Which everyday substance did ancient Romans use as a mouthwash and tooth whitener?

Ancient Romans genuinely used urine as a mouthwash and tooth whitener, valuing it specifically for its ammonia content, which acted as a cleaning and whitening agent on teeth and could also be used in processing cloth. Portuguese urine was reputedly considered especially potent for this purpose, and the practice became significant enough economically that it was taxed as an imported commodity by the Roman state.

Wine and honey, while both common substances in Roman daily life, were not the specific product prized and taxed for its ammonia content in this way, that distinction belonged specifically to urine, however unusual it may sound to modern ears.

The Roman fondness for this practice inspired the well-known jab from the poet Catullus, who mocked a rival for having unusually white teeth, implying he must be using entirely too much urine to achieve it, showing that even Romans found the habit a bit much.