What Is Bac Water? Bacteriostatic Water Uses and Mixing
What is bac water? It is short for bacteriostatic water for injection: sterile water with a small amount of benzyl alcohol (0.9% in the Hospira 30 mL vial) that slows the growth of bacteria once the bottle is opened. Its only job is to dissolve or thin out another drug, such as a freeze-dried peptide, before it is injected.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert
Short answer
- What is in it?
- Water for injection plus a preservative. On the Hospira label, that preservative is benzyl alcohol.
- What is it for?
- Mixing or thinning drugs given into a vein, a muscle or under the skin. It is not a medicine by itself.
- Who should not get it?
- Newborn babies. The label warns against it because of the benzyl alcohol.
- Does it need the fridge?
- No. The label stores it at room temperature, 20 to 25 C.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert
What is in bacteriostatic water?
Two things: water made for injection, and a preservative. In the Hospira product, the preservative is benzyl alcohol. Its 30 mL plastic vial has 0.9% (9 mg in each mL). Its 20 mL glass vial has 1.1% (11 mg in each mL).
Not every maker uses benzyl alcohol. An older Fresenius Kabi label used two parabens instead: 0.12% methylparaben and 0.012% propylparaben. So read the label, not just the name on the box.
The Hospira label gives a pH of 5.7, with a range of 4.5 to 7.0. That means it is slightly acidic. The word bacteriostatic means it slows bacteria down rather than killing them. It does not make a dirty vial or powder clean. The CDC adds that the preservative has no effect on viruses and does not fully protect against bacteria.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert; Fresenius Kabi / APP bacteriostatic water label (parabens); CDC page on unsafe injection practices
What is bac water used for?
The label gives it one use. It is only for diluting or dissolving drugs that go into a vein, a muscle or under the skin, following the instructions of the drug's own maker.
Several approved drugs come with it or call for it. Three examples from their US labels:
- EGRIFTA WR (tesamorelin, a peptide) comes in a box with bac water. The label says to use only that water, 1.3 mL for each vial.
- Novarel (hCG) is mixed with bac water that has 0.9% benzyl alcohol.
- Serostim (a growth hormone) mixes its 4 mg vial with 0.5 to 1 mL of bac water.
Research peptides arrive as a dry powder, so they are mixed the same way. The water you add sets how strong each mL is. Our guides show what that means in syringe units for a 20 mg retatrutide vial and a 50 mg GHK-Cu vial.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert; DailyMed: tesamorelin (EGRIFTA WR) kit label; NOVAREL (chorionic gonadotropin) label, DailyMed; SEROSTIM (somatropin) label, DailyMed
How do you read a bac water label?
We read 9 top search results in full on 22 October 2026, and none walked through a label line by line. Here is a real one, the Hospira 30 mL vial, read line by line. Each line is printed on the vial, on the tray of 25 vials, or in the leaflet that comes with them.
| Label line | Where | What it means |
|---|---|---|
| BACTERIOSTATIC WATER for Injection, USP | Vial | The product name. USP tells you it meets the official US Pharmacopeia rules for this product. |
| 30 mL Multiple-dose | Vial | How much is in the bottle, and that it is made to be entered more than once. |
| 0.9% benzyl alcohol added | Vial | The preservative. 0.9% is 9 mg in every mL. Hospira’s 20 mL glass vial says 1.1% instead. |
| NOT FOR USE IN NEONATES | Vial and tray | Never for newborn babies. Benzyl alcohol has been linked to a deadly reaction in newborns called gasping syndrome. |
| LOT and EXP | Vial | The batch number and the expiry date of the sealed bottle. |
| Rx only | Tray | In the United States it is sold on prescription. |
| FOR DRUG DILUENT USE ONLY | Tray | It exists to dissolve or thin out another drug. It is not meant to be injected on its own. |
| NDC 0409-3977-03 | Tray | The US drug code for this exact product and pack: a tray of 25 vials. Each single vial carries its own code, 0409-3977-01. |
| Store at 20 to 25°C | Package insert | Keep it at room temperature, which is 68 to 77 F. The label does not ask for a fridge. |
| The pH is 5.7 (4.5 to 7.0) | Package insert | The water is slightly acidic, and each batch must fall in that range. |
What is not on the label
There is no 28-day line. We read all seven US bac water labels listed on DailyMed, from Hospira and six other companies, and none prints a number of days after opening. The 28 days people quote comes from CDC guidance on opened multi-dose vials, covered below.
A bottle sold as mixing water or reconstitution solution may not list a preservative at all. If the preservative line is missing, you cannot tell from the bottle that it is bac water. To check a bottle and a peptide together, see bacteriostatic water quality checks.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert; DailyMed listings for bacteriostatic water, read 22 October 2026; Nine top results for "what is bac water" and "bac water" read in full on 22 October 2026: thepeptide.app, asterlane.ghost.io, peptideplotter.com, wittmerrejuvenationclinic.com, peppal.app, arpovohealth.com (two pages), thepeptidecatalog.com, nbinno.com; Gasping syndrome and benzyl alcohol, N Engl J Med 1982, PMID 7133084; CDC page on unsafe injection practices
Is bac water the same as sterile water or saline?
No. Sterile water for injection has no preservative, and its label sells it only in single-dose containers. The Hospira bac water label itself points to it for newborns: when water is needed for a newborn's medicine, only preservative-free sterile water should be used. Saline is salt water, a different product again.
The full side by side, including pH, is in bac water vs sterile water.
Sources: Pfizer / Hospira Sterile Water for Injection label; DailyMed: Hospira bacteriostatic water vial, tray and insert
How long does bac water last?
- Sealed: until the EXP date on the vial.
- Opened: the labels print no limit. The CDC tells clinics to write the date on an opened multi-dose vial and discard it by day 28, unless the maker gives another date, and never after the expiry date.
- With a drug mixed in: the drug's label decides, and the answers differ a lot. The EGRIFTA WR label keeps its mixed vial at room temperature and throws it out after 7 days. The Novarel label says to refrigerate its mixed solution and use it within 30 days.
Does bac water need to be refrigerated? No, the label keeps it at 20 to 25 C. The full storage rules, before and after opening, are in does bacteriostatic water need to be refrigerated.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert; CDC page on unsafe injection practices; DailyMed: tesamorelin (EGRIFTA WR) kit label; NOVAREL (chorionic gonadotropin) label, DailyMed
How do you mix a drug with bac water?
The Hospira label sets out the handling rules. In plain words:
- Use germ-free handling every time a needle goes into the bottle.
- Use the amount of water the drug's maker gives. The bac water label does not set it.
- Mix well and use it promptly.
- Do not keep a mixed drug for later unless its maker's label allows that.
- Look before you use it. The liquid should be clear, with no specks and no change in color.
- Do not use the bottle unless the liquid is clear and the seal is intact.
Labels for approved peptides add their own steps. EGRIFTA WR, for example, says to swirl the vial and not to shake it.
One 30 mL bottle goes a long way. It holds 30 full 1 mL fills, 15 fills of 2 mL, or 10 fills of 3 mL.
To turn the water you add into syringe units, use the peptide reconstitution calculator.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert; DailyMed: tesamorelin (EGRIFTA WR) kit label
Who should not use bac water, and can you inject it alone?
Not for newborns. Benzyl alcohol has been linked to a deadly reaction in newborn babies, first described in 1982 as gasping syndrome. The label says solutions with benzyl alcohol must not be used in newborns.
The Hospira label lists these other limits:
- It is not for replacing lost body fluid.
- It is not for epidural or spinal anesthesia.
- Given into a vein on its own, with nothing dissolved in it, it can break down red blood cells (hemolysis). The label says other ingredients must first bring it to about the same strength of dissolved substances as body fluids.
- In pregnancy, the label says it should be given with added drugs only if clearly needed.
So bac water is not something to inject by itself. It is a mixing liquid. Anyone who reacts to benzyl alcohol should raise it with a clinician before using a drug mixed in it.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert; Gasping syndrome and benzyl alcohol, N Engl J Med 1982, PMID 7133084
Where can you get bac water?
In the United States, the label marks it Rx only, so pharmacies hand it out on prescription. Other countries set their own rules. Whatever the source, read the label lines above before you use a bottle.
Disclosure: Some links on this page are referral links. Peptides Vietnam may earn a fee at no cost to you. Editorial content is never sold.
In Vietnam, Peptara Labs sells bacteriostatic water for mixing peptides. Check its preservative line against the table above when it arrives.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert
Common questions
What does bac water stand for?+
Bac water is short for bacteriostatic water for injection. The word bacteriostatic describes its job: it slows the growth of bacteria. The benzyl alcohol in it does that job, which is why one bottle can be entered more than once. It does not stop every germ: the CDC says the preservative does nothing against viruses.
Can you inject bac water by itself?+
No. The Hospira tray label says it is for drug diluent use only, and the insert says to use it only as a diluent or solvent. Given into a vein with nothing dissolved in it, the label says it can damage red blood cells (hemolysis). It is also not for fluid replacement or for epidural or spinal use.
How long is bac water good for after opening?+
The bac water labels we read print no number of days after opening. The CDC sets 28 days for an opened multi-dose vial unless its maker states otherwise, and never beyond the printed expiry. A drug mixed in it follows that drug’s own label instead.
Does bac water need to be refrigerated?+
No. The Hospira label stores it at 20 to 25 C, which is room temperature. Once another drug is mixed in, that drug’s label decides. For example, the Novarel label says its mixed solution should be refrigerated and used within 30 days.
Is bac water the same as sterile water?+
No. Both are sterile, but only bac water has a preservative. The sterile water label sells it only in single-dose containers. The Hospira bac water label also says that when water is needed for newborns, only preservative-free sterile water should be used.
How much bac water do you add to a peptide?+
The bac water label leaves that to the maker of the drug being mixed. For example, the EGRIFTA WR label uses 1.3 mL for one vial of tesamorelin. For research peptides with no label, the amount of water only sets how strong each mL is, and a reconstitution calculator turns that into syringe units.
Sources: DailyMed: Hospira bacteriostatic water vial, tray and insert; Pfizer / Hospira Sterile Water for Injection label; CDC page on unsafe injection practices; DailyMed: tesamorelin (EGRIFTA WR) kit label; NOVAREL (chorionic gonadotropin) label, DailyMed
Related reading
Bac Water vs Sterile Water
The preservative, pH and the newborn rule
Does Bac Water Need the Fridge?
Storage before and after opening
Peptide Reconstitution Calculator
Water and vial size to syringe units
Bac Water for 20mg Retatrutide
Units for every trial dose
Bac Water for 50mg GHK-Cu
Units for 50 mg and 100 mg vials
Bac Water Quality Checks
What good and bad water looks like
Peptides Vietnam offers this page as general background, not medical advice. It sums up what US drug labels and CDC guidance say about bacteriostatic water. For any drug you mix, follow that drug's label and ask a licensed pharmacist.