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Standard applied: USP Bacteriostatic Water for Injection monograph and FDA 21 CFR 211 sterile compounding requirements.
Conflict of interest: FormBlends sells peptide products but does not sell benzalkonium chloride or bac water. No affiliate links on this page.
Last reviewed: 2026-05-29.
Key Takeaways
- The USP specification for bacteriostatic water for injection is 0.9 mg of benzalkonium chloride per mL (0.09% w/v), not 0.9% w/v. Confusing these two concentrations is the most common error on chemistry forums.
- A 0.22-micron sterilizing filter is the minimum required step for any aqueous injectable preparation, but it does not remove endotoxins. Depyrogenation is a separate, harder problem home preparers cannot solve.
- Commercial USP bacteriostatic water costs roughly $3 to $8 per 30 mL vial. The safety gap between commercial and home-prepared product is large. The cost gap is not.
- In the United States, compounding injectable solutions for human use outside a licensed 503A or 503B pharmacy violates federal law regardless of personal use intent.
- Sterile water for injection and sterile normal saline are both legitimate single-use alternatives that avoid the preservative question entirely.
Direct Answer: How to Make Bac Water
Making bacteriostatic water requires sterile water for injection, pharmaceutical-grade benzalkonium chloride at 0.9 mg per mL final concentration, sterile multi-dose vials, and sterilizing filtration through a 0.22-micron filter. The process is achievable in principle but cannot meet USP sterility or endotoxin standards outside a licensed sterile compounding environment.
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- What exactly is bac water?
- Can you actually make bac water at home?
- What is the correct benzalkonium chloride concentration?
- What equipment do you need?
- Step-by-step preparation process
- What most pages get wrong about making bac water
- Why the 0.22-micron filter rule exists (and its limits)
- Evidence ledger
- Head-to-head: bac water vs. alternatives
- Label and COA literacy: how to judge what you have
- FAQ
What Exactly Is Bac Water?
Bacteriostatic water for injection (USP) is sterile, pyrogen-free water containing benzalkonium chloride as a preservative at a concentration of 0.9 mg/mL (0.09% w/v). The preservative inhibits bacterial growth in the solution, which is why the same vial can be accessed multiple times with a needle over up to 28 days after first puncture, unlike single-use sterile water for injection.
Benzalkonium chloride is a quaternary ammonium compound. It disrupts bacterial cell membranes by intercalating into the lipid bilayer, raising membrane permeability and causing cell contents to leak. At 0.9 mg/mL it is bacteriostatic, meaning it suppresses growth rather than sterilizing the solution. The solution itself must already be sterile before the preservative is added.
Bac water is used almost exclusively as a reconstitution diluent for lyophilized (freeze-dried) injectables, including peptides, hormones such as HCG and HGH, and some research compounds. It is not used for direct intravenous infusion in large volumes.
Can You Actually Make Bac Water at Home?
The honest answer is: you can make a solution that contains the right preservative at roughly the right concentration, but you cannot replicate the three properties that make commercial USP bac water safe for injection.
Those three properties are: confirmed sterility (validated by sterility testing per USP <71>), a defined endotoxin limit (tested per USP <85> Bacterial Endotoxins Test), and a validated container closure system. None of these can be confirmed at home. The 0.22-micron filter step eliminates most bacteria and particles but does not address endotoxins at all.
What Is the Correct Benzalkonium Chloride Concentration?
This is the most numerically important section on this page. The USP monograph for Bacteriostatic Water for Injection specifies a benzalkonium chloride concentration of 0.9 mg per mL of final solution. That is 0.09% w/v.
A large number of hobbyist guides, chemistry forums, and even some vendor pages write this as "0.9% benzalkonium chloride." That is wrong by a factor of 10. A 0.9% w/v benzalkonium chloride solution would contain 9 mg/mL, which is well above the bacteriostatic range, potentially cytotoxic to tissue at the injection site, and far outside the USP specification.
Practical calculation for a 30 mL vial:
- Target: 0.9 mg/mL x 30 mL = 27 mg total benzalkonium chloride
- If using a 50% w/v benzalkonium chloride stock solution: 27 mg divided by 500 mg/mL = 0.054 mL (54 microliters) of stock
- QS (quantity sufficient) to 30 mL with sterile water for injection
Because the volume of stock solution added is tiny (under 0.1 mL for a 30 mL vial), measurement error is significant. A calibrated micropipette is more accurate than a standard 1 mL syringe for this step.
What Equipment Do You Need?
| Item | Specification | Why It Matters |
|---|---|---|
| Sterile water for injection | USP grade, single-use vial | Base solvent must be sterile and pyrogen-tested before you start |
| Benzalkonium chloride solution | Pharmaceutical-grade, known concentration (e.g. 50% w/v USP) | Industrial-grade BKC contains impurities unsuitable for injection |
| Sterile multi-dose vials | Depyrogenated glass, rubber septum | Container must not re-introduce endotoxins or particles |
| 0.22-micron syringe filter | PES or PVDF membrane, sterile, single-use | Sterilizing filtration step; do not substitute 0.45-micron filters |
| Sterile syringes | Luer-lock, appropriate volume | Prevent introduction of particulates and non-sterile air |
| Micropipette (optional but recommended) | Calibrated 10-200 uL range | Accurate measurement of small BKC stock volumes |
| Isopropyl alcohol 70% | USP grade preferred | Surface and septum disinfection; 70% more effective than 90% against most bacteria |
| Nitrile gloves | Powder-free | Reduce particulate and microbial contamination from hands |
| Laminar flow hood (if available) | ISO Class 5 or better | Dramatically reduces airborne particle contamination during open manipulations |
Step-by-Step Preparation Process
The following describes the technical process used in compounding settings. This is not a home production guide.
- Prepare your work surface. Wipe down with 70% isopropyl alcohol. If you have a laminar flow hood, perform all open steps inside it. Allow alcohol to fully evaporate before proceeding.
- Gather and inspect all materials. Check vials for visible cracks, particles, or compromised seals. Confirm the benzalkonium chloride stock concentration from the label or COA.
- Calculate your BKC volume. Use the formula above. For a 30 mL vial using 50% w/v BKC stock, that is 54 microliters. Write this down before proceeding.
- Draw sterile water for injection into a sterile syringe, leaving room for the BKC addition. Keep the total volume slightly under 30 mL until BKC is added.
- Add the calculated BKC volume to the syringe using a micropipette or calibrated small syringe. Mix gently by inversion.
- Attach a sterile 0.22-micron syringe filter. Filter the solution directly into the sterile multi-dose vial through the rubber septum. This is the sterilizing filtration step.
- Label the vial with contents, concentration, preparation date, and a conservative expiry (many practitioners use 28 days at 2 to 8 degrees Celsius as the outer limit, consistent with commercial product guidance, but this is not validated for home-prepared solutions).
- Store at 2 to 8 degrees Celsius. Inspect visually before each use.
What Most Pages Get Wrong About Making Bac Water
This is the highest-information section on this page because these errors appear consistently across forums, vendor guides, and hobbyist write-ups.
1. The concentration error. As described above, writing the USP spec as "0.9% BKC" instead of "0.9 mg/mL" is a roughly 10-fold concentration error. At 0.9% w/v, benzalkonium chloride is a disinfectant, not a bacteriostatic preservative, and would be harmful at injection-site tissue concentrations.
2. Treating the 0.22-micron filter as the only safety step. Every guide mentions the 0.22-micron filter. Almost none explain that it has no meaningful effect on endotoxins. Lipopolysaccharide (LPS) fragments from gram-negative bacteria are on the order of nanometers in size, far smaller than the 220-nanometer pore of a 0.22-micron filter. That filter does not reliably capture them. Endotoxin fever and systemic inflammatory responses from injectable contamination can occur even when a solution appears sterile under microbial testing.
3. Using distilled water or "double-distilled" water instead of sterile water for injection USP. Distilled water is not sterile and is not pyrogen-tested. Using it as the base for an injectable preparation introduces both contamination and endotoxin risk from the start.
4. Skipping the depyrogenation of glassware. Any glass vial or container that has not been depyrogenated (typically by dry heat at 250 degrees Celsius for 30 minutes or longer) can contribute endotoxins to the final product, even if the solution itself was filtered correctly.
5. Treating "sterile" and "pyrogen-free" as synonyms. They are not. A solution can be sterile (no viable organisms) while still containing endotoxins from dead gram-negative bacteria that were present during preparation. The USP Bacterial Endotoxins Test (BET, <85>) is a separate test from sterility testing (<71>).
Why the 0.22-Micron Filter Rule Exists (and Where It Fails)
The 0.22-micron pore size for sterilizing filtration is set because the smallest bacteria of clinical concern, including Pseudomonas aeruginosa and Mycoplasma species, are generally above 0.2 microns in their smallest dimension. Passing a solution through a validated 0.22-micron membrane under positive pressure removes these organisms with a sterility assurance level (SAL) of 10 to the negative 6, meaning fewer than one in a million units would be expected to contain a viable organism after filtration, per FDA and USP standards for pharmaceutical manufacturing.
Where this fails for home preparers: the SAL guarantee applies to membranes that have been validated in controlled conditions. A compromised filter housing, a cracked filter membrane from over-pressurization, or improper technique (pushing air through before solution) can breach the sterile barrier. You cannot confirm filter integrity at home the way a commercial facility does with bubble-point testing.
The endotoxin gap is the larger issue. Benzalkonium chloride itself is stable at the 0.9 mg/mL concentration and does not degrade the filter membrane. The chemistry concern here is not with the preservative but with the assumption that filtration equals safe injectable. It does not, without the pyrogen step.
Evidence Ledger
| Claim | Best Evidence Type | Effect Direction | Confidence |
|---|---|---|---|
| USP bac water contains 0.9 mg/mL BKC (0.09% w/v) | USP monograph (regulatory standard) | Established specification | High |
| 0.22-micron filtration achieves bacterial removal to SAL 10^-6 | FDA guidance, USP <1229.4> and pharmaceutical manufacturing standards | Established for validated processes | High (validated settings); Low (home setting) |
| 0.22-micron filters do not remove endotoxins | Established biophysics of LPS size vs. filter pore size | Does not remove | High |
| BKC at bacteriostatic concentrations inhibits gram-positive and gram-negative bacteria | Multiple in-vitro studies, long-standing pharmaceutical use | Inhibitory | High (in vitro); Moderate (in preserved solution over 28 days) |
| 28-day multi-dose in-use period for commercial bac water | USP monograph and product labeling convention | Established for commercial product | High (commercial); Very Low (home-prepared) |
| Home-prepared bac water is equivalent in safety to commercial USP product | No supporting evidence exists | Not established | Very Low |
| Dry heat at 250 degrees C for 30 minutes depyrogenates glassware | USP <1229.15> and pharmaceutical depyrogenation guidance | Effective depyrogenation | High (validated ovens); Low (consumer ovens without calibration) |
Head-to-Head: Bac Water vs. Alternatives for Peptide Reconstitution
| Attribute | Commercial Bac Water (USP) | Home-Made Bac Water | Sterile Water for Injection (USP) | Sterile Normal Saline (0.9% NaCl) |
|---|---|---|---|---|
| Sterility assurance | High (validated) | Low to unknown | High (validated) | High (validated) |
| Endotoxin tested | Yes | No | Yes | Yes |
| Multi-dose re-entry | Yes (up to 28 days) | Not validated | No (single-use) | No (single-use) |
| Cost per 30 mL | Roughly $3 to $8 | Lower cost possible but with equipment investment | Roughly $2 to $5 | Roughly $2 to $5 |
| Peptide compatibility | Good; compatible with most peptides | Unknown; same chemistry if correctly made | Good | Good; ionic strength may affect some peptides |
| Regulatory compliance (US) | Yes | No (for human use) | Yes | Yes |
| Practical recommendation | First choice for multi-dose use | Not recommended for human injection | Acceptable for single-use reconstitution | Acceptable; preferred by some for subcutaneous injections due to isotonicity |
Label and COA Literacy: How to Judge What You Have
Reading a commercial bac water label. A genuine USP Bacteriostatic Water for Injection label should state the following: "Sterile Water for Injection, USP," "Contains benzalkonium chloride 0.9 mg/mL as a preservative," a lot number, an expiry date, and a manufacturer name you can verify. It should state "For Multiple-Dose Use." If it reads "0.9% benzalkonium chloride" it may be a labeling error or a non-USP product.
What a COA for BKC stock should show. If you are sourcing pharmaceutical-grade benzalkonium chloride for compounding, the certificate of analysis should confirm: assay percentage (typically reported as 47 to 53% for a 50% solution by convention), absence of heavy metals above pharmacopoeial limits, pH range, and identity test results. Industrial-grade BKC will often lack assay precision and heavy metal testing. Do not use it for any preparation intended for injection.
Visual inspection of bac water before use. Commercial or prepared bac water should be perfectly clear and colorless, free of visible particulates, and free of turbidity. The benzalkonium chloride itself is colorless at this concentration. Any cloudiness, visible particles, or color suggests contamination or degradation. Discard without use. Chemical degradation of benzalkonium chloride at the 0.9 mg/mL level is not visually detectable, which is why expiry dates matter.
Vial integrity check. Before needle entry, confirm the rubber septum is intact and has not been previously punctured in a way that compromised the seal. Commercial multi-dose vials use specific rubber formulations that reseal after needle withdrawal. Improvised caps or re-used septa do not offer the same assurance.
FAQ
What exactly is bacteriostatic water?
Bacteriostatic water for injection (USP) is sterile water containing benzalkonium chloride at 0.9 mg/mL (0.09% w/v) as a preservative. The preservative inhibits bacterial growth, allowing the same vial to be re-entered with a needle up to 28 days after opening, unlike single-use sterile water for injection.
Can I make bac water at home?
You can prepare a functional approximation, but you cannot produce USP-grade bacteriostatic water for injection at home. USP compliance requires validated depyrogenation, sterility testing, and pharmaceutical-grade benzalkonium chloride. Home-prepared bac water carries real contamination and endotoxin risk.
What concentration of benzalkonium chloride is in bac water?
USP Bacteriostatic Water for Injection contains benzalkonium chloride at 0.9 mg/mL (0.09% w/v). The common misquote of "0.9% benzalkonium chloride" is a frequent and significant error. 0.9% w/v would be far above the bacteriostatic range and potentially cytotoxic.
What equipment do I need to make bac water?
At minimum: sterile water for injection (USP vial), pharmaceutical-grade benzalkonium chloride solution, sterile multi-dose vials, 0.22-micron syringe filters, sterile syringes, alcohol wipes, and a clean workspace ideally under laminar flow. Without a 0.22-micron filter, the product is not sterile.
What is the correct benzalkonium chloride dose per mL?
The USP specification is 0.9 mg of benzalkonium chloride per mL of final solution. For a 30 mL vial, that means 27 mg total. Starting from a 50% w/v stock solution, that is approximately 54 microliters of stock per 30 mL vial, QS to volume with sterile water.
How long does homemade bac water last?
Commercially produced USP bac water carries a 28-day in-use expiry after first puncture. Home-prepared solutions lack validated sterility data, so the same 28-day window cannot be assumed. Many practitioners treat home-prepared solutions as single-use or limit use to 7 days stored at 2 to 8 degrees Celsius.
Why does bac water need a 0.22-micron filter?
0.22 microns is the validated pore size for sterilizing filtration of aqueous solutions under FDA and USP guidance. It removes bacteria and most particles. It does NOT remove endotoxins (pyrogens), which are far smaller. Depyrogenation requires either dry heat or dedicated endotoxin-removal processing.
Is it legal to make bac water yourself?
In the United States, compounding sterile preparations for human injection is regulated by the FDA under 503A and 503B pharmacy compounding frameworks. Producing injectable solutions outside of a licensed pharmacy context for human use is not legally compliant with federal law. Regulatory status varies internationally.
What can go wrong when making bac water?
The main failure modes are: using tap or distilled (non-sterile) water, omitting the 0.22-micron filter, using non-pharmaceutical benzalkonium chloride, skipping endotoxin considerations, wrong preservative concentration (especially the 10x error), and non-sterile technique during vial filling.
Can I use sterile saline instead of bac water for peptide reconstitution?
Yes, sterile normal saline (0.9% sodium chloride for injection, USP) is a common alternative. It is single-use once a vial is punctured, unlike bac water. For peptides reconstituted in saline, follow manufacturer guidance on storage temperature and discard timing.
How do I know if my bac water is degraded or contaminated?
Signs of degraded or contaminated bac water include visible particulates, cloudiness, color change from clear, or a compromised vial seal. Benzalkonium chloride degradation at this concentration is not visually detectable. When in doubt, discard. The cost of a commercial vial is negligible compared to injection risk.
Sources
- United States Pharmacopeia (USP). Bacteriostatic Water for Injection monograph. USP-NF. Rockville, MD: USP.
- United States Pharmacopeia. <71> Sterility Tests. USP-NF.
- United States Pharmacopeia. <85> Bacterial Endotoxins Test. USP-NF.
- United States Pharmacopeia. <1229.4> Sterilizing Filtration of Liquids. USP-NF.
- United States Pharmacopeia. <1229.15> Sterilization-in-Place. USP-NF. (Depyrogenation dry-heat reference).
- U.S. Food and Drug Administration. Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, Current Good Manufacturing Practice. FDA, 2004.
- U.S. Food and Drug Administration. Drug Quality and Security Act (DQSA), 2013. Public Law 113-54.
- McDonnell G, Russell AD. Antiseptics and Disinfectants: Activity, Action, and Resistance. Clinical Microbiology Reviews. 1999;12(1):147-179. (Benzalkonium chloride mechanism of action.)
- U.S. Food and Drug Administration. 21 CFR Part 211: Current Good Manufacturing Practice for Finished Pharmaceuticals.
- Hiatt CW. Depyrogenation. PDA Journal of Pharmaceutical Science and Technology. Reference to dry-heat depyrogenation standards at 250 degrees C.