"BAC water" is the common shorthand for Bacteriostatic Water for Injection, USP. It is the liquid most often used to dissolve lyophilised peptide powder. This guide sticks to what the FDA-labelled product says and what regulators publish. It is an educational reference and gives no compound-specific dosing.
Guide
BAC Water (Bacteriostatic Water): What It Is, Label Facts and Handling Limits
What bacteriostatic water contains, what its FDA label says about use, neonates, storage and endotoxin limits, and the 28-day multi-dose rule.
By Sam, MBA. Human-researched and AI-assisted with full editorial review. Last reviewed October 2026.
What it is
Bacteriostatic Water for Injection, USP is sterile, nonpyrogenic water for injection with a preservative added. One manufacturer's label describes it as containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) of benzyl alcohol as a bacteriostatic preservative.[1] A bacteriostatic agent is one that stops bacteria from reproducing; it does not necessarily kill them, and surviving bacteria can multiply again if the agent is removed.[6] It is therefore a preservative, not a sterilant.
The label states it is indicated only for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection.[1] The only labelled use is as a diluent.
How it is supplied
The same label lists multiple-dose vials: a 30 mL plastic Fliptop vial at 9 mg/mL benzyl alcohol (NDC 0409-3977-03) and a 20 mL fill in a 30 mL glass Fliptop vial at 11 mg/mL benzyl alcohol (NDC 0409-1093-04).[1] The storage statement is to store at 20 to 25 °C (68 to 77 °F).[1] Check the NDC and strength on the bottle you hold against the label of that manufacturer; other manufacturers' products may differ.
The neonate warning
The label carries a boxed warning: NOT FOR USE IN NEONATES. It explains that benzyl alcohol, the preservative, has been associated with toxicity in neonates.[1] Preserved diluent is therefore not interchangeable with preservative-free sterile water for every use.
When the label calls for sterile water instead
Not every lyophilised peptide product uses BAC water. Two labels for the same peptide show both choices:
- EGRIFTA SV (2 mg vial): use only the supplied Sterile Water for Injection, add 0.5 mL, give immediately after reconstitution and throw away any unused solution and diluent.[5]
- EGRIFTA WR (11.6 mg vial): use only the supplied Bacteriostatic Water for Injection, add 1.3 mL; one reconstituted vial provides daily doses for 7 consecutive days, after which unused solution is discarded.[4]
The first label is a single-use design and the second allows 7 days of daily withdrawals.[4][5] The principle for any compound is to use the diluent that the product's own documentation names. If no documentation exists, no number on a website, including this one, substitutes for it. See how to reconstitute peptides.
The 28-day rule
CDC's multi-dose vial guidance, which cites USP General Chapter 797, says the vial should be dated and discarded within 28 days.[2] The EGRIFTA WR instructions for use say the same for the supplied bottle of Bacteriostatic Water for Injection: throw it away 28 days after use begins.[4] CDC also advises never to leave a needle inserted in a vial septum for multiple uses and to use a new sterile syringe and needle each time.[2]
Practical consequences, all derived from those sources:
Does BAC water change the peptide vial's shelf life?
The Pfizer label states not to store reconstituted solutions of drugs for injection unless otherwise directed by the manufacturer of the solute.[1] In other words, the diluent's own 28-day limit does not extend the life of a dissolved drug.[2][1] The EGRIFTA WR label's 7-day discard after mixing is a product-specific figure for that drug, not a property of the diluent.[4] For most research peptides there is no manufacturer statement, so no label-derived shelf life is available.
Endotoxin limits
FDA's archived inspection guide describes the bacterial endotoxins test as the Limulus Amebocyte Lysate (LAL) test, defines the endotoxin unit (EU) as the unit of endotoxin activity, and states that Water for Injection has an allowable endotoxin limit of 0.25 EU/mL, with Bacteriostatic Water for Injection given a slightly higher limit of 0.5 EU/mL.[3] That guide dates from 1985,[3] and the current pharmacopeial limits should be checked against the current USP chapter. The relevance here is that a diluent is itself a tested pharmaceutical product with an endotoxin specification, which is one reason to use labelled, lot-numbered diluent rather than improvised water. A peptide COA that reports an endotoxin result should be read against the sample's own context; see how to read a COA.
Common questions
Is bacteriostatic water the same as sterile water?
No. Both are sterile, but the bacteriostatic product contains benzyl alcohol and is supplied for multiple withdrawals, while sterile water for injection is labelled as the diluent in single-use designs such as the EGRIFTA SV kit.[1][5]
Can I use it past 28 days if it looks clear?
The CDC guidance cited here says to discard multi-dose vials within 28 days.[2] Appearance is not evidence of sterility.
Is there a calculator for diluent volume?
Yes. The BAC water tool and the reconstitution calculator compute volumes. They do not choose a diluent for you; the product's label does.
Evidence note
Every figure above comes from an FDA-type label, a CDC page or an FDA inspection guide, which are grade A or regulator documents on this site's scale. The limitations are the age of the 1985 endotoxin guide[3] and the fact that label wording differs by manufacturer.
References
- Pfizer / Hospira: Bacteriostatic Water for Injection, USP labeling
- CDC: Injection safety, clinical safety recommendations (multi-dose vials)
- FDA Inspection Technical Guide: Bacterial endotoxins / pyrogens (archived 1985 guide)
- EGRIFTA WR (tesamorelin) US prescribing information, FDA, revised March 2025
- EGRIFTA SV (tesamorelin) US label, DailyMed
- Wikipedia: Bacteriostatic agent
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