How Long Does Bacteriostatic Water Last?
Last updated: September 23, 2026 · Reading time: 4 minutes · For research use only
| State of the vial | Where to keep it | How long |
|---|---|---|
| Sealed | Room temperature, 20 to 25 °C, out of sunlight | Until the printed date |
| Punctured once or more | Fridge, 2 to 8 °C (common lab practice) | 28 days after first puncture |
| Mixed with a peptide | Fridge, 2 to 8 °C | Set by the peptide's data sheet |
Before opening: room temperature
A sealed vial of bacteriostatic water needs no fridge. Pharmaceutical labels ask for controlled room temperature, 20 to 25 °C (68 to 77 °F), and the vial stays good until the expiration date printed on it. Keep it out of direct sun and out of places that get hot, such as a car in summer or a shelf above a radiator.
After the first puncture: 28 days
The clock starts the first time a needle goes through the stopper. USP chapter 797 sets 28 days as the limit for an opened multiple-dose container, unless the manufacturer states otherwise. Write the date on the vial the day you open it, because nobody remembers it three weeks later.
The limit exists because benzyl alcohol at 0.9% is bacteriostatic. It stops bacteria from multiplying. It does not sterilize what each needle brings in, so the risk grows with every draw. Wiping the stopper with a 70% isopropyl alcohol swab before each puncture keeps that risk low. It does not reset the 28 days.
Does it need to be refrigerated after opening?
The label on pharmaceutical vials still says room temperature after opening. Most research labs move an opened vial to the fridge anyway, at 2 to 8 °C, because cold slows down anything that got past the preservative. Both ways, the vial goes in the bin on day 28. Once the water has been added to a peptide, the fridge is no longer optional. The dissolved peptide is what degrades, and its own data sheet gives the storage time.
Signs a vial has gone bad
- The liquid is cloudy, tinted or has particles floating in it.
- The glass is cracked or chipped, or the aluminum seal is loose.
- The rubber stopper is torn, cored or pushed into the vial.
- The printed expiration date has passed.
- The first puncture was more than 28 days ago.
Bacterial contamination is usually invisible at first, so a clear vial is not proof that it is fine. Go by the dates.
Pick the vial size you will finish
Whatever is left on day 28 gets thrown away, so a 30 mL vial only saves money if you draw 30 mL within four weeks. Most peptide work uses 1 to 3 mL per reconstitution, which is why a 10 mL vial is the size most labs finish on time. The mixing volumes per vial size are in the reconstitution guide, and the difference from plain sterile water is covered in what bacteriostatic water is.
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Bacteriostatic Water USP, 10 mL, 0.9% benzyl alcohol, $5 per vial
Each batch tested for sterility, endotoxin and benzyl alcohol content. COA on request.
Buy Bacteriostatic Water, $5 →Frequently asked questions
Does bacteriostatic water need to be refrigerated?
Not while it is sealed. An unopened vial is stored at controlled room temperature, 20 to 25 °C (68 to 77 °F), until the date printed on the label. After the first puncture many labs keep it at 2 to 8 °C to slow any growth that gets past the preservative. The fridge does not extend the 28-day limit.
How long does bacteriostatic water last once opened?
28 days from the first time the stopper is punctured. That limit comes from USP chapter 797 for multiple-dose containers and applies even if most of the liquid is still in the vial.
Can you use bacteriostatic water after 28 days?
It should be discarded. The 0.9% benzyl alcohol stops bacteria from multiplying but does not kill everything that enters with each needle. After 28 days of punctures that margin is no longer counted on. A fresh 10 mL vial costs $5, far less than a spoiled peptide.
How do you know if bac water is bad?
Discard it if the liquid is cloudy, tinted or has visible particles, if the glass is cracked, if the stopper is torn or pushed in, if the printed date has passed, or if the vial was first punctured more than 28 days ago. Contamination is often invisible, so the dates matter more than the look.
Can you freeze bacteriostatic water?
No. Freezing can crack the glass or loosen the seal, and it adds nothing to the shelf life. Keep sealed vials at room temperature and opened vials in the fridge door or main compartment, away from the freezer plate.
This article is for laboratory and research use only. It is not medical advice. Always follow local regulations and consult a licensed healthcare professional for any medical question.