Obviously, people shouldn't do that. But some antibiotics go through the body almost unchanged. When penicillin was first developed they used to recycle the antibiotic from the patient's urine over and over because they had so little of it.
Because of this so-called "first path" effect you end up taking enough that hopefully some (and enough) "gets to the required location" and not too much gets where it's not wanted (most of what counts as "side effects"). The approval process focuses on this and ignores anything excreted, which shouldn't be surprising: with few exceptions the process is #1 safety and #2 minimal clinically effective dosing.
* former pharmaceutical scientist; have designed protocols approved by the FDA.
I'm not sure this is obvious or even particularly well-publicized.
The "official recommendation" in the U.S. for many meds is that they be flushed down the toilet, especially scheduled/controlled substances. The FDA maintains a "flush list" [1] of medications that you are specifically instructed to dispose of by flushing. However, even for medications not on the official flush list, it's common to be informed by reasonable authorities that you should dispose of them by flushing -- e.g. I had to sign paperwork at my doctor's office affirming that I'd responsibly dispose of my unused ADHD meds by flushing them down the toilet before my dr would write the prescription. This was seconded by the drug/alcohol training I was given as a condition of attending college, which stated that you should flush all unused medication.
[1]: https://www.fda.gov/drugs/disposal-unused-medicines-what-you...
Back in Europe, you could just hand off your unused meds to the local pharmacy and rest assured they'll be handled more responsibly.
That's its whole purpose. Just look at the name "antibiotic"...
* Ignoring minute traces of minerals that are lost through transpiration