[alyx]

Excuse my ignorance, but aren't antibiotics meant for humans, tested on us as well?

I think the issue here is that antibiotics being given to animals were not studied for these scenarios. For instance, how they may impact milk and meat quality. Not to mention that they are given to animals in much less "regulated" way, compared to people.

[illivah]

At some point, yes. But that's not the first place they're tested. The first place their tested could simply be a petri dish, as penicillin was. Then it goes up the ranks into mammals more and more similar to us - rats, pigs, monkeys, etc. Usually it's not every step, because there really isn't a big reason to and that can slow development and increase costs of life-saving drugs.