Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative, sold in multiple-dose vials. Sterile water for injection has no preservative and is single-dose: once a vial is entered, the rest is discarded. That preservative is the whole difference, and it decides whether a vial can be used more than once.
Notice: Research peptides are sold for in-vitro laboratory research only. They are not FDA-approved, not for human or animal consumption, and this page is not medical advice.
Side-by-side comparison
| Bacteriostatic Water for Injection, USP | Sterile Water for Injection, USP | |
|---|---|---|
| What it is | Sterile water + benzyl alcohol | Sterile water only |
| Preservative | 0.9% (9 mg/mL) benzyl alcohol, some labels also list 1.1% | None ("no antimicrobial agents or other preservatives") |
| Container | Multiple-dose vial | Single-dose vial |
| After first use | Repeated withdrawals allowed | Discard the unused portion immediately |
| Neonatal warning | "NOT FOR USE IN NEONATES" | Not applicable (no benzyl alcohol) |
| Label storage | 20 to 25°C (68 to 77°F) | 20 to 25°C (68 to 77°F) |
Sources: Hospira bacteriostatic water label and American Regent sterile water label on DailyMed, checked September 26, 2026.
What bacteriostatic water contains
The Hospira label for Bacteriostatic Water for Injection, USP describes it as water with "0.9% (9 mg/mL) or 1.1% (11 mg/mL) of benzyl alcohol added as a bacteriostatic preservative," with a pH of 5.7 (range 4.5 to 7.0) (DailyMed). A second label, for a 30 mL vial distributed by Henry Schein, uses the same wording (DailyMed).
"Bacteriostatic" means the preservative inhibits bacterial growth. It does not make the water self-sterilizing. It lowers the risk from the small contamination that each needle entry can introduce, which is why the product can be sold in a "multiple-dose container from which repeated withdrawals may be made" (DailyMed).
The same label warns that intravenous administration of bacteriostatic water "without a solute may result in hemolysis." It is a diluent, not a product meant to be used on its own.
What sterile water contains
American Regent's label describes Sterile Water for Injection, USP as "water for injection sterilized and packaged in single dose vials. It contains no antimicrobial agents or other preservatives." It adds: "Unused amount should be discarded immediately following withdrawal of any portion of vial contents" (DailyMed).
The label also marks it "NOT FOR DIRECT INJECTION" because it is hypotonic. Like bacteriostatic water, it is meant as a diluent.
Multi-use vs single-use: why it matters
With no preservative, nothing slows the growth of any microbe introduced when a needle goes through the stopper. That is why sterile water is single-dose. CDC's rule for single-dose vials is that leftover contents are not kept for later use (CDC).
For peptide work this has a practical effect. If you reconstitute several vials over a few weeks from one diluent vial, a single-dose sterile water vial is meant to be discarded after the first draw. A bacteriostatic water vial is designed for repeated entries.
The 28-day rule: where it comes from
The 28-day figure is widely repeated, but it is not printed on the two bacteriostatic water labels we checked on DailyMed. It comes from CDC's clinical guidance on multi-dose vials:
"Once a multi-dose vial is opened (e.g., needle-punctured) the vial should be dated and discarded within 28 days unless the manufacturer states another date for that opened vial." (CDC)
CDC adds that this date should never go past the manufacturer's original expiration date. In practice:
- Write the date on the diluent vial the first time you puncture it.
- Discard it 28 days later, or sooner if the manufacturer says so.
- Discard it earlier if it looks cloudy, has particles, or may have been contaminated.
The 28-day rule applies to the opened diluent vial. It is not a stability claim about a peptide dissolved in that water, which depends on the peptide itself. See how to store peptides.
The benzyl alcohol neonatal warning
Both bacteriostatic water labels state "NOT FOR USE IN NEONATES" and explain: "Due to the potential toxicity of benzyl alcohol in neonates, solutions containing benzyl alcohol must not be used in this patient population" (DailyMed).
The history is documented in an FDA review file. It records that in 1982 investigators linked flush solutions containing 0.9% benzyl alcohol to severe metabolic acidosis and respiratory depression with gasping, "leading to the death of 16 infants in neonatal intensive care units," and that in May 1982 "FDA in conjunction with the American Academy of Pediatrics (AAP) and CDC issued a bulletin" warning against benzyl alcohol-preserved fluids in newborns (FDA). The same document notes that infants are more vulnerable because their metabolic pathways are immature. The original report appeared in the New England Journal of Medicine in 1982.
For lab work, the point is simpler: bacteriostatic water is not "just water." It contains a preservative with known toxicity in some settings.
When each is used in labs
| Situation | Typical choice | Why |
|---|---|---|
| Reconstituting a vial that will be entered repeatedly | Bacteriostatic water | Preservative supports repeated withdrawals |
| One-time stock solution used immediately | Sterile water | No preservative needed if used at once |
| Assays where benzyl alcohol could interfere | Sterile water or a buffer | Avoids adding an extra chemical |
| Peptides that don't dissolve in water | Buffer or dilute acid per supplier | Solubility, not sterility, is the problem |
Supplier handling guides for research peptides typically start with water. Sigma-Aldrich suggests "sterile distilled water or sterile dilute acetic acid (0.1%)" for stock solutions (Sigma-Aldrich), and Bachem advises matching buffer pH to the peptide's charge (Bachem).
Buying diluent: check the label
- Look for "USP." Both products above are sold as USP-grade for injection, with a lot number and expiration date.
- Confirm the product name. Bacteriostatic water and bacteriostatic sodium chloride are different products.
- Check the preservative line. It should state the benzyl alcohol percentage.
- Note the container type. "Single-dose" and "multiple-dose" mean what they say.
Doing the math
Diluent choice does not change the concentration arithmetic: peptide mass ÷ volume added. A 5 mg vial with 2 mL of either diluent is 2.5 mg/mL. Use the peptide reconstitution calculator or the BAC water calculator, and see how to reconstitute peptides for the full procedure.
Start with a vendor you can verify
The diluent is the easy part. Whether the vial holds what its label says depends on the vendor. Our ranking of the best place to buy peptides online scores US vendors on testing, COAs and labeling. It is an editors' ranking, not a sort by score (how we rank).
Related: how long reconstituted peptides last.
Sources are linked inline. Label details checked on DailyMed September 26, 2026.