01 What it is
Most peptides arrive as a dry powder because they keep far better that way. To use one, you add water. Plain sterile water is meant for a single use, but every time a needle goes through the rubber stopper there is a small chance of bringing in bacteria. The 0.9% benzyl alcohol in BAC water stops bacteria from growing, which is why one mixed vial can safely last for weeks of doses.
02 Common uses
03 How it’s used
Draw the water with a fresh syringe and let it run slowly down the inside wall of the peptide vial. Never squirt it onto the powder. Swirl gently, don’t shake. 2ml is the standard amount for a 3ml vial, and up to 3ml makes a gentler, less stingy shot. Wipe the stopper with alcohol before every draw, store the bottle at room temperature away from light, and discard it 28 days after first opening or sooner if it turns cloudy. Pre-filled pens don’t need it.
Ask how much water to add for your vial, and what that means for the units you draw.
04 What to watch for
Use bacteriostatic water, not plain sterile water, for any vial you will draw from more than once. Don’t use it if it looks cloudy, has particles, or the seal was broken. A small number of people react to benzyl alcohol. It is not for use in newborns, and anyone pregnant should ask a doctor. Redness, warmth or swelling at an injection site that gets worse after 48 hours needs checking.
05 Evidence & status
Standard diluentNot a treatment. It is a standard pharmaceutical diluent used in clinics and hospitals.
In the US, Bacteriostatic Water for Injection, USP is a regulated pharmaceutical product.
Sources
- BACTERIOSTATIC WATER- bacteriostatic water injection, solution (Hospira, Inc.)DailyMed, U.S. National Library of Medicine, 2026 · Prescribing information
- Preventing Unsafe Injection Practices (Injection Safety: Clinical Safety)U.S. Centers for Disease Control and Prevention, 2024 · Reference
- Single-dose or multi-dose? Not all vials are created equal.U.S. Centers for Disease Control and Prevention, 2024 · Reference
- General Chapter <797> Pharmaceutical Compounding - Sterile Preparations: Frequently Asked Questions (updated December 11, 2023)United States Pharmacopeia (USP), 2023 · Reference
Status and sources last checked November 3, 2026. Something out of date? Tell us.
06 How long a mixed vial lasts
The 28-day rule comes from how multi-dose vials are handled in clinics: the CDC and the pharmacy standard USP <797> set 28 days from first puncture as the beyond-use date for a multi-dose container unless the manufacturer says otherwise. The 0.9% benzyl alcohol in bacteriostatic water is what makes a multi-dose vial reasonable at all; it stops bacteria growing between draws. It does nothing for the peptide itself, which is why a mixed vial still lives in the fridge and out of light.
That 28 days is about contamination, not potency. Some peptides lose activity faster once mixed; the dossiers say so where it is known.
07 Bacteriostatic vs sterile water
Sterile Water for Injection has no preservative and is meant for a single use: open it, use it, discard the rest. Bacteriostatic Water for Injection adds 0.9% benzyl alcohol so a vial can be entered repeatedly over 28 days. For a peptide vial that will be drawn from many times, bacteriostatic water is the right one; sterile water is fine only if the whole mixed vial is used at once.
The one group for whom the difference is a safety issue is newborns: benzyl alcohol has been linked to a serious toxicity (“gasping syndrome”) in neonates, and the label says bacteriostatic water is not for use in them. A small number of adults react to benzyl alcohol at the injection site.
Mixing a vial, step by step →08 What it can and can’t be used with
It is a diluent for compatible injectable products; the label says to check that the drug being mixed is compatible with benzyl alcohol, and nearly all peptide powders are. It is not for intravenous use on its own and not for large volumes: the label limits it to small-volume dilution. The practical rule is the one on the guides: 2–3ml into a peptide vial, added down the inside wall, swirled rather than shaken.
09 Where it stands legally
In the US, Bacteriostatic Water for Injection, USP is an FDA-regulated prescription product; the Hospira label on this page is marked Rx only. It is also widely sold by laboratory and research suppliers. This site does not sell or supply it, and nothing here changes what your pharmacy or supplier is permitted to do.
10 FAQ
BAC water or sterile water?
For a vial you will draw from more than once, bacteriostatic. Plain sterile water has no preservative and is meant for a single use.
How much do I add?
2ml is standard for a 3ml peptide vial; anywhere from 2 to 3ml is fine. More water means fewer mg per unit, so redo the math in the calculator.
How long does an opened bottle last?
The usual rule for an opened multi-dose bottle is 28 days. Write the date on it when you first open it.
Can I use sterile water instead?
Only if you use the whole mixed vial at once. For a vial you will draw from over days or weeks, use bacteriostatic water.
Why 28 days?
It is the standard beyond-use date for multi-dose vials in clinical practice (CDC and USP <797>), based on contamination risk, not on how long the peptide stays active.
Does more water change my dose?
No. It changes how many units you draw for the same dose. The calculator does the arithmetic.
Is benzyl alcohol safe?
At 0.9% for small-volume dilution in adults, it has decades of use. It is not for newborns, and a few people get site irritation from it.
Educational information only — not medical advice. The Daily Stack does not sell or supply any products. Talk to a licensed healthcare professional about your own health decisions.