Tests of intelligence and brain function were the same whether or not people drank fluoridated water growing up, a highly anticipated, long-term study found.
The new research, published Monday in the respected Proceedings of the National Academy of Sciences, is the first to measure community water fluoridation exposure during childhood in the U.S. and any potential impact on cognition up to age 80. The results contradict claims made by Health Secretary Robert F. Kennedy Jr. that fluoride is “industrial waste” associated with IQ loss.
Dr. Scott Tomar, head of the department of population oral health at the University of Illinois in Chicago, called the new study “quite significant.”
“I think that the public can be assured,” Tomar, who was not involved with the new research, said. “There is no association with community water fluoridation and any measure of IQ or neurodevelopment.”
Fear over a link to lower IQ scores has been cited by a growing number of communities across the country that prohibit the addition of fluoride to drinking water. Two states — Utah and Florida — have enacted statewide bans. Several other states, including Kentucky, Louisiana, Missouri and Oklahoma, have similar legislation pending.
Opponents of water fluoridation have often pointed to small studies that suggested a possible link between the mineral and kids’ IQ. Those studies were conducted in China or other countries with much higher fluoride concentrations than allowed in the U.S.
The optimal level of fluoride in drinking water to prevent cavities is 0.7 milligrams per liter, according to the Centers for Disease Control and Prevention. That amounts to 3 drops in a 55-gallon barrel. The legal limit in U.S. drinking water is 4.0 milligrams per liter.
The lack of high-quality data prompted the new research from Rob Warren, a sociologist and population health expert at the University of Minnesota. His is the first robust U.S.-based study of water fluoridation’s possible effects on intelligence and brain power from the teen years into older adulthood.
He used data from the Wisconsin Longitudinal Study that’s followed 10,317 people in the state since they graduated from high school in 1957. Participants took IQ tests at age 16, then did cognitive testing later in life, at ages 53, 64, 72 and 80. The original purpose of the data wasn’t to look at fluoride, so Warren’s team didn’t have urine or blood tests to measure exact levels of fluoride. They estimated their exposure based on records of when community water fluoridation began in certain areas, and on locations of untreated wells.
“I was curious about the short-term effects on adolescent cognition,” Warren said, “but also cognitive functioning later in life. Because if there’s negative consequences for early life IQ, you might expect long- term effects.”
His team found no difference, at any stage of life, between people who grew up with water fluoridation in Wisconsin and those who did not. Since 1995, 86 Wisconsin communities have stopped adding fluoride to municipal water systems, according to the state’s Department of Natural Resources.
The new research builds on research Warren published in December 2025, which found no link between community water fluoridation in early life and tests of brain function at age 60.
Dr. Bruce Lanphear, a professor of health sciences at Simon Fraser University in Canada, said Warren’s latest research is “one of the more rigorous attempts to examine fluoridation and cognition across the life course.” Lanphear published a 2019 study that found IQ levels to be slightly lower in 3- and 4-year-old children whose mothers had higher measures of fluoride in their urine while pregnant.
There is a limitation in the new study: The researchers didn’t measure how much fluoride individuals actually consumed, Lanphear wrote in an email.
“It infers exposure from place of residence,” Lanphear wrote. “It also cannot account for total intake from sources such as infant formula, toothpaste, or diet. If you don’t measure individual exposure, you risk missing the real signal.”
Warren said his studies shouldn’t be interpreted as the final word on the matter, and should prompt additional research. “There’s now good reason to doubt the claim that fluoride causes reduction in IQ.”
More recently, the Trump administration has backed off demonizing fluoride. In March, Dr. Jay Battacharya, who is currently leading the CDC, told a House Appropriations subcommittee that “fluoride is essential for oral health,” although he maintained that too much “can have neurological and developmental impacts.”
But the issue has already instilled panic in some families who now refuse to allow their children to be treated with fluoride in dentist offices.
“I’ve never seen as much pushback to fluoride as I have in these last few years,” Dr. Meg Lochary, a pediatric dentist in Union County, North Carolina, said. Union County stopped water fluoridation in 2024.“People are very, very wary of it.”
Water fluoridation has been heralded as one of the top public health initiatives of the last century for its ability to fight cavities.
Major public health groups, including the American Dental Association, the American Academy of Pediatrics and the CDC, support the use of fluoridated water. All cite studies that show it reduces tooth decay by 25%.
“One of the main reasons kids miss school is because of toothaches,” Tomar said. “In the most severe cases, it can proceed to become an infection that then spreads to other parts of the body.”
Dr. Susan Fisher-Owens, professor of pediatrics and preventive and restorative dental sciences at the University of California San Francisco, said a growing body of research is showing links between poor oral health and chronic diseases that develop later in life, like diabetes and dementia.
The addition of fluoride in community water systems is a “low-cost, safe way to help protect people,” she said.
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