How long does bac water last once opened?
28 days from the first time a needle goes through the stopper, for a 10 mL bottle and a 30 mL one alike: its 0.9% benzyl alcohol holds bacterial growth down for about that long. A sealed bottle keeps until its printed expiry, usually two to three years from manufacture.
Last edited · written by PepPrep · not medical advice
28 days from the first time a needle goes through the stopper. That figure comes from the manufacturers and from USP guidance for multi-dose vials, and it is the same for a 10 mL bottle and a 30 mL bottle. The clock starts on the first puncture, not on the day you bought it and not on the day you first used most of it. That clock belongs to the water and its preservative. How long whatever you dissolve in it stays good is a second, separate clock, set by that product's own labelling — the shelf-life guide keeps the two apart.
Why 28 days and not longer
Bacteriostatic water contains 0.9% benzyl alcohol, which stops bacteria multiplying — it does not sterilise what is already in there. Every puncture is a chance to introduce something, and the preservative holds the line for about a month of ordinary use. After that the assumption is that it no longer can, so the rest is thrown away. It is a rule about risk, not about the water going off on a given morning.
Unopened, it lasts years
A sealed bottle is good until the expiry printed on it, usually two to three years from manufacture, at room temperature and out of direct light. It does not need refrigerating before opening. The shelf-life guide covers the three different ways supplies expire.
Once opened: fridge or shelf?
Either is acceptable within the 28 days. Most people keep the opened bottle in the fridge because it is beside the vials it is used with, and because a cold bottle is a bottle nobody has left on a sunny windowsill. What matters more than the temperature is that the rubber stopper gets an alcohol pad before every draw and that nothing but a fresh sterile needle goes through it.
Write the date on it
The single most useful habit: when you first puncture the bottle, write the date on the label with a marker or a fridge-safe label. Twenty-eight days later is easy to work out; "when did I open this?" three weeks in is not.
Buy the size you will finish
A 30 mL bottle is cheaper per millilitre and exactly as useless as a 10 mL bottle on day 29. If you mix once a fortnight, the small bottle wastes less. See bac water vs sterile water for the full comparison, including the no-preservative version that has to be used the day it is opened.
Supplies mentioned: Mixing & Water
Sources: DailyMed (opens in a new tab)
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.