The water has its own label
What is bacteriostatic water?
Bacteriostatic water for injection is sterile water with benzyl alcohol as a preservative. It is commonly used to reconstitute lyophilized peptides, but it is a separate product with its own label, storage rules, and in-use limits.
The product
Bacteriostatic water is a USP-defined diluent.
Bacteriostatic Water for Injection, USP is a sterile, nonpyrogenic preparation of water containing 0.9% benzyl alcohol as a bacteriostatic preservative. It is packaged in single-dose or multi-dose vials and used to dilute or dissolve medications for injection. Read the DailyMed label.
The benzyl alcohol inhibits bacterial growth, which is why bacteriostatic water can be used with multi-dose vials. Sterile water for injection does not contain a preservative and is typically single-use.
The comparison
Bacteriostatic water vs. sterile water vs. saline.
- Bacteriostatic water
- Sterile water with 0.9% benzyl alcohol. Multi-dose use. Follow label for in-use period (often 28 days after first puncture).
- Sterile water
- Sterile water without preservative. Typically single-use. Discard after one puncture unless instructions specify otherwise.
- Normal saline (0.9% NaCl)
- Sterile saline solution. Some products require saline instead of water. Check the product instructions.
- Reconstitution solution
- A generic term. Can mean bacteriostatic water, sterile water, saline, or a manufacturer-supplied diluent. Read the label.
Do not assume all “reconstitution solution” products are interchangeable. Some peptides require a specific diluent for stability or compatibility. Use what the product instructions name.
The dating
Unopened vs. in-use: two different windows.
Unopened bacteriostatic water is stable until the expiration date printed on the packaging when stored at controlled room temperature (typically 20–25°C / 68–77°F). That expiration date applies only to the sealed vial.
Once the vial is punctured, the in-use period begins. Most bacteriostatic water labels specify 28 days after first use when stored properly. After that period, discard any remaining water regardless of how much is left.
Record the date of first puncture on the vial. If no label space exists, use tape or a marker. That date is when the 28-day clock starts.
The boundary
The water label does not set the peptide's discard date.
A common misconception: because bacteriostatic water has a 28-day in-use limit, people assume a peptide reconstituted with it also lasts 28 days. That is not how it works.
The 28 days applies to the water product before mixing. Once the water is added to a peptide, the beyond-use date comes from the peptide product instructions, the compounding pharmacy, or the prescriber. That date may be shorter or longer than 28 days depending on the peptide's stability profile.
If your peptide product does not specify a beyond-use date after reconstitution, ask the dispensing pharmacy or prescriber. Do not assume 28 days because that is what the water label says.
For more on reconstituted peptide timing, see the reconstituted peptide fridge life guide.
The storage
Store bacteriostatic water at controlled room temperature.
The DailyMed label specifies storage at controlled room temperature, typically 20–25°C (68–77°F), with excursions permitted to 15–30°C (59–86°F). Do not freeze bacteriostatic water.
Keep the vial away from direct light and heat. After first puncture, continue storing at room temperature and discard at the end of the in-use period specified on the label.
The safety record
Keep the water lot with the peptide record.
When you reconstitute a peptide, record the bacteriostatic water lot number, first-puncture date, and discard date alongside the peptide vial details. If a problem arises, you can trace which water was used.
CDC injection-safety guidance calls for single-dose vials when available, proper aseptic technique, and discarding multi-dose vials according to manufacturer recommendations or within 28 days of first use. Read the CDC guidance.
Related reading
Reconstitution guides and calculators
Calculator
Peptide reconstitution calculator
Enter vial amount and liquid volume to calculate concentration and U-100 syringe units.
How to Reconstitute Peptides
A label-first guide to diluent, technique, and recordkeeping.
Do Peptides Always Need BAC Water?
Product forms, ready-to-use devices, and when BAC water is not required.
Reconstituted Peptide Fridge Life
What the label says, what to record, and when to ask a pharmacist.
How to Reconstitute BPC-157
A label-first checklist for BPC-157 reconstitution.
More resources
Storage and travel
Common questions
Bacteriostatic Water FAQ
What is bacteriostatic water used for?
Bacteriostatic water for injection is used to dilute or dissolve medications intended for injection. The benzyl alcohol preservative inhibits bacterial growth, making it suitable for multi-dose vials when the product instructions specify it. Not every injectable uses bacteriostatic water; follow the diluent named in your product instructions.
How long does bacteriostatic water last?
Unopened bacteriostatic water is stable until the expiration date on the packaging. After first puncture, the in-use period depends on the manufacturer's label, typically 28 days when stored properly. This is the water's own in-use limit; it does not set the beyond-use date for a peptide mixed with it.
Is reconstitution solution the same as bacteriostatic water?
Not necessarily. 'Reconstitution solution' or 'diluent' can refer to bacteriostatic water, sterile water, normal saline, or a manufacturer-supplied mixing solution. Some products come with their own diluent that differs from bacteriostatic water. Read the product label to confirm which diluent is required.
Does bacteriostatic water expire after opening?
Yes. Once the vial is punctured, the in-use period begins. Most bacteriostatic water labels specify 28 days after first use when stored at controlled room temperature. Discard any remaining water after that period, regardless of how much is left.
Can I use bacteriostatic water after 28 days?
Follow the label. If the label says discard 28 days after first puncture, do not use it beyond that date. The preservative slows bacterial growth but does not guarantee indefinite sterility. When in doubt, use a new vial.
Does the bacteriostatic water label set the peptide's discard date?
No. The water label applies to the water product before mixing. Once the water is added to a peptide, the beyond-use date comes from the peptide product instructions, not the diluent. A 28-day water limit does not mean every mixed peptide lasts 28 days.
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