Bacteriostatic water vs sterile water — is the difference actually important?

The reconstitution guide distinguishes between bacteriostatic and sterile water and I do not understand why it matters if both are sterile to begin with.

Both are sterile in the bottle. The difference is what happens after you put a needle through the stopper.

Sterile water has nothing in it. The moment you have breached the seal you are relying entirely on your own technique to have introduced nothing, and you are relying on that again every subsequent time you draw from it.

Bacteriostatic water contains benzyl alcohol, usually around 0.9%. It does not sterilise anything and it will not rescue a contaminated vial. What it does is stop small numbers of organisms multiplying, which is what turns "a few cells got in" into "the vial is now a culture" over a week in a fridge.

So: single use, immediately, either is fine. Multi-dose vial you will come back to for a fortnight, the distinction is real.

The two exceptions to bacteriostatic being the default are worth knowing because they are the ones that catch people out.

Neonates — benzyl alcohol is contraindicated, this is a well documented and historically nasty problem. And anyone with a known benzyl alcohol sensitivity, which is uncommon but exists.

Neither is likely to be you, but they are the reason the guide says "usually" rather than "always", and a guide that says always is a guide that has not thought about it.

From twenty years of multi-dose vials: the thing that actually protects you is not which water you chose, it is swabbing the stopper every single time and letting it dry.

Every single time. Not "it looks clean". Not "I did it an hour ago". And letting it dry matters — alcohol does its work while evaporating, and stabbing through a wet stopper achieves close to nothing.

The bacteriostatic water is a safety net under that habit. It is not a replacement for it, and I have met people who treated it as one.

Also: it does not fix a vial that was already contaminated when it arrived, which is the failure mode nobody plans for because you cannot see it.

Bacteriostatic just means the organisms sit there instead of doubling. Sitting there is still in your arm eventually.

Understood. Static not cidal, buys time on a multi-dose vial, does not substitute for swabbing, does not rescue anything already wrong.