Bacteriostatic Water: What It Is, How It Is Used, and the 28-Day Rule
Bacteriostatic water is sterile water for injection that contains 0.9 percent benzyl alcohol, 9 mg per mL, as a preservative. It comes in a multiple-dose vial for diluting or dissolving injectable drugs. The CDC says to date a multi-dose vial when it is first punctured and discard it within 28 days unless the manufacturer states another date. It must not be used in newborns.[1][2]
What is bacteriostatic water?
Bacteriostatic Water for Injection, USP is a sterile, nonpyrogenic preparation of water for injection that contains 0.9 percent (9 mg per mL) or 1.1 percent (11 mg per mL) benzyl alcohol, added as a bacteriostatic preservative. The label gives its pH as 5.7, within a range of 4.5 to 7.0, and says it is supplied in a multiple-dose container from which repeated withdrawals may be made.[1] Bacteriostatic means the preservative limits the growth of bacteria. It does not sterilize a vial that has already been contaminated, and the CDC notes that a preservative does not affect viruses and does not give complete protection against bacterial contamination.[2] The label says to use it only to dilute or dissolve drugs, following the instructions of the drug's manufacturer.[1] People often call it bac water. The 0.9 percent in its name refers to benzyl alcohol, not to the sodium chloride in 0.9 percent saline.
What is bacteriostatic water used for?
Its labeled use is narrow. Bacteriostatic Water for Injection is indicated only for diluting or dissolving drugs for intravenous, intramuscular, or subcutaneous injection, according to the instructions of the manufacturer of the drug.[1] A real example is Egrifta WR, an FDA-approved peptide made of tesamorelin. One presentation is packaged with a 30 mL multiple-dose vial of Bacteriostatic Water for Injection, and its label says to use only the diluent provided to mix the powder.[4] The water label also says that parenteral preparations containing benzyl alcohol should not be used for fluid replacement, so it is not a drink and not an infusion fluid.[1] It is not a general-purpose liquid for wounds or other tasks. If the instructions for your medicine do not name bacteriostatic water, ask your pharmacist which diluent applies. Many peptides sold online for research have no label at all, and the choice of diluent is then not backed by any approved instructions.
How long is bacteriostatic water good after opening?
The CDC says that once a multi-dose vial is opened, for example by a needle puncture, the vial should be dated and discarded within 28 days unless the manufacturer states another date.[2] That is the number most people mean by the 28-day rule. One detail is easy to miss: the Hospira bacteriostatic water label we checked gives no in-use period at all. The 28-day figure comes from CDC guidance for multi-dose vials, not from that label.[1] Before the first puncture, the printed expiration date applies. Write the date you first opened the vial on the label, and use a new sterile needle and syringe for every entry, because the CDC says a used syringe and needle are both contaminated.[2] If a vial looks cloudy, has particles, or has a damaged seal, the label says not to use it.[1] This 28-day limit is about the water bottle. It does not say how long a mixed peptide lasts, which depends on the drug.
What is the difference between bacteriostatic water and sterile water?
The two differences are the preservative and the container. Sterile Water for Injection, USP, in the Hospira label we checked, contains no bacteriostat, antimicrobial agent, or added buffer and is supplied only in single-dose containers.[5] Bacteriostatic water contains benzyl alcohol and is supplied in multiple-dose vials.[1] That is why sterile water is used once. Its label says not to reuse single-dose containers and to discard the unused portion.[5] Bacteriostatic water can be entered more than once, within the CDC limit. The preservative also brings a restriction. The bacteriostatic water label says that where water is needed for preparing medications for newborns, only preservative-free Sterile Water for Injection should be used.[1] Neither product is a stand-in for the other by default. Each label says the water is meant for diluting drugs as the drug's own manufacturer instructs.
| Feature | Bacteriostatic Water for Injection | Sterile Water for Injection |
|---|---|---|
| Contains | Water plus 0.9% or 1.1% benzyl alcohol | Water only; no bacteriostat or antimicrobial |
| Preservative | Yes | No |
| Container | Multiple-dose vial | Single-dose container |
| After first puncture | Date it; CDC says discard within 28 days unless the maker says otherwise | Discard the unused portion |
| Newborns | Not for use | Preservative-free water is what the bacteriostatic label points to |
| Label storage | 20 to 25 degrees C (68 to 77 degrees F) | 20 to 25 degrees C (68 to 77 degrees F) |
| When to use | Only as the drug's instructions say | Only as the drug's instructions say |
Can you use saline instead of bacteriostatic water?
Only if the label of the drug you are mixing says so. The bacteriostatic water label says it is for diluting drugs according to the instructions of the drug's manufacturer, and it warns that some drugs for injection may be incompatible in a given vehicle, or when combined in a vehicle that contains benzyl alcohol. It tells the reader to consult a pharmacist.[1] The same warning applies in reverse: a diluent is not interchangeable just because it is also clear and sterile. The Egrifta WR label, for example, says to use only the diluent provided and does not offer a substitute.[4] For a product with no label, there is no approved instruction to follow. The safe course is to ask the pharmacist who dispensed the product which diluent it needs, instead of choosing from an online chart. This page does not recommend a diluent for any compound.
Who should not use bacteriostatic water?
Newborns must not receive it. The label states NOT FOR USE IN NEONATES and says that because of the potential toxicity of benzyl alcohol, solutions containing it must not be used in that population.[1] The warning has a history. In 1982 the CDC reported 16 neonatal deaths at two medical centers that were thought to be caused by the benzyl alcohol preservative in bacteriostatic saline used to flush catheters. The babies developed metabolic acidosis and gasping respirations, and the FDA recommended that such solutions and diluents with this preservative not be used for newborns.[3] The current label also says parenteral preparations with benzyl alcohol should not be used for fluid replacement, or in epidural or spinal anesthesia procedures, and that intravenous use without a drug added may cause hemolysis.[1] If a newborn is in the household, or anyone may be sensitive to preservatives, ask a pharmacist first.
How do you store bacteriostatic water?
The label says to store it at 20 to 25 degrees Celsius, which is 68 to 77 degrees Fahrenheit, and it does not ask for refrigeration.[1] Use aseptic technique for every entry into the vial, which the label states for both single and multiple withdrawals.[1] Wipe the rubber stopper with a fresh alcohol swab, and use a new sterile needle and syringe each time, because the CDC says that once used, both are contaminated and must be discarded.[2] Date the vial when you first puncture it and discard it within 28 days unless the manufacturer states another date.[2] A quick piece of arithmetic shows why the date, not the volume, usually ends a bottle. A 30 mL vial holds 15 draws of 2 mL, so anyone mixing fewer than 15 vials in 28 days reaches the date before the water runs out. Storage for the mixed peptide is a separate question, covered in the storage guide.
Frequently asked questions
Is bacteriostatic water the same as sterile water?
No. Bacteriostatic water contains benzyl alcohol as a preservative and comes in multiple-dose vials. Sterile water has no preservative and comes in single-dose containers that are discarded after one use.
Does bacteriostatic water need to be refrigerated?
The Hospira label says to store it at 20 to 25 degrees Celsius, which is room temperature, and does not call for refrigeration. Check the label on your own product if it comes from a different maker.
Can you drink it or use it for anything else?
No. It is labeled only as a diluent for injectable drugs, and the label says it should not be used for fluid replacement. Do not drink it or use it as an infusion fluid.
What does the 0.9 percent mean?
It means 0.9 percent benzyl alcohol, which is 9 mg per mL. It is not the same as 0.9 percent saline, which is sodium chloride in water and has no relation to the preservative.
Why is the limit 28 days?
The CDC says an opened multi-dose vial should be dated and discarded within 28 days unless the manufacturer states another date. The bacteriostatic water label itself gives no in-use period.
How do I remember when I opened a vial?
Write the date on the vial the first time you puncture it, with a marker or a label, and check that date before every use. This guide does not claim any app timer for it.
Sources
- DailyMed, National Library of Medicine (Hospira, Inc.). Bacteriostatic Water for Injection, USP. Accessed October 3, 2026.
- Centers for Disease Control and Prevention. Clinical Safety: Preventing Unsafe Injection Practices. Accessed October 3, 2026.
- CDC, Morbidity and Mortality Weekly Report. Neonatal Deaths Associated with Use of Benzyl Alcohol, United States (1982). Accessed October 3, 2026.
- DailyMed, National Library of Medicine (Theratechnologies). Egrifta WR (tesamorelin) prescribing information. Accessed October 3, 2026.
- Pfizer Labeling (Hospira, Inc.). Sterile Water for Injection, USP. Accessed October 3, 2026.
Written and maintained by the team that builds PepTrac. Every calculator on this site runs the same tested math as the app. Read how we research, cite, and update pages in our editorial policy.
