never exceed the original expiration date [4]. Sterile water in single-use containers has no in-use period; the unused portion is discarded after the first entry [2].
Good practice is to write the date of first puncture on the vial label and follow the product's own labeling where it gives a different period. Labeling for the prescription product also says not to use the solution unless it is clear and the seal is intact [1]. More detail on storage and dating is in bacteriostatic water shelf life and storage.
Is bacteriostatic water or sterile water hypotonic?
Both are. Neither contains a salt or other solute to balance osmotic pressure. One sterile water label lists its osmolarity as 0 and describes it as hemolytic "due to its hypotonicity" [3], meaning cells placed in it take up water, swell and can burst. Bacteriostatic water labeling likewise says it must be made approximately isotonic before use [1]. If a protocol needs an isotonic vehicle, neither water is the right choice on its own; that comparison is covered in bacteriostatic water vs saline.
How are distilled and deionized water different?
Distilled and deionized (DI) water describe how water was purified, not whether it is sterile. FDA inspection guidance lists eight types of water used in drug manufacturing, separating purified water, used as an ingredient, from finished, packaged products such as sterile water and bacteriostatic water [5]. It also notes there are no absolute microbial standards for water "other than water intended to be sterile" and flags deionized water systems for particular attention because of microbial contamination [5].
FDA guidance on high purity water systems gives a sense of scale: for purified water systems, the agency considers any action limit over 100 CFU/mL unacceptable, while systems producing the water used for sterile products are expected to yield water that is "essentially sterile" [6]. A purified water system can operate within its limits and still contain viable organisms. The same guidance notes that distilled and deionized water have similar ionic quality, so conductivity cannot be used to judge microbiological quality [6]. Put simply, low conductivity does not mean sterile.
For lab work where sterility matters, such as dissolving lyophilized (freeze-dried) materials you intend to store and sample from over time, distilled or DI water from a bench system or retail jug is not a substitute for a sealed, sterile product. It is fine for rinsing glassware and making non-sterile buffers.
Which one should a lab choose?
- Choose bacteriostatic water when you will draw from the same vial more than once and the protocol permits 0.9% benzyl alcohol [1].
- Choose sterile water in single-use containers when the protocol specifies preservative-free water or benzyl alcohol is a known incompatibility [1, 2].
- Choose distilled or DI water only for non-sterile tasks; it is not a finished sterile product [5].
- Whatever you choose, ask for the lot number and a Certificate of Analysis. Our quality and testing process explains what Batch documents for each batch, and how to read a certificate of analysis walks through the fields.
Key takeaways
- The only formulation difference is the preservative: 0.9% benzyl alcohol in bacteriostatic water, none in sterile water [1, 2].
- Bacteriostatic water comes in multiple-draw vials; sterile water comes in single-use containers [1, 2].
- Opened multiple-draw vials are typically discarded within 28 days unless labeling says otherwise [4].
- Both waters are hypotonic [1, 3].
- Distilled and deionized water are purified, not sterile [5, 6].
Frequently asked questions
Can sterile water be used in place of bacteriostatic water?
For a single draw, the water itself is equivalent apart from the preservative [1, 2]. The difference is reuse: sterile water containers are labeled for single use, and the unused portion should be discarded [2], so they are not suited to repeated withdrawals over days.
Can bacteriostatic water be used in place of sterile water?
Only when the protocol allows benzyl alcohol. Labeling notes that some materials may be incompatible in a vehicle containing benzyl alcohol [1], so check the material's instructions first.
Does bacteriostatic water expire after 28 days?
An unopened vial is good until its printed expiration date. The 28-day period starts at first puncture and is the CDC default unless the manufacturer states a different in-use date; it never extends past the printed expiration date [4].
Is distilled water the same as sterile water?
No. Distillation and deionization remove dissolved impurities, but water from those systems is not packaged and labeled as sterile, and FDA guidance accepts measurable microbial counts in purified water systems [5, 6].
Are the FDA-labeled versions sold over the counter?
No. The labels for both the bacteriostatic and the sterile water products are marked "Rx only" [1, 2]. Batch is supplied for laboratory and research use only.
For laboratory and research use only. Not for human or veterinary use. Not intended to diagnose, treat, cure, or prevent any disease.
Sources
- Bacteriostatic Water for Injection, USP (Hospira, Inc.) - prescribing information — DailyMed, U.S. National Library of Medicine. Accessed October 24, 2026.
- Sterile Water for Injection, USP (Hikma Pharmaceuticals USA Inc.) - prescribing information — DailyMed, U.S. National Library of Medicine. Accessed October 24, 2026.
- Sterile Water for Injection, USP in Flexible Plastic Container (Hikma Pharmaceuticals USA Inc.) — DailyMed, U.S. National Library of Medicine. Accessed October 24, 2026.
- Preventing Unsafe Injection Practices — U.S. Centers for Disease Control and Prevention. Accessed October 24, 2026.
- Water for Pharmaceutical Use (Inspection Technical Guide No. 46) — U.S. Food and Drug Administration. Accessed October 24, 2026.
- Guide to Inspections of High Purity Water Systems (7/93) — U.S. Food and Drug Administration. Accessed October 24, 2026.