The Romans used human urine as a mouthwash and teeth whitener. This may seem somewhat disgusting but it was effective.
However, even in Roman times, it was not universally popular, as an extract from a poem by Roman poet Catullus, who was not fond of Egnatius, reads:
Egnatius, because he has snow-white teeth, smiles all the time. If you’re a defendant in court, when the counsel draws tears, he smiles: if you’re in grief at the pyre of pious sons, the lone lorn mother weeping, he smiles.
Whatever it is, wherever it is, whatever he’s doing, he smiles: he’s got a disease, neither polite, I would say, nor charming. So a reminder to you, from me, good Egnatius.
If you were a Sabine or Tiburtine or a fat Umbrian, or plump Etruscan, or dark toothy Lanuvian, or from north of the Po, and I’ll mention my own Veronese too, or whoever else clean their teeth religiously, I’d still not want you to smile all the time: there’s nothing more foolish than foolishly smiling.
Now you’re Spanish: in the country of Spain what each man pisses, he’s used to brushing his teeth and red gums with, every morning, so the fact that your teeth are so polished just shows you’re the more full of piss.