1. The Short Answer
Store unopened bacteriostatic water at room temperature, 20 to 25°C (68 to 77°F), and use it before the printed expiration date.1 Once the stopper has been punctured, discard the vial 28 days later regardless of that date. Refrigeration is only needed after the water has been mixed with a drug, and then the drug's label decides the rules.
That is the whole rule set in three sentences. The rest of this guide explains why each part exists, because most storage mistakes come from applying the right rule to the wrong state of the vial.
2. Three States, Three Rules
| State of the vial | Temperature | How long | Rule comes from |
|---|---|---|---|
| Sealed, never punctured | 20 to 25°C (68 to 77°F) | Until the printed expiration date | FDA-approved product label |
| Punctured, plain water | Room temperature, per label | 28 days from first entry, or the expiration date if sooner | USP <797> and CDC |
| Mixed into a drug vial | Whatever the drug's label says, often 2 to 8°C | Whatever the drug's label says | The drug's own label or datasheet |
Sources: Hospira label for storage temperature1; USP and CDC for the 28-day limit (section 4).
3. How to Store Unopened Bacteriostatic Water
The FDA-approved Hospira label gives one storage instruction: 20 to 25°C (68 to 77°F), which is USP controlled room temperature.1 A kitchen cabinet or a closed drawer in a climate-controlled room meets that. Places that do not:
- A car in summer or winter. Cabin temperatures leave the labeled range quickly.
- A windowsill or a shelf in direct sun, which also adds heat.
- Above a stove, dishwasher or radiator.
- A garage, shed or unheated mailbox for days at a time.
Keeping vials in their carton is a good habit even though the storage statement is about temperature. It protects the label and flip cap, keeps the lot number and expiry with the vial, and shields it from light.
What the expiration date actually promises
FDA describes a drug's expiration date as the period during which the product is known to remain stable, meaning it keeps its strength, quality and purity, when it is stored according to its labeled storage conditions.4 Federal manufacturing rules tie the two together: expiration dates must be determined by stability testing and related to the storage conditions stated on the labeling.5
For bacteriostatic water, “strength, quality and purity” covers the benzyl alcohol content that makes it bacteriostatic, sterility, pH, endotoxin levels and a container that still seals. The date is a promise with a condition attached. A vial that spent a week in a hot car has left the conditions the promise was made under, even if the date has not passed.
4. After Opening: the 28-Day Rule
The first needle puncture starts a separate clock. Under USP <797>, an opened or needle-punctured multiple-dose container is discarded 28 days after first entry unless the manufacturer specifies otherwise.2 CDC gives the same instruction: date the vial when first opened and discard it within 28 days unless the manufacturer specifies a different date.3
The printed expiration date still applies, and whichever comes first wins. A vial opened a week before it expires is done in a week, not in 28 days. Why the number is 28 and how to track it across several vials is covered in our 28-day puncture rule guide, so it is not repeated here.
Buy the size you will use in 28 days. The 28-day limit, not the expiration date, is what usually ends a vial's life. If you draw only a few milliliters a month, most of a 30 mL vial goes in the bin. A 3 mL vial exists for exactly that situation. The trade-offs between sizes are in 10 mL vs 3 mL bacteriostatic water.
5. Should Bacteriostatic Water Be Refrigerated?
On its own, no. The label's storage instruction is room temperature, and it does not call for refrigeration.1 Two points get confused here:
- Refrigerating an opened vial does not extend the 28 days. The USP and CDC limit counts from first entry; neither ties it to storage temperature.23 Cold storage is not a way to stretch a vial to 40 or 60 days.
- Mixed drugs are a different product. Once bacteriostatic water is inside a drug vial, the drug's label takes over, and those labels usually do call for refrigeration.
The bacteriostatic water label itself draws that line: “Do not store reconstituted solutions of drugs for injection unless otherwise directed by the manufacturer of the solute.”1 The second point is easy to see in real labels, which give different answers for the same diluent:
- Novarel (HCG) mixed with Bacteriostatic Water for Injection: refrigerate at 2 to 8°C and use within 30 days.6
- Pregnyl (HCG) mixed with its benzyl alcohol-preserved solvent: refrigerate, discard after 60 days, do not freeze.7
- Zomacton 5 mg (somatropin) mixed with its bacteriostatic saline diluent: refrigerate at 2 to 8°C for up to 14 days.8
Same preservative, three different time limits. The drug decides, which is why a general “bac water lasts X days in the fridge” figure is not meaningful once something has been dissolved in it. CDC's Project Firstline makes the same point about multi-dose vaccine vials:
“Follow the vaccine maker's instructions for storage and for how long you can use the vaccine after you first access the vial, which varies by product. Beyond-use or improperly stored vials must be thrown away.”
6. Freezing, Heat and Light
Freezing. The label gives a room-temperature range and no instruction to freeze, so a frozen vial is outside the conditions its expiration date covers.5 There is also a physical reason: ice is about 9% less dense than liquid water, so the same water takes up more room once frozen.9 In a filled, sealed vial that pressure lands on the stopper and the glass, which can loosen the seal or crack the vial in ways that are hard to see. Treat a vial that has frozen as compromised. For mixed drugs, some labels say so explicitly; Pregnyl's is one.7
Heat. Stability testing for expiration dating is done under defined, controlled conditions that include heat, and a vial kept well above its labeled range for days is outside what the date was tested against.4
Light. The label's storage statement is about temperature. Keeping the vial in its carton still costs nothing and keeps it out of direct sun, which is also a heat source.
7. Storage Myths We Found in Top Search Results, Checked Against the Label
Before updating this guide in September 2026, we read the pages that rank for bacteriostatic water storage and shelf life and listed every storage instruction they gave. Four claims came up repeatedly that the FDA-approved label, USP or CDC do not support. We name the claim, not the site.
| Claim we saw | What the primary sources say |
|---|---|
| “After the first puncture, move the vial to the fridge at 2 to 8°C.” | The Hospira label has one storage instruction, 20 to 25°C (68 to 77°F), for the vial whether or not it has been entered.1 |
| “The 28 days assumes the opened vial is refrigerated.” | USP <797> and CDC count 28 days from first entry and do not tie the limit to temperature.23 Cold storage neither earns nor extends it. |
| “An unopened vial is good for 2 to 3 years.” | The label gives no fixed shelf life. Each vial's date comes from its maker's stability testing and is printed on the vial.5 Read the date; do not assume one. |
| “Light degrades the benzyl alcohol, so light protection is required.” | The label's storage statement is about temperature only and has no light-protection instruction.1 Keeping the carton on is sensible, not a label requirement. |
8. Expired Bacteriostatic Water
FDA's consumer advice is direct: if a medical product has expired, do not use it, because once the date has passed there is no guarantee it is safe and effective. The agency specifically notes that some expired products are at risk of bacterial growth.10 For a product whose whole purpose is to stay sterile and resist contamination across repeated punctures, that is the relevant risk.
Past the date, three things are no longer backed by data: that the benzyl alcohol is still at the stated concentration, that the stopper still seals the way it did when tested, and that the contents are still sterile. None of these can be checked by looking at the vial.
What about FDA's shelf-life extension program?
Articles on expired medicines often mention that FDA has extended expiration dates for some drugs. That is the Shelf-Life Extension Program (SLEP), set up in 1986. It is administered by the Department of Defense, applies to certain federally stockpiled products, and extends dates only after FDA tests specific lots for stability, identity, strength, quality and purity, and only for products stored under their labeled conditions. FDA states that SLEP remains limited to federal stockpiles.11
So the program shows that some products can outlast their dates when stored perfectly and then retested. It does not mean an individual vial in a drawer has been retested, and it gives no basis for using one.
9. Signs a Vial Should Be Discarded
CDC's guidance is to inspect a vial before use and to discard a multiple-dose vial when its beyond-use date is reached or any time its sterility is in question.12 In practice, discard when any of these apply:
| What you see | What it may mean |
|---|---|
| Cloudiness or haze | Microbial growth or a contaminant; bacteriostatic water should be clear and colorless |
| Floating particles or flakes | Glass, stopper fragments (coring) or growth |
| Any color | Contamination or a leached component |
| Damaged, pushed-in or heavily cored stopper | The seal that keeps the contents sterile may be broken |
| Cracked or chipped glass | Loss of container integrity |
| Missing or loose flip cap on a new vial | The vial may have been tampered with or entered |
| No first-use date written on an opened vial | The 28-day limit cannot be confirmed |
| Frozen, or left in heat for days | Outside the labeled storage conditions |
A clear vial is not proof of a sterile one. Bacterial contamination at levels that can cause infection is often invisible, which is why the time limits exist alongside the visual check.
10. Disposal
Used needles and syringes go into a sharps container. For the vial itself, FDA's general guidance on expired medicines favors take-back locations or mail-back envelopes where available.10 For more on what bacteriostatic water is and why its preservative behaves the way it does, see what is bacteriostatic water.
11. Summary
- Sealed: 20 to 25°C, good until the printed date, as long as it stayed in that range.
- Punctured: 28 days from first entry or the printed date, whichever is sooner. No refrigeration needed.
- Mixed with a drug: the drug's label sets temperature and time (30, 60 or 14 days in the examples above).
- Do not freeze. Do not use after expiry. SLEP applies only to tested federal stockpiles.
- Discard anything cloudy, discolored, particulate, cracked, undated or of doubtful sterility.
Frequently Asked Questions
Should bacteriostatic water be refrigerated?
Not on its own. The FDA-approved label stores bacteriostatic water at 20 to 25 C (68 to 77 F) and does not call for refrigeration. Refrigeration becomes relevant only after it is mixed with a drug, and then the drug's label decides the temperature and time.
Does bacteriostatic water expire?
Yes. Every vial carries an expiration date supported by the manufacturer's stability testing under the labeled storage conditions. A punctured vial also has a second, shorter limit: discard it 28 days after first entry unless the manufacturer states otherwise, even if the printed date is years away.
Can you use expired bacteriostatic water?
No. FDA advises against using any expired medical product because there is no guarantee it is still safe and effective, and some expired products can support bacterial growth. For bacteriostatic water, sterility, preservative content and container integrity are only backed up to the printed date.
How long is bacteriostatic water good for after opening?
28 days from the first needle puncture, under USP <797> and CDC guidance, unless the manufacturer specifies a different period. Write the date on the vial when you first enter it and discard it on day 28 even if liquid is left.
Can you freeze bacteriostatic water?
The label gives a room-temperature storage range and no freezing instruction, so freezing takes the vial outside the conditions its expiration date is based on. Water expands as it freezes, which can stress the stopper seal and the glass. A vial that has frozen should be discarded.
How do you know if bacteriostatic water has gone bad?
Discard it if the liquid is cloudy, discolored or has visible particles, if the stopper is damaged or the glass cracked, if it is past the expiration date or 28 days past first puncture, or any time you cannot be sure it stayed sterile. Contamination is often invisible, so a clear vial is not proof it is safe.
References & Primary Sources
Every technical claim on this page is sourced to a pharmacopeial monograph, a federal regulatory body, or peer-reviewed literature. Links open the primary source directly.
- Regulatory / pharmacopeialBacteriostatic Water for Injection, USP: FDA prescribing information (Hospira, Inc.)
- Regulatory / pharmacopeialGeneral Chapter ⟨797⟩ Pharmaceutical Compounding: Sterile Preparations
- Public health authorityPreventing Unsafe Injection Practices
- Regulatory / pharmacopeialExpiration Dates - Questions and Answers
- Regulatory / pharmacopeial21 CFR 211.137: Expiration dating
- Regulatory / pharmacopeialNOVAREL (chorionic gonadotropin) for injection: Prescribing Information
- Regulatory / pharmacopeialPregnyl (chorionic gonadotropin) for injection: prescribing information
- Regulatory / pharmacopeialZOMACTON (somatropin) for injection: prescribing information (Ferring Pharmaceuticals Inc.)
- Public health authorityWater Density
- Regulatory / pharmacopeialDon't Be Tempted to Use Expired Medicines
- Regulatory / pharmacopeialExpiration Dating Extension
- Public health authorityMulti-Dose Vial Safety Reminders