What bacteriostatic water is
Bacteriostatic water is sterile water containing roughly 0.9% benzyl alcohol as a preservative. The preservative is what allows a vial to be entered more than once, because it inhibits bacterial growth in the remaining solution. Plain sterile water has no preservative and is typically treated as single-use.
| Bacteriostatic water | Sterile water for injection | |
|---|---|---|
| Preservative | ~0.9% benzyl alcohol | None |
| Typical use | Multi-dose vials | Single use |
| Once opened | Commonly labelled for up to 28 days | Discard after use |
| Storage | Per the manufacturer label | Per the manufacturer label |
Choosing a diluent volume
There is no single correct volume. It is a trade-off, and it is set with your healthcare professional. The mechanics, though, are fixed:
- More water gives a lower concentration and a larger draw for the same dose. Larger draws are read against more of the barrel, which typically makes small measurement errors proportionally smaller.
- Less water gives a higher concentration and a smaller draw. That keeps injection volume down, but a dose may land within the first few units of the barrel, where a half-unit misread is a larger share of the total.
- Vial capacity is a hard limit. A 2 mL vial cannot take 3 mL of diluent, whatever the arithmetic says.
Reverse-calculating from a target concentration
To hit a specific concentration, divide the vial strength by it. For a 10 mg vial at a target of 5 mg/mL, that is 10 ÷ 5 = 2 mL of bacteriostatic water.
Diluent volume needed to reach a target concentration.
| Vial strength | Target 2 mg/mL | Target 5 mg/mL | Target 10 mg/mL |
|---|---|---|---|
| 5 mg | 2.5 mL | 1 mL | 0.5 mL |
| 10 mg | 5 mL | 2 mL | 1 mL |
| 15 mg | 7.5 mL | 3 mL | 1.5 mL |
| 30 mg | 15 mL | 6 mL | 3 mL |
A convenient trick: reconstituting so that the concentration in mg/mL equals the vial strength in mg divided by 1 mL makes the unit count read directly. At 1 mL of water in a 10 mg vial, 100 units is the entire vial, so 10 units is exactly 1 mg.
