Does bac water go bad? My bottle has been open 3 weeks

Bacteriostatic water is 0.9% benzyl alcohol + sterile water. Once opened, stability is usually 28 days per manufacturer spec. After that, the bacteriostatic agent concentration drops enough that it's no longer reliably preserving. Fresh bottles are like $3. Just pitch it.

ok, pitching. thanks.

I draw a little line on my bac vial + date when I open it. never have to guess.

3 weeks is fine, you're well under the 28 day window. People way overthink this stuff. I've used bac water that's been sitting for 6+ weeks and never had issues, but if you're paranoid just grab a fresh bottle since they're cheap.