It’s about time since opening, not volume
A 30mL vial of bacteriostatic water isn’t limited by how much is left in it; it’s limited by how long it’s been since the vial was first pierced. Most manufacturer guidance for multi-dose bacteriostatic water gives a usable window of around 28 days after first opening, refrigerated, regardless of how much volume remains.
Why time matters more than the amount used
Each time a vial is pierced to draw water, there’s a small opportunity for contamination, even with careful technique. The benzyl alcohol in bacteriostatic water inhibits bacterial growth but doesn’t eliminate that cumulative risk indefinitely, which is why the guidance is time-based rather than volume-based.
What to track
Writing the date a bacteriostatic water vial was first opened directly on the vial, the same habit recommended for reconstituted peptide vials, removes any guesswork about whether it’s still within its usable window later.
Storage between uses
Refrigeration is standard advice for an opened bacteriostatic water vial, alongside keeping the rubber stopper clean and swabbing it with alcohol before each draw.
When to replace it
Once a vial is past its recommended window from first opening, or if it shows any visible cloudiness or particles, replacing it rather than continuing to use it is the more cautious choice, especially since it’s a relatively low-cost item compared with the peptide it’s used to reconstitute.