Internet suggests that it actually has to be ingested by the insects, not just have contact with their exoskeleton. So for example it doesn't work on bedbugs since they won't eat it.
Don't breathe it in though.
The ants would happily eat either peanut butter or honey by itself. But they avoided the mixtures. I did spend a couple of days trying different strengths until I ran out of boric acid.
Then I gave up and bought a tiny dropper bottle of ant poison. Put a single drop on each ant trail I could see (around 5 drops total). Two days later, I didn't have an ant issue anymore.