If you have a medicine that comes as a powder, it has to be mixed with a liquid before you can inject it. Two liquids come up again and again: bacteriostatic water and sterile water. They sound alike, and both are clear and sterile, but they are not interchangeable. Here is what sets them apart and how to know which one your medicine calls for.
Key takeaways
- Both are sterile water made for injection. The difference is a preservative.
- Bacteriostatic water contains benzyl alcohol (usually 0.9%), which slows bacterial growth so the vial can be used more than once.
- Sterile water for injection has no preservative and is usually a single-use product.
- Bacteriostatic water must not be used in newborns, according to its label.
- Use the diluent your medicine's label or pharmacist names. Do not swap on your own.
What is bacteriostatic water?
Bacteriostatic Water for Injection, USP, is sterile water with a small amount of benzyl alcohol added. Pfizer's (Hospira) label describes it as containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol "added as a bacteriostatic preservative."
"Bacteriostatic" means it slows or stops bacteria from growing. It does not kill everything, and it does not make a vial safe to use forever. What it does is lower the risk that comes with putting a needle through the same rubber stopper several times.
That is why it is sold in multiple-dose vials. The label says it is made so that "repeated withdrawals may be made" to dilute or dissolve drugs for injection under the skin, into the muscle or into a vein.
What is sterile water for injection?
Sterile Water for Injection, USP, is water that has been purified and sterilized, with nothing added. Because there is no preservative, once the vial is opened, any bacteria that get in can grow. For that reason, preservative-free sterile water is usually packaged for single use, and leftover liquid is thrown away.
Some medicines are designed to be mixed with plain sterile water. This can be because a preservative would interact with the drug, or because the mixed product is meant to be used all at once.
Side-by-side comparison
| Feature | Bacteriostatic water | Sterile water for injection |
|---|---|---|
| Base | Sterile water | Sterile water |
| Preservative | Benzyl alcohol (often 0.9%) | None |
| Typical vial type | Multiple-dose | Usually single-dose |
| Use after opening | Repeated withdrawals, within the time your label or pharmacist allows | Usually used once, leftover discarded |
| Newborns | Not for use, per label | Preservative-free option the label points to for newborns |
| Who decides which to use | The medicine's label and your pharmacist | The medicine's label and your pharmacist |
Safety notes from the label
The bacteriostatic water label includes a few warnings worth knowing:
- Not for newborns. Benzyl alcohol has been linked to serious toxicity in newborns. The label says only preservative-free sterile water should be used for them.
- Not given alone into a vein. The label warns that putting bacteriostatic water into a vein without a dissolved drug "may result in hemolysis," which is damage to red blood cells. It is a mixing liquid, not something to inject by itself.
- Check before use. Do not use it if the liquid is not clear or the seal is broken.
- Mixed drugs follow their own rules. The label says not to store mixed drugs unless the drug's maker says it is OK.
If you have ever had a reaction to benzyl alcohol, tell your prescriber and pharmacist.
How long does an opened vial last?
For opened multi-dose vials, the CDC points to USP standards, which say to discard the vial within 28 days unless the manufacturer gives a different date. That date should never go past the printed expiration date.
Two things to keep in mind:
- The water vial and the mixed medicine vial are different. Your mixed medicine may need to be used much sooner than 28 days. Its own label or your pharmacist's instructions decide that. Our post on how long a mixed peptide vial lasts covers this.
- Write the date on the vial the first time you open it. The PepSteady app can also log the opening date and remind you when it is time to replace it.
Which one does your medicine need?
The answer comes from the medicine, not from personal preference. Look in three places:
- The package insert or label. FDA-approved powders, such as tesamorelin (Egrifta) or hCG, come with mixing directions that name the diluent. Some include the diluent in the box.
- Your pharmacy label. A compounding pharmacy should tell you exactly what to mix with and how much.
- Your pharmacist. If anything is unclear, call before mixing. It is a quick question for them.
Changing the liquid can change how stable the medicine is and how long it is good for. It can also matter for people with sensitivities. So even if both waters are "sterile," use the one you were told to use.
Handling tips for either type
- Wash your hands and wipe the rubber stopper with an alcohol pad each time.
- Use a new, sterile needle and syringe for every draw. CDC is clear that used needles and syringes are contaminated.
- Store the water as its label directs. The bacteriostatic water label lists room temperature, 68 to 77°F (20 to 25°C).
- Never use a vial that looks cloudy, has particles, or has a damaged seal.
When you are ready to mix, our step-by-step guide on how to reconstitute a peptide vial walks through the process.
The bottom line
Bacteriostatic water and sterile water are both sterile, but only bacteriostatic water has a preservative that allows more than one use. Each has its place. Follow the diluent named on your medicine's label or by your pharmacist, date every vial you open, and ask before you substitute.
Sources
- Pfizer: Bacteriostatic Water for Injection, USP prescribing information
- Pfizer Medical: Sterile Water for Injection, bacteriostatic vial
- CDC: Injection safety for clinicians (single-dose and multi-dose vials)
Frequently asked questions
What is the difference between bacteriostatic water and sterile water?
Both are sterile water made for injection. Bacteriostatic water also contains a small amount of benzyl alcohol, a preservative that slows bacterial growth so the vial can be entered more than once. Sterile water for injection has no preservative and is usually meant for one-time use.
Can I use sterile water instead of bacteriostatic water?
Only if the medicine's label or your pharmacist says so. Some products must be mixed with one specific diluent, and changing it can affect stability, storage time or safety. Ask before you substitute.
How long can a bacteriostatic water vial be used after opening?
CDC, citing USP standards, says opened multi-dose vials are generally discarded within 28 days unless the manufacturer gives a different date. Your pharmacist or the product label may give a shorter time.