Black tea, a staple in many U.S. households, is the world’s most consumed beverage; yet it may contain higher concentrations of fluoride than previously thought. This could pose problems for excessive tea drinkers, say researchers at the Medical College of Georgia, in Augusta.

Their recent study discovered that the fluoride concentrations in black (not green or white) tea  have been underestimated, and also may be linked to incidences of skeletal fluorosis, a disease caused by excessive fluoride consumption, characterized by joint and bone pain and damage. Study participants had a 10- to 30-year history of consuming one to two gallons of black tea a day. The problem is exacerbated when the tea is steeped in fluoridated drinking water or when other fluoride sources, such as treated toothpaste, are also present, as they add to the ingestion of that mineral.

The researchers are quick to also say that black tea remains a healthy beverage for those who enjoy the beverage in moderation—between two and four cups a day.