Bacteriostatic Water vs Sterile Water vs Saline
Bacteriostatic water is sterile water with 0.9% benzyl alcohol in it, which is what lets an opened bottle be drawn from for about 28 days instead of once. Sterile water and saline have no preservative and are single-use. Which one your vial needs is on its instructions; how much you add is a choice, and this page explains what that choice changes.
Last edited · written by PepPrep · not medical advice
"Bac water" is just the short name for bacteriostatic water. They are the same product, and the short form is what most people actually say and type. You will see both on labels, so it's worth knowing that neither one is a different or lesser grade.
Both are sterile. The difference is one ingredient:
- Bacteriostatic water (bac water) is sterile water plus 0.9% benzyl alcohol. That trace of alcohol stops bacteria from growing after the vial's seal is first punctured, which is why an opened vial is generally considered usable for about 28 days. If you'll draw from the same vial multiple times, this is the one you want.
- Sterile water for injection has no preservative at all. It's perfectly clean until the moment you open it — and then the clock runs out fast. It's meant for mix-it-and-use-it-once situations, or for people avoiding benzyl alcohol.
- Sterile saline (0.9% sodium chloride) is sterile salt water matching your body's own salt level. Some product instructions specifically call for it; if yours do, use it. If instructions for something that will be injected name saline, that means sodium chloride for injection, a pharmacy item; the saline pods on this site are irrigation/inhalation grade, for nasal sprays.
| Water | Preservative | After opening | Buy it when… |
|---|---|---|---|
| Bacteriostatic | 0.9% benzyl alcohol | ~28 days | You'll draw from the vial more than once (most people) |
| Sterile water | None | Use immediately | Single-use mixing, or avoiding benzyl alcohol |
| Saline 0.9% | None (salt only) | Use immediately | Nasal sprays; for anything injected, only sodium chloride for injection, and only when the instructions name it |
Rule of thumb: multi-dose vial → bac water. Single-use → sterile water. Nasal spray, or instructions that say saline → saline of the grade they name (for injection if it will be injected).
The glossary entry for bacteriostatic water has the short definition, and bacteriostatic water vs sterile water lines the two up side by side, specification by specification.
Is bac water the same as saline?
No. Both are sterile, but they are different liquids:
- Bacteriostatic water is sterile water with 0.9% benzyl alcohol, a preservative, so a bottle can be drawn from for weeks.
- Saline, also sold as sodium chloride 0.9% or normal saline, is sterile water with 0.9% salt. The single-dose vials have no preservative and are used once.
The matching "0.9%" is a coincidence of numbers: in bac water it is benzyl alcohol, in saline it is salt. Neither stands in for the other. The product's own instructions say which liquid it is meant to be mixed with, and saline is the one nasal-spray recipes usually name. Bacteriostatic saline also exists (salt water with the same benzyl alcohol preservative), but it is a separate product and much harder to find.
Two more names you will see: water for injection is sterile water, with no salt and no preservative. Hospira is a manufacturer's name rather than a different kind of water. What makes any bottle bacteriostatic water is the preservative on its label, usually 0.9% benzyl alcohol, so that is the line to check.
"How much bac water do I add?"
This is the most common question about bac water, and the answer surprises people: there is no single correct amount. The volume you add is a choice, not a fixed property of what's in the vial. Adding more water doesn't change how much substance is in there — it only spreads that same amount across a larger volume, so each syringe marking is worth less.
What the choice actually controls is how easy the measurement is to read. Too little water and your measurement lands on a sliver of a marking; too much and you're injecting more liquid than you need to. The usual aim is a number that falls on a clean, easy-to-read part of the syringe.
Two things decide it for you:
- What your product's own instructions say. If they specify a volume, use that — it overrides every rule of thumb, including this page.
- What lands on a readable marking. Our reconstitution calculator works this out: put in the vial size, the volume you're considering, and the amount you want per draw, and it shows you where that falls on a U-100 syringe. Try a couple of volumes and keep whichever gives you the tidiest number.
What this page will not do is tell you how much to take. That's between you and whoever prescribed or supplied it. The calculator converts an amount you have already decided on into syringe markings — it's arithmetic, not advice.
Bottle sizes, and which to buy
Bac water is usually sold in 30 mL and 10 mL bottles, and occasionally 3 mL.
- 30 mL is the common size and the one most listings mean by default. It costs less per millilitre, but the 28-day clock starts when you first puncture it, the same as any other size.
- 10 mL is the small one, and it suits people who mix occasionally. A big bottle you can't finish inside a month isn't a saving.
- 3 mL is worth knowing about if you mix rarely — a small bottle you finish beats a large one you throw away.
Buy the size you'll realistically use within about a month of opening, not the best price per millilitre. Where to buy peptide supplies covers which source suits which item, and why the bottled waters are the one line on this site with a pharmacy answer.
Common questions
Can I use sterile water instead of bacteriostatic water?
For a vial used all at once, yes. For a vial you keep, no: nothing stops bacteria growing in it once opened.
How long does bacteriostatic water last once opened?
About 28 days from the first puncture; write the date on the vial. Unopened, until the printed expiry.
Is saline the same as sterile water?
No. Saline is 0.9% salt in water; sterile water has nothing dissolved in it. Use whichever your product's instructions name.
Does bacteriostatic water need to be refrigerated?
Not before opening; room temperature and out of light is fine. Once opened most people keep it in the fridge beside the vials.
Supplies mentioned: Mixing & Water
Check your product's label: DailyMed (opens in a new tab) in the US; official sources for other countries.
Equipment only: nothing on this page is advice about what to inject, how much, or whether a product is fit to use. That comes from the product's labelling and whoever supplied it.