If you're old enough, you remember this stuff, rust brown and murky. My Dad used to apply it on a swab to my throat whenever I got a sore throat. It was an all-purpose antiseptic, going on scrapes and cuts in liberal swathes.
It turns out my dentist is insane. She suggested very strongly that I use a Betadine solution in my Waterpik, which I use to irrigate under the gums as a measure against gingivitis. But I use a product called Peroxyl, which is sort of like Listerine, with Hydrogen Peroxide in it's mix. Apparently that's not toxic enough. "We use Betadine here when we scale teeth. Just mix it four-to-one with water."
So I dutifully got some (after the second check-up, when she insisted again). And tonight I tried it out. That is the most foul substance in the Universe! I was spitting for the next hour, and my mouth still feels unnatural. I won't be doing that every night.