Ancient Romans Used Urine as Mouthwash — and the Science Behind It Actually Makes Sense

An Unlikely Dental Hygiene Practice

It is easy to dismiss ancient health practices as simple ignorance — the product of cultures that lacked the scientific knowledge to make better choices. But the Roman use of urine in oral hygiene is a more nuanced story. When examined through the lens of chemistry, the practice has a rational basis, even if the theory behind it was articulated differently than a modern scientist would express it.

Human urine is primarily water but contains urea, creatinine, and various salts. When urine is stored or aged, bacteria metabolize the urea, converting it to ammonia. Ammonia is a base — it reacts with acids and acts as an effective cleaning and whitening agent. This is why ammonia-based compounds are still found in some modern household cleaners and, historically, in industrial textile processing. The Romans who used urine as a mouthwash were, in chemical effect, rinsing with a dilute ammonia solution.

A Commodity Worth Taxing

The use of urine extended well beyond personal dental hygiene in Rome. Urine was collected — from public urination vessels placed at street corners throughout Roman cities — for use in the textile industry, where it was used as a mordant to fix dyes and as a cleaning agent for wool and cloth. The tanners who processed animal hides used urine to soften the material. Laundries, called fulleries, used urine as a cleaning agent for togas and other garments.

This commercial demand made urine a valuable commodity. The Emperor Vespasian, who ruled from 69 to 79 AD, imposed a tax on the collection and sale of urine — specifically on the commercial trade between the collectors and the laundries. When his son Titus reportedly complained that it was undignified to tax urine, Vespasian held a coin under his son's nose and asked whether it smelled bad. This exchange gave rise to the Latin phrase Pecunia non olet — "Money does not smell" — which has passed into modern Italian, Spanish, and French as a proverb about the morally neutral nature of profit.

Roman Dental Practices in Context

Roman dental hygiene, while different from modern practice, was more sophisticated than is sometimes assumed. Romans used various abrasive tooth powders made from crushed shells, bone, pumice stone, and salt to clean teeth and remove surface stains. Recipes for tooth powder recorded in texts such as Pliny the Elder's Natural History include ingredients now known to have genuine cleaning properties, alongside others that were simply traditional.

The wealthy could consult dental practitioners — the Romans had professionals who extracted teeth, fitted false teeth made from animal bone or ivory held in place with gold wire, and treated toothache with various herbal preparations. The use of urine as a mouthwash was simply one component of a broader oral care tradition that combined genuinely effective chemistry (the ammonia), practical abrasion (tooth powders), and the accumulated folk knowledge of a civilization that had been managing everyday health for centuries.

The Gap Between Knowledge and Understanding

What the Roman urine mouthwash story illustrates is the gap that can exist between effective empirical knowledge and correct theoretical explanation. Roman practitioners knew that urine whitened teeth; they attributed this to a vague notion of the liquid's cleaning properties, or to divine or humoral principles, rather than to ammonia chemistry. The observation was correct. The mechanism was not understood. This pattern — correct empirical observation, incorrect theoretical framework — runs throughout the history of medicine and technology. The useful knowledge was real, even when the explanation was wrong. Modern science has not eliminated this pattern; it has only moved the boundary of where understanding ends and useful approximation begins.