Guides / Mixing and syringes

Bacteriostatic water vs sterile water vs saline

All three are sterile. The difference is the preservative: bacteriostatic water has one so a vial can be used more than once, and the other two don't.

By PeptideTracker editorial team · Updated November 8, 2026 · 12 sources

[00] Short answer

  1. Bacteriostatic water is sterile water with 0.9% (9 mg/mL) benzyl alcohol added as a preservative, in a multiple-dose vial1.
  2. Sterile water for injection has no preservative and comes only in single-dose containers. The label says to throw away what's left2.
  3. 0.9% saline has 9 mg of salt per mL. Plain saline is preservative-free and single-dose3; bacteriostatic saline adds 0.9% benzyl alcohol4.
  4. An opened multiple-dose vial is discarded within 28 days unless the maker gives another date5–6.
  5. Benzyl alcohol labels say not for use in newborns1,4.

What's the difference between bacteriostatic water and sterile water?

Bacteriostatic water contains 0.9% benzyl alcohol, a preservative that lets you draw from the same vial more than once. Sterile water for injection contains no bacteriostat or antimicrobial agent and is single-dose only1–2. Saline is the same idea with salt added.

Four common diluents, from their US labels (Hospira)
DiluentWhat's in each mLPreservativeContainerAfter opening
Bacteriostatic Water for InjectionWater plus 9 mg benzyl alcohol (0.9%)1Benzyl alcoholMultiple-dose1Repeated withdrawals allowed1
Sterile Water for InjectionWater only2NoneSingle-dose2Don't reuse. Discard unused portion2
0.9% Sodium Chloride Injection9 mg sodium chloride3NoneSingle-dose3Don't reuse. Discard unused portion3
Bacteriostatic 0.9% Sodium Chloride9 mg sodium chloride plus 9 mg benzyl alcohol4Benzyl alcoholMultiple-dose4Repeated withdrawals allowed4

One Hospira bacteriostatic water vial (20 mL in a 30 mL glass vial) uses 1.1% (11 mg/mL) benzyl alcohol instead of 0.9%1. Check the strength on your own bottle.

None of these is meant to be injected on its own. Both water labels say they're only for diluting or dissolving a drug, and the label on the largest sterile water vial carries the words "not isotonic" and "hemolytic"1–2.

UK note: in the UK, plain sterile water is labeled "water for injections" or "sterilised water for injections". The SmPC we checked says "for single use"12.

Which water should you use to mix a medicine?

Use the diluent your medicine's label names. The bacteriostatic water label says it's for mixing drugs according to the drug maker's instructions, and warns that some drugs don't mix well with benzyl alcohol1. Approved products show how much the choice varies:

Diluents named on approved labels (US prescribing information)
ProductDiluent on the labelHow it's used
EGRIFTA SV (tesamorelin)Sterile Water for Injection8Inject right after mixing. Throw away unused solution and water8
EGRIFTA WR (tesamorelin)Bacteriostatic Water for Injection9One vial gives daily doses for 7 days, then discard9
Zomacton 5 mg (somatropin)Bacteriostatic saline with 0.9% benzyl alcohol10Up to 14 days in the fridge after mixing10
Zomacton 10 mg (somatropin)Bacteriostatic water with 0.33% metacresol10Up to 28 days in the fridge after mixing10

Bacteriostatic doesn't always mean benzyl alcohol. Zomacton 10 mg comes with water preserved with metacresol instead.

Both EGRIFTA labels say to use only the diluent provided8–9. Research peptides have no label, so nobody has published which water suits them. For the mixing steps, see how to reconstitute peptides.

How long can you use bacteriostatic water once it's opened?

Up to 28 days after the first needle goes in, unless the maker gives another date. CDC says an opened multi-dose vial should be dated and thrown away within 28 days6, and USP <797> sets the same 28-day limit for multiple-dose containers unless their labeling says otherwise5.

  • The 28 days is a default. The Zepbound multi-dose vial, which contains benzyl alcohol and phenol, has its own rule: throw it away 30 days after first use or after 4 weekly doses, even with medicine left7.
  • The preservative has limits. CDC notes it has no effect on viruses and doesn't give complete protection against bacteria6. Clean the stopper and use a new needle every time.
  • Single-dose means once. CDC says not to keep leftovers from a single-dose vial, even for the same person6.
  • The water and the mixed vial can have different clocks. See how long mixed peptides last.

Is the benzyl alcohol in bac water safe?

For adults, the bac water label says reactions to injected 0.9% benzyl alcohol are not known to occur in people1. The firm warnings are about newborns. Benzyl alcohol has been linked to toxicity in neonates, so the label says solutions containing it must not be used for them1.

  • Why newborns. In 1982 the CDC reported 16 deaths in preterm babies whose catheters were flushed every day with bacteriostatic saline. Estimated intake was 99 to 405 mg/kg a day11. Labels call this the "gasping syndrome"10.
  • Pregnancy and breastfeeding. The Zomacton label says fetal exposure is unlikely because the mother breaks benzyl alcohol down quickly. It still tells pregnant or breastfeeding women to mix with plain saline instead, use one dose per vial, and discard the rest10.
  • Allergy. The same label says not to use the benzyl alcohol diluent if you have a known hypersensitivity to it10.
  • Other limits on the label. Not for epidural or spinal use, and not for fluid replacement1.

Questions

Can I use sterile water instead of bacteriostatic water?

Only if your medicine's label allows it, and only for one dose. Sterile water has no preservative, so its label says don't reuse the container and throw away what's left.

Is bacteriostatic water the same as saline?

No. Bacteriostatic water is water with 0.9% benzyl alcohol. Saline is 0.9% salt (9 mg per mL). Bacteriostatic saline has both salt and 0.9% benzyl alcohol.

What percentage of benzyl alcohol is in bac water?

Usually 0.9% (9 mg per mL). One Hospira vial uses 1.1% (11 mg per mL), so check your own bottle.

How long is bacteriostatic water good for after opening?

Up to 28 days after first puncture, per CDC and USP <797>, unless the maker gives a different date. Never past the printed expiry.

Can I use bac water if I'm pregnant?

Ask your prescriber. One drug label (Zomacton) says fetal exposure is unlikely but still tells pregnant or breastfeeding women to mix with plain saline and use one dose per vial.

Why can't bac water be used for babies?

Benzyl alcohol has been linked to deaths in preterm newborns. A 1982 CDC report described 16 cases after catheters were flushed with bacteriostatic saline.

Sources

Checked against US labels for Hospira Bacteriostatic Water, Sterile Water, 0.9% Sodium Chloride and Bacteriostatic 0.9% Sodium Chloride for Injection, USP <797> FAQs (2023), CDC injection safety guidance, and drug labels that name a diluent. How we check

  1. 1Bacteriostatic Water for Injection, USP (Hospira): prescribing information. revised 11/2019; description, contraindications, warnings, adverse reactions
  2. 2Sterile Water for Injection, USP (Hospira), single-dose vials: label (DailyMed). revised 03/2026
  3. 30.9% Sodium Chloride Injection, USP (Hospira), preservative-free: label (DailyMed). revised 08/2021
  4. 4Bacteriostatic 0.9% Sodium Chloride Injection, USP (Hospira): label (DailyMed). revised 02/2019
  5. 5USP <797> Pharmaceutical Compounding: Sterile Preparations, FAQs (updated 11 Dec 2023). FAQs 193 and 194
  6. 6CDC, Preventing Unsafe Injection Practices. single-dose and multi-dose vials, 26 Mar 2024
  7. 7Zepbound (tirzepatide) injection, US prescribing information (DailyMed). Eli Lilly, Aug 2026; sections 11 and 16.2
  8. 8EGRIFTA SV (tesamorelin for injection) US prescribing information (DailyMed). Theratechnologies, Jul 2026; section 2
  9. 9EGRIFTA WR (tesamorelin for injection) US prescribing information (DailyMed). Theratechnologies, Jul 2026; section 2
  10. 10Zomacton (somatropin) for injection, US prescribing information (DailyMed). Ferring, Apr 2026; sections 2.4, 5.13, 8.1 and 8.4
  11. 11CDC. Neonatal deaths associated with use of benzyl alcohol, United States. MMWR 1982;31(22):290-1. 11 Jun 1982
  12. 12Sterilised water for injections (Bio Products Laboratory), UK SmPC (emc). revised 04/2025

This guide explains what labels and published studies say. It isn't medical advice and doesn't replace your prescriber or pharmacist.

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