Water fluoridation does not affect IQ, study finds

“Fluoridation does not appear to have any cognitive effects throughout life,” concludes sociologist Rob Warren of the University of Minnesota. With colleagues from Michigan and Wisconsin, he studied the impacts on the brain, particularly on intelligence, of this measure long recommended to combat tooth decay.

Their study published Monday in the journal PNAS comes at a time when the US Department of Health is waging an offensive against water fluoridation and several US states have banned it or are in the process of doing so. Montreal has not escaped this movement against fluoride, the Quebec metropolis having stopped fluoridating water in the Dorval and Pointe-Claire treatment plants at the beginning of 2025. And this, despite a request from the Regional Public Health Department to continue to do so.

Mr. Warren specializes in the factors that impact children’s cognitive development. He decided to do this study when he noted that most analyzes on the subject concluded that the impacts of water fluoridation on intelligence are difficult to apply in the United States.

The study of PNAS covers 10,300 Wisconsin residents who completed high school in 1957. One-third had never been exposed to drinking water fluoridation, and three other groups had been exposed from birth or from ages 8 or 14. They had an IQ test at age 16 and then cognitive tests between ages 53 and 80. No cognitive effects were noted between the four groups.

Data on exposure to water fluoridation came from local municipal archives.

Different effect on boys?

These results, however, go against a Canadian study published in 2019 in a journal of the American Medical Association (JAMA Pediatrics). Its authors concluded that fluoridation had an effect on IQ. It was based on 500 mothers and their children, who were assessed at 3 or 4 years of age. Montrealers were part of the study.

The effect on the children’s IQ was less than four points, when comparing the extremes in terms of the concentration of fluoride in the mothers’ urine. These extremes correspond to mothers living in a place without water fluoridation, and having the least fluoride in their urine (5^e percentile), and to those living in an area with water fluoridation and having the most fluoride in their urine (95^e percentile).

“Some will say that losing three to five IQ points has little impact, but is it worth one less cavity? says Bruce Lanphear, an epidemiologist at Simon Fraser University and author of the 2019 study.

Water fluoridation affected boys more than girls, for poorly understood reasons, according to Dr.^r Lanphear.

“We never have water fluoridation as a factor to consider in our patients,” notes Renée St-Pierre, psychologist at the Brain, Development and Behavior clinic at the Montreal Children’s Hospital. She notes that the study of D^r Lanphear took into account socioeconomic factors, but not mothers’ IQ, and estimates that a drop of four IQ points has “population, but not clinical” impacts.

Impact on caries

Water fluoridation aims to reduce the risk of cavities. “In places where drinking water is fluoridated, the majority of fluoride consumed comes from the water,” says Mr. Warren. But there is also fluoride in toothpaste and it is applied at the dentist. »

In 2025, a study published in the JAMA proposed that abandoning water fluoridation in the United States would increase the number of cavities by 7.5%.

Edwin Chan, a dentist at the Montreal Children’s Hospital, notes that a study showed cavities became more prevalent in Calgary after water fluoridation was abandoned in 2011. That led to its reintroduction in 2025, after a referendum where 62 per cent of the population approved fluoridation. A 2021 study showed that 65% of 7-year-olds in Calgary had cavities, compared to 55% of those in Edmonton, which continued to fluoridate water.

The D^r Chan notes that a minority of parents of his young patients are concerned about fluoride in their children’s toothpaste and that many are unaware if they are using fluoridated toothpaste. He estimates that half of toothpastes contain fluoride.

By 2024, the Biden administration had concluded that water fluoridation was associated with a low risk of lower IQ.