What is bacteriostatic water, and how is it different
One ingredient separates it from every other water in a pharmacy, and that ingredient is the only reason a vial can be entered more than once.
Bacteriostatic water is water for injection with a preservative added. That is the whole difference, and it is the difference that decides how your vial behaves. The FDA label (Hospira, DailyMed) defines it as:
a sterile, nonpyrogenic preparation of water for injection containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) of benzyl alcohol added as a bacteriostatic preservative
Note that there are two strengths, not one. Plenty of write-ups state 0.9% as though it were the only formulation. The same label lists both.
The one difference that matters
Put the two labels side by side and the practical consequence writes itself. These are quotations, not paraphrases:
| Bacteriostatic water | Sterile water for injection | |
|---|---|---|
| Preservative | 0.9% or 1.1% benzyl alcohol | “contains no bacteriostat, antimicrobial agent or added buffer” |
| Container | “a multiple-dose container from which repeated withdrawals may be made” | “supplied only in single-dose containers” |
| Leftovers | Stays usable across withdrawals | “Do not reuse single-dose containers. Discard unused portion.” |
Sterile-water quotations are from the Medefil label.
Why that decides everything about your vial
A vial you intend to draw from twenty-five times over several weeks is a multi-dose vial, and a multi-dose vial needs a preservative. Without one, every re-entry through the stopper is an opportunity for something to grow in a liquid that will sit at fridge temperature for weeks.
Here is what that looks like in an approved product. EGRIFTA SV (tesamorelin) is a reconstituted peptide with an FDA label, and its label specifies sterile water, not bacteriostatic:
Reconstitute 1 vial with 0.5 mL of diluent … Administer 0.35 mL immediately following reconstitution … Throw away any unused solution … Do not freeze or refrigerate the reconstituted solution.
One dose, then discard, no refrigeration. That is the behaviour sterile water forces. The pattern of one vial lasting weeks in the fridge is not how any approved reconstituted peptide is labelled — it depends entirely on the preservative in bacteriostatic water. If your water has no benzyl alcohol on it, none of the multi-week arithmetic on this site applies to your vial.
How it is sold, and how much you need
The Hospira label lists 30 mL plastic vials at 0.9% and 20 mL glass vials at 1.1%. Repackagers list other volumes, so treat those two as the reference presentations rather than the only ones.
The amount you need is not a number we can put on this page, because it depends on your vial and the mark you want your draw to land on — that is what the calculators are for. What we can tell you is the shape of the answer: a single blend vial typically takes somewhere between 1 and 3 mL, so one 30 mL vial of bacteriostatic water covers several peptide vials rather than being consumed by one. Work out your own volume.
What the label says it is not for
Three restrictions are printed on the bottle, and they are worth reading before you assume bacteriostatic water is interchangeable with any other clear liquid:
- “NOT FOR USE IN NEONATES.” Benzyl alcohol is the reason. This is a label warning, in capitals, not a footnote.
- Not for fluid replacement. The label states that preparations with benzyl alcohol “should not be used for fluid replacement”.
- Not for epidural or spinal use. Also explicit on the label.
The indication itself is narrow: “indicated only for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection”. Its measured pH is 5.7, with an allowed range of 4.5 to 7.0.
What we cannot tell you
How long your mixed vial stays good. The preservative is what makes repeated withdrawals possible, but no label anywhere states a shelf life for a reconstituted vial of any of the blends this site covers, kept at 2–8 °C in a household fridge. The figure of 28 days that circulates comes from compounding-pharmacy convention, not from a study of these peptides. Why we print no expiry date.
Nor can we tell you what to buy or where. This site has no shop, no vendor list and no affiliate links, and that is not going to change.