Medieval teeth have yielded an unexpected archaeological treasure: evidence of mercury-based medicine administered to leprosy patients nearly 900 years ago. Researchers led by Dr. Elena Fiorin analyzed microscopic calculus—the hardened plaque that accumulates on teeth during life—from 76 individuals buried in two medieval leprosaria, discovering significantly elevated mercury levels that point to a deliberate and sustained medical treatment regimen lost to history.
This discovery matters because it reveals how medieval caregivers grappled with one of history's most stigmatized diseases. For centuries, leprosy patients were isolated in leprosaria—specialized hospitals and burial grounds—yet almost no written medical records survive to explain how they were actually treated. St Leonard's Leprosarium in Peterborough, England, founded in 1125 to serve the Cathedral Abbey, and St Thomas d'Aizier in France, a property of the Abbey of Fécamp dating to the late 11th century, left no documentation of their medical practices. Until now, leprosy care remained a silent chapter in medieval medicine.
The research required entirely new methodology. Rather than examining bones, teeth, or hair as previous studies had done, Dr. Fiorin's team pioneered the use of dental calculus to trace mercury exposure—a first on such a large archaeological scale. "Dental calculus is hardened plaque that forms on teeth during life," Dr. Fiorin explained. "Although it might seem insignificant, it is a highly informative material because it can trap tiny particles from what a person ate, drank, inhaled, or was exposed to." Because calculus forms in the mouth while a person is alive, it captures substances that enter the body directly, making it particularly useful for identifying repeated medical exposures that may have been applied in or around the mouth.
The evidence is striking. Individuals buried in leprosaria showed significantly higher mercury levels in their dental calculus compared to people buried in non-leprosaria cemeteries—St Pierre in Thaon, France, and St Michael in Leicester, England. Crucially, the researchers analyzed 45 soil samples from the graves themselves to rule out environmental contamination from local sources like mining. They found that mercury was incorporated during life, not afterward. One dramatic case illustrates the pattern: a female buried at a leprosarium contained 3.8 mg/kg of mercury in her calculus, while a male interred beside her showed 0.4 mg/kg—both clear evidence of exposure, yet the surrounding soil contained only trace amounts, eliminating environmental explanations.
Mercury, or quicksilver, was highly valued in medieval medicine for treating skin diseases and infections, including leprosy. The metal was volatile and highly toxic, yet physicians applied it in ointments mixed with fats and oils, rubbed onto the skin. The calculus samples hint that some patients may have been exposed to mercury vapor or ingested medicinal preparations containing it—a form of early pharmacological intervention, however toxic by modern standards.
What emerges from dental calculus is a portrait of institutional care and medical innovation amid crisis. Medieval leprosaria, far from being mere warehouses for the afflicted, employed available treatments. Their practitioners recognized the disease, attempted remedies, and recorded nothing in writing—their knowledge died with them. Now, centuries later, the teeth of their patients tell the story of medical ambition, the willingness to experiment with dangerous substances in pursuit of healing, and the limits of medieval science. The study transforms leprosaria from forgotten margins of medieval society into laboratories of hope.