Bacteriostatic water is sterile water with one preservative. The water is USP Water for Injection, the preservative is 0.9% benzyl alcohol, and that one additive lets a single vial be punctured again and again for up to 28 days. It is also sold as BAC water, bac water and bacteriostatic water for injection (BWFI). In the US it is a prescription diluent used to dissolve injectable drugs, and it is the standard liquid for mixing peptides.
Most people who look it up need one of three things: a place to buy it, a fair price, or the amount to add to a vial. A 10 mL vial costs $10 to $15 online and a 30 mL vial costs $12 to $25, while a pharmacy fills it only against a prescription. This guide covers where to buy bacteriostatic water, which size to pick, how much to add to a peptide vial, how to mix it, and how to store it.
Quick Answer
Bacteriostatic water (BAC water) is sterile water with 0.9% benzyl alcohol. It is the standard liquid for dissolving freeze-dried peptides like retatrutide, tirzepatide, semaglutide, BPC-157, and GHK-Cu. A pharmacy fills it only with a prescription, so most people order a 3 mL, 10 mL or 30 mL vial from an online supplier for $7.50 to $25. How much you add is a choice: peptide mg divided by mL of water gives mg per mL, so a 10 mg vial plus 2 mL makes 5 mg/mL, and on a U-100 insulin syringe 1 mL is 100 units. Let the water run slowly down the inside wall of the peptide vial, then swirl gently instead of shaking. Date the water vial at its first puncture and discard it 28 days later. The mixed peptide goes in the fridge at 2 to 8°C.
🔑 Key Takeaways
- It is sterile water plus one preservative. Bacteriostatic water is USP Water for Injection with benzyl alcohol, and that alcohol holds back bacterial growth so one vial survives repeated draws.
- It is a prescription product, and that decides where you can buy it. CVS, Walgreens and Walmart keep it behind the pharmacy counter, so most people order it online. The buying section walks through each route.
- The biggest vial is rarely the cheapest. A 30 mL vial wins on price per mL and usually loses on money spent. The reason is a date, not a volume.
- There is no single correct amount to add. The water you add sets the concentration: peptide mg divided by mL of water, then 1 mL equals 100 units on a U-100 syringe. The chart below covers the common vial sizes.
- The water and the peptide are stored differently. Most guides tell you to refrigerate opened BAC water. The Hospira label says 20 to 25°C, and the 28-day limit applies either way. The storage section shows which vial goes where.
- Not every peptide wants it. A few benzyl-alcohol-sensitive peptides, including some hCG and NAD+ preparations, do better with preservative-free sterile water used within 24 hours.
Start with what is actually in the vial. It explains every buying, mixing and storage rule that follows.
What is bacteriostatic water?
Bacteriostatic water is sterile water with a preservative. The base is USP Water for Injection, a purified, nonpyrogenic water made to United States Pharmacopeia standards. The additive is benzyl alcohol at 0.9%, which works out to 9 mg per milliliter. That preservative is the entire difference between BAC water and plain sterile water, and it is why a vial can take a needle day after day for roughly a month.
The finished product sits at a pH of about 4.5 to 7.0, carries no buffers or salts, and ships in multi-dose vials, glass or plastic, most often 3 mL, 10 mL, or 30 mL, sealed with a rubber stopper and an aluminum crimp. People also call it bac water, bacteriostatic water for injection, or BWFI. When someone asks what bac water is made of, the answer is short: USP water plus 0.9% benzyl alcohol, nothing else.
Peptides arrive as a dry, freeze-dried (lyophilized) powder because they are far more stable dry than wet. To use one, you draw bacteriostatic water into a syringe and add it to the powder vial, which dissolves the cake into a clear, stable liquid you can dose. That step is called reconstitution, and BAC water is the standard choice because the preservative protects the reconstituted peptide across the days or weeks you draw from it.
The reference product is Bacteriostatic Water for Injection, USP from Hospira, a Pfizer company. Its only labeled job is diluting or dissolving drugs that are injected. Our Hospira bacteriostatic water guide (PDF) covers the label and NDC numbers, and the bacteriostatic water uses guide (PDF) lists the drugs whose labels call for it.
Where to buy bacteriostatic water
There are three places to buy it. One fact explains all of them: the Hospira vial is labeled Rx only, the same wording printed on any prescription drug. It is not a supplement and not a general pharmacy item, and that is why shopping for it feels stranger than it should.
At a pharmacy: CVS, Walgreens and Walmart
If you searched for bacteriostatic water near me, the pharmacy counter is the only local option. CVS, Walgreens and Walmart do not put it on a shelf. Their pharmacies can dispense it against a prescription, but stock varies by store, so call ahead. Ask for Bacteriostatic Water for Injection, USP by its full name, because sterile water and saline flush look similar and neither one is a substitute.
With a prescription this is the cheapest route. GoodRx lists coupon prices from $4.68 and an average retail price of $13.43 for the most common version. The simplest way to get one is to ask your prescriber to add it when they write for the medication you will be mixing.
On Amazon and other marketplaces
Amazon is the fastest route and the hardest to verify. When we searched Amazon for bacteriostatic water on November 11, 2026, the first results were bottles titled "Premium Glass Bottle", "Filtered Water Solution" and "Deionized Water", next to an eyewash additive and an aquarium product. None of the first ten was labeled Bacteriostatic Water for Injection, and the Hospira vial was not among them.
Some of those bottles may be fine. The trouble is that a marketplace listing rarely shows a lot number or a Certificate of Analysis, sellers rotate, and you cannot check any of it once the vial is in your hand.
From an online supplier
This is where most home users end up. Specialty sellers ship 3 mL, 10 mL and 30 mL vials without a pharmacy counter, and a few resell the Hospira vial itself. Quality varies more inside this group than between the other two routes, so paperwork is the only useful filter: a lot number, a stated benzyl alcohol strength, and a Certificate of Analysis you can read before you pay.
The vial in the offer box on this page is PureBac. Every lot is tested on seven markers, benzyl alcohol is verified at 0.9%, and the Certificate of Analysis for each lot is public. Its honest limitation is price: at $1.50 per mL it is not the cheapest water, it is sold online only, and there is no 30 mL size. Our page on where to buy BAC water for peptides shows what that batch testing covers.
Affiliate disclosure: PeptideDeck earns a commission if you buy PureBac through the offer boxes on this page, at no extra cost to you.
What bacteriostatic water costs
Price it per mL, not per vial. A 30 mL vial at $19 sounds worse than a 10 mL vial at $12 until you divide, and it sounds better than it is once you count how much you will use before the 28 days run out.
These are the single-vial prices listed on November 11, 2026 by six online sellers of bacteriostatic water, plus the pharmacy route.
PureBac, the vial in the offer box, is $14.99 for 10 mL and $7.99 for 3 mL. The 10 mL drops to $13.49 each when you buy ten, and shipping is free over $150.
Notice what the table does not say. The lowest price per mL belongs to the 30 mL vial, and for most people it is the wrong buy. The next section explains why.
Which size to buy: 3 mL, 10 mL or 30 mL
Thirty milliliters is the most searched size. For most home users it is also the most wasteful one. The problem is a date, not a volume.
An opened multi-dose vial is discarded 28 days after the first puncture. That rule comes from CDC guidance that follows USP <797>, and it does not care whether you used 2 mL or 25 mL. On day 29, whatever is left goes in the bin.
Run it on a real month. A typical mix uses 2 mL of water per peptide vial, and one person on one peptide mixes two or three vials in 28 days. That is 4 to 6 mL of water.
What a 30 mL vial really costs
Buy 30 mL at $18.95. Use 6 mL in 28 days. Discard 24 mL.
The listed price was $0.63 per mL. The price of the water you used was $3.16 per mL.
The same seller lists a 10 mL vial at $11.50. It covers the same month for $7.45 less.
So match the vial to what you will use in 28 days, not to the price per mL.
The unintuitive move is two 10 mL vials instead of one 30 mL. The second vial's 28-day clock does not start until you puncture it, so two sealed vials give you two separate windows. One large vial gives you one.
What to check before you buy
Every vial looks the same. The liquid is clear, the label says sterile, and the seller says pharmaceutical grade. Two checks separate a documented vial from a repackaged one.
0.9% or 1.1% benzyl alcohol
Bacteriostatic Water for Injection, USP does not come in one preservative strength. The Hospira label lists two.
Both say Bacteriostatic Water for Injection, USP on the front. Both are packed in trays of 25 for hospitals and pharmacies, so every single vial sold online has been split out of a tray, and the batch paperwork tends to stay with whoever split it.
This is where a Certificate of Analysis stops being a marketing word. One that lists a measured benzyl alcohol strength answers the question. A product page that says 0.9% with nothing behind it is repeating a number.
Red flags
Walk away if you see:
- No lot number. Every legitimate vial has one. Without it there is no traceability and no real Certificate of Analysis.
- A certificate you cannot see before buying. "Available on request" is a promise. "Published per lot" is a document.
- One certificate for every batch. A certificate that is not tied to your lot says nothing about the vial in your hand.
- A squeeze bottle or a dropper top. The real product ships in a sealed multi-dose vial with a crimped metal ring over a rubber stopper. Anything you can pour is not it.
- No stated benzyl alcohol strength. After the table above, you know why this matters.
- A price far below the rest with no paperwork. The water is not what costs money. The testing is, so a very cheap vial usually skipped it.
- Vague origin language. "Pharmaceutical grade" and "lab tested" are unregulated phrases. A manufacturer name and a lot number are not.
How bacteriostatic water actually works
The word bacteriostatic is the key. Bacteriostatic means it stops bacteria from multiplying, as opposed to bactericidal, which kills them outright. Benzyl alcohol slips into bacterial cell membranes and disrupts them enough that the population cannot grow. It does not sterilize a contaminated vial, but it buys you a usable multi-dose window that plain sterile water cannot offer.
That is the practical payoff. With benzyl alcohol on guard, you can puncture the same vial of reconstituted peptide many times over several weeks. Without it, anything you mix should be treated as single-use and discarded within about a day. For home peptide work, where one vial is dosed over weeks, that preservative earns its place on every single draw.
BAC water vs sterile water vs saline
People constantly ask whether bac water is the same as sterile water. It is not, and the difference matters. The table below lines up the four liquids you will see mentioned, plus 0.6% acetic acid, which shows up for a few poorly soluble peptides.
For most home reconstitution, bacteriostatic water is the right pick because it is preserved and built for repeated draws. Sterile water is the backup when a peptide does not tolerate benzyl alcohol, and saline or acetic acid are situational. If you wonder whether reconstitution solution is the same as bac water, the answer is usually yes: most peptide reconstitution solutions sold for home use are simply bacteriostatic water under a different label.
Full comparison: bacteriostatic water vs sterile water vs saline.
How much bacteriostatic water to use for peptides
This is the question almost everyone is really here for. The honest answer is that there is no single fixed amount of BAC water for a peptide vial. The volume you add is a choice, and that choice sets your concentration. Add less water and the solution is more concentrated, so each dose is a tiny volume. Add more water and the solution is dilute, so small doses are easier to measure precisely but the vial empties faster.
Two short formulas cover everything:
- Concentration: milligrams per mL = peptide mass (mg) divided by BAC water added (mL). A 10 mg vial plus 2 mL of BAC water is 5 mg/mL.
- Dose volume: mL to draw = desired dose (mg) divided by concentration (mg/mL). On a U-100 insulin syringe, 1 mL is 100 units, so 0.1 mL is 10 units.
A handy shortcut for unit math: micrograms per unit = (peptide mg multiplied by 10) divided by BAC water mL. So 5 mg in 2 mL gives 25 mcg per unit. Best practice is to pick a water volume that makes your target dose land on a round unit count, like 250 mcg landing on exactly 10 units rather than 13.7 units.
The chart below runs the common vial sizes against the common BAC water volumes, so you can see the concentration and what a sample dose draws to on a standard U-100 insulin syringe.
Skip the math, use the calculator
For exact numbers on your specific vial and target dose, run it through our peptide reconstitution calculator. Enter the peptide milligrams, the BAC water volume, and your dose, and it returns the concentration and the exact units to draw. The full peptide calculator suite covers dosing and cost too. This beats guessing on questions like a tirzepatide bac water calculator or how many units 2 mL of bac water is.
Per-peptide BAC water amounts
Here are sensible starting points for the peptides people search most, framed as pick your concentration, then this is your BAC water volume.
- Retatrutide (5, 10, 12, 30 mg): For a 10 mg retatrutide vial, 2 mL of BAC water gives 5 mg/mL, so a 2 mg dose draws to 40 units. A 5 mg vial plus 1 mL is also 5 mg/mL. A 30 mg vial plus 3 mL gives 10 mg/mL, where 5 mg draws to 50 units. See our retatrutide dosing schedule for titration.
- Tirzepatide (10, 15, 30, 60 mg): A 30 mg tirzepatide vial plus 3 mL is 10 mg/mL, so a 2.5 mg dose is 25 units and 5 mg is 50 units. A 60 mg vial plus 3 mL gives 20 mg/mL. Our tirzepatide reconstitution guide walks through each strength.
- Semaglutide (5, 10 mg): A 10 mg vial plus 2 mL is 5 mg/mL, so a 0.25 mg starter dose is 5 units. Add only 1 mL for 10 mg/mL if you want 0.25 mg to land on 2.5 units.
- BPC-157 (5, 10 mg): A 5 mg vial plus 2 mL is 2.5 mg/mL, so a 250 mcg dose is exactly 10 units. A 10 mg vial plus 2 mL is 5 mg/mL, where 250 mcg is 5 units. More detail in our BPC-157 dosage guide.
- GHK-Cu (50, 100 mg): A 50 mg vial plus 5 mL is 10 mg/mL, so a 2 mg dose is 20 units. A 100 mg vial plus 5 mL is 20 mg/mL, where 2 mg is 10 units.
- Tesamorelin (10 mg): Plus 2 mL is 5 mg/mL, so a 1 mg dose is 20 units. Plus 1 mL is 10 mg/mL, where 1 mg is 10 units.
- Ipamorelin and CJC-1295 (5 mg): Plus 2 mL is 2.5 mg/mL, so 100 mcg is 4 units, 200 mcg is 8 units, and 300 mcg is 12 units.
- NAD+ (500 mg): Plus 5 mL is 100 mg/mL, so a 50 mg dose is 0.5 mL, which is 50 units. NAD+ is one of the peptides that some people reconstitute with preservative-free sterile water instead, covered in the exceptions section below.
Notice the pattern: the same vial in different water volumes changes concentration, not the total amount of peptide. A 5 mg vial holds 5 mg whether you add 1 mL or 5 mL. You are only deciding how big each injection is and how many doses you can draw before the vial runs out.
If you are pairing it with a GLP protocol, our roundup of peptides for weight loss connects the dosing back to goals.
How to mix peptides with BAC water
Reconstitution takes about a minute per vial once you have done it once. Here are the steps, whether you are mixing retatrutide, tirzepatide, semaglutide, or a repair peptide.
- Bring both vials to room temperature. Let the cold peptide powder and the BAC water sit out for 15 to 20 minutes so condensation does not form.
- Swab both stoppers. Wipe the rubber top of the BAC water vial and the peptide vial with a fresh alcohol pad for 10 to 15 seconds each, and let them dry.
- Draw your chosen water volume. Pull the BAC water amount you decided on into an insulin syringe, for example 2 mL for a 10 mg vial.
- Add it down the glass wall. Insert the needle and let the water run slowly down the inside of the vial rather than blasting it directly onto the powder cake. This protects the peptide.
- Swirl, do not shake. Roll the vial gently between your fingers until the solution turns fully clear. Shaking or vortexing can stress and degrade the peptide.
- Label and refrigerate. Write the peptide name, the concentration, and the date on the vial, then store it in the refrigerator at 2 to 8 C right away.
That same routine answers searches like how to mix retatrutide with bac water or how to reconstitute peptides with bac water. The technique does not change by peptide; only the water volume you chose changes the dose math.
How to store BAC water and reconstituted peptides
Storage trips people up because there are three different states with three different rules. The table puts them side by side so you are never guessing whether bac water needs to be refrigerated.
So does BAC water need to be refrigerated? Before opening, no, a sealed vial is stable at room temperature. After opening, the label still says 20 to 25 C. Many people keep the opened vial in the fridge anyway, which does no harm but does not extend the 28-day limit. Your reconstituted peptide is different: keep it cold, and if a protocol stretches past a month, freezing aliquots at -20 C extends it to 2 to 4 months. You can freeze bacteriostatic water, but it is rarely worth it for an unopened vial that already lasts a couple of years. Avoid repeated freeze and thaw cycles, since each one degrades a slice of the peptide.
How long does BAC water last after opening?
The headline number is 28 days. Once you puncture a vial of bacteriostatic water, the CDC, following USP chapter <797>, says to date it and discard it within 28 days unless the manufacturer gives another date. The Hospira label itself gives no 28-day figure; it only says to store the vial at 20 to 25 C. That window is counted from the first needle puncture, not from the day you bought it or opened the box. A sealed, unopened vial is good until its printed expiration date, usually 18 to 24 months out.
Why 28 days? Benzyl alcohol slows bacterial growth but does not stop contamination forever, and every puncture is a chance to introduce something. The 28-day rule is a ceiling that assumes clean technique. Several things shorten it well below 28 days:
- Heavy puncture counts. The rubber stopper can stop resealing reliably after roughly 15 to 20 sticks, so a vial used many times a day ages faster.
- Sub-spec product. Water made with less than the full 0.9% benzyl alcohol can fail before the 28 days are up, which is why a verified concentration matters.
- A lost first-puncture date. If you cannot remember when you opened it, you cannot trust the clock, so treat it as expired.
This is the practical answer to how long bac water is good for, in the fridge or otherwise. With clean technique, plan on 28 days. With sloppy technique, plan on less.
Signs your BAC water has gone bad
Does bac water go bad or expire? Yes, both. Beyond the date math, your eyes are the final check. Discard a vial of bacteriostatic water, or a reconstituted peptide, if you see any of these.
Throw it out if you notice:
- Cloudiness or haze. Clear water that turns milky or foggy is a contamination signal.
- Floating particles or specks. Anything visible suspended in the liquid means it is no longer clean.
- Color change or a film. Yellowing, tint, or a surface skin are all discard triggers.
- A compromised seal. A loose crimp, a cracked vial, or a stopper that no longer reseals.
- An unknown age. Past 28 days from opening, or a vial you cannot date, goes in the bin regardless of how it looks.
Clear and colorless is the only acceptable look. When in doubt, the cost of a fresh vial is far lower than the cost of injecting something contaminated.
When not to use bacteriostatic water
BAC water is the default, but a handful of peptides do not get along with benzyl alcohol. For these, the safer solvent is preservative-free sterile water, used as a single-dose mix and discarded within about 24 hours.
If a peptide vial or its instructions call for a specific diluent, follow that. For everything else in the GLP and repair-peptide world, bacteriostatic water is the standard solvent.
Is bacteriostatic water safe?
For the tiny volumes used to reconstitute peptides, benzyl alcohol exposure is very small, and bacteriostatic water has a long, documented safety record in clinical compounding. One important caution appears on every label: benzyl alcohol is not for use in newborns. High benzyl alcohol exposure has been linked to a serious condition in premature and newborn infants known as gasping syndrome, which is why the FDA-approved labeling warns against using benzyl-alcohol-preserved water in neonates.
A few other basics are worth stating plainly. Bacteriostatic water is hypotonic, so it is not meant to be pushed intravenously on its own without being mixed into an isotonic solution. Aseptic technique is the biggest factor in whether a vial stays safe after opening, so swab the stopper every time. Bacteriostatic water is a prescription drug product in the United States, and any decision about peptides themselves belongs with a qualified, licensed healthcare provider who knows your history.
Reactions to the preservative itself, including benzyl alcohol allergy, are covered in our bacteriostatic water side effects guide (PDF).
Frequently Asked Questions
References
- DailyMed (U.S. National Library of Medicine). Bacteriostatic Water for Injection, USP, label (Hospira, Inc.), including 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol, storage at 20 to 25 C, the USP Water for Injection base, and the neonatal benzyl alcohol caution. dailymed.nlm.nih.gov.
- DailyMed (U.S. National Library of Medicine). Novarel (chorionic gonadotropin for injection) label, Ferring Pharmaceuticals: supplied with 10 mL Bacteriostatic Water for Injection, USP (0.9% benzyl alcohol); refrigerate and use within 30 days after reconstitution. dailymed.nlm.nih.gov.
- DailyMed (U.S. National Library of Medicine). Sterile Water for Injection, USP, label: preservative-free, single-dose use, for comparison with bacteriostatic water. dailymed.nlm.nih.gov.
- Centers for Disease Control and Prevention. Preventing Unsafe Injection Practices: an opened multi-dose vial is dated and discarded within 28 days unless the manufacturer states another date (from USP <797>). cdc.gov.
- Gershanik J, Boecler B, Ensley H, McCloskey S, George W. The gasping syndrome and benzyl alcohol poisoning. N Engl J Med. 1982;307(22):1384-1388 (PMID 7133084). pubmed.ncbi.nlm.nih.gov.
- PubChem (U.S. National Library of Medicine). Benzyl Alcohol (CID 244): identity, properties, and use as an antimicrobial preservative. pubchem.ncbi.nlm.nih.gov.
- U.S. Food and Drug Administration. Inactive Ingredient Database, benzyl alcohol as an approved injectable preservative. accessdata.fda.gov.
- United States Pharmacopeia. General Chapter <797> Pharmaceutical Compounding, Sterile Preparations: beyond-use dating for multiple-dose containers. usp.org.
- GoodRx. Bacteriostatic Water prices: coupon price from $4.68 and average retail price of $13.43 for the most common version, checked November 2026. goodrx.com.
- Single-vial prices listed on November 11, 2026 by six online sellers: Bacteriostatic Water Store, BacterioWater, BAC Water Depot, Alpha Liquids, Bacteriostatic Water Supplies and PureBac. Listing a seller here is not an endorsement.
- Amazon.com search results for bacteriostatic water, first page, viewed November 11, 2026.
