How Long Does BAC Water Last? What a 120-Day Test Found

Bacteriostatic water, usually shortened to BAC water, is sterile water with a small amount of benzyl alcohol added. The benzyl alcohol is a preservative: it holds back the growth of bacteria that get in when a needle goes through the stopper. That is what lets one vial be used more than once.

The question people ask most about it is when to throw a vial away. Thirty days? Sixty? Ninety? A group called PTDS paid for a test to put some numbers on one part of that question, and an independent lab, Janoshik, ran the measurements.

What the test did

PTDS took three vials of Hospira bacteriostatic water, the same brand throughout, and kept each one under different conditions for 120 days. One sat at room temperature and was punctured only on the days it was tested. One sat at room temperature and was punctured every week as well. One was kept in the refrigerator and punctured every week.

On days 0, 30, 60, 90 and 120 the lab measured how much benzyl alcohol each vial still held. The label strength for this product is 0.9%.

What it found

For the first 90 days, very little happened. All three vials read between 0.883 and 0.918 at every test, close to the 0.9% on the label. The readings wobble up and down a little from one test to the next. That wobble comes from the measurement, not the water: benzyl alcohol does not appear in a sealed vial, so a reading that rises from one month to the next is the method varying.

At day 120 all three vials read lower. The two room-temperature vials came in at 0.816 and 0.817. The refrigerated vial came in at 0.844.

Did weekly punctures matter?

Not in this test. The room-temperature vial that was punctured every week finished at 0.817. The one punctured only for testing finished at 0.816. Those two results are the same for any practical purpose.

That only speaks to the preservative. Every puncture is also a chance for bacteria to get in, and this test did not look at that.

Did the refrigerator help?

A little, perhaps. The refrigerated vial held slightly more benzyl alcohol at day 120 than the two at room temperature: 0.844 against 0.816 and 0.817.

It is one vial against two, and the gap is small compared with how much the method varies, so this is a hint and not a finding. Hospira's label says to store the product at room temperature, 20 to 25°C.

How big was the fall?

From day 0 to day 120 the readings fell by 5.0% in the refrigerated vial, 7.6% in the first room-temperature vial and 8.4% in the second. The video reports that the test method itself can vary by up to 8%.

So the fall is about the same size as the uncertainty in the measurement. What makes it worth noticing is that all three vials moved the same way at the same time, after three months of sitting still. That pattern suggests a real decline beginning somewhere after day 90. It does not prove one, and with no test after day 120 nobody knows whether the readings would have kept falling or come back up.

What the test did not measure

This is the part that matters most. The test measured preservative. It did not measure sterility. No one checked whether anything was growing in the vials.

A vial with plenty of benzyl alcohol can still be contaminated. Benzyl alcohol slows bacteria down; it does not sterilise, and it does little against some organisms. A dirty stopper or a reused needle can put in more than the preservative can handle. So these results cannot tell you that a vial is safe to use at 60, 90 or 120 days. They tell you only that the preservative was still mostly there.

What official guidance says

The US Centers for Disease Control and Prevention says an opened multi-dose vial should be thrown away within 28 days unless the manufacturer specifies a different date. That rule exists because of contamination risk, which is exactly what this test left out. Nothing in these results is a reason to set it aside.

The limits of this test

One vial per condition. With a single vial there is no way to tell a real difference between conditions from ordinary vial-to-vial variation.

One brand. The results say nothing about other makes of bacteriostatic water, including the unbranded vials many research-chemical sellers supply, which may not contain 0.9% benzyl alcohol to begin with.

No sterility testing, as above.

No published paper. The data was shared on a public forum, not peer reviewed, and we are reporting it second-hand from a video. The video itself notes that the report is unclear about its units.

The short version

In three vials of Hospira BAC water, the preservative stayed at roughly label strength for 90 days, whether the vial was refrigerated or not and whether it was punctured weekly or not. By day 120 it had dipped by 5 to 8%.

That is useful to know, and credit goes to PTDS for paying for it. It is not a test of whether old BAC water is safe. For that, the 28-day rule for opened vials still stands. If you are new to mixing vials, the reconstitution guide on this site walks through the basics.