They washed their mouths out with piss
Piss was somewhat of a hot commodity in Ancient Rome. In fact, people made careers out of collecting piss! Some people collected it from public urinals, others would literally go door-to-door asking people to fill up a cup. But… why?
Piss was used for several things back in those days. It was used to wash clothes. People literally drenched their clothes in piss, and stomped on it with the hopes of having fresh clean clothes by the time they were done. Another way in which urine was used was as… wait for it… mouthwash. People used to rinse their teeth in piss to freshen their breath and whiten their teeth.