Landmark study affirms fluoride’s link to lowered IQ, adds to debate

Fluoride exposure is consistently linked with lowered IQ, according to a landmark analysis of more than 70 published studies on the subject.

The paper, published January 6 in the journal JAMA Pediatrics and authored by scientists with the National Institutes of Health, found a significant inverse relationship between measures of fluoride exposure and IQ in 64 of 74 studies.

The study, the largest meta-analysis of its kind, found that those exposed to high levels of fluoride have measurably lower IQs, equivalent to a difference of nearly 7 IQ points, compared to those in the low fluoride groups. This conclusion came from 59 studies. Most, but not all, of these examined people who live in areas with naturally-occurring levels of fluoride higher than that used in fluoridation.

Importantly, however, the paper found the link between fluoride and IQ loss persisted even at low levels of fluoride, as measured in human urine samples.

The meta-analysis is an offshoot of the National Toxicology Program’s investigation into fluoride’s likely impact on neurodevelopment and cognition, published in August 2024, and comes at a time of heightened scrutiny of, and debate over, the practice of fluoridation of public drinking water.

In a seven-year-long court case that wrapped up in September 2024, US District Judge Edward Chen of the Northern District of California, ordered the US Environmental Protection Agency (EPA) to take regulatory action to eliminate the “unreasonable risk” that water fluoridation presents. The fluoridation of water at 0.7 ppm “poses an unreasonable risk of reduced IQ in children,” Chen wrote in his decision.

The issue remains divisive, though a growing number of epidemiologists and experts on neurotoxins have spoken out against the practice. Many dental researchers and groups, such as the American Dental Association, remain outspoken supporters of fluoridation.

The study seriously challenges the merits of the longstanding US policy of water fluoridation, in which fluoride is added to the water supply as a method of reducing cavities, usually to a concentration of around 0.7 parts per million (ppm), according to Bruce Lanphear, an expert on environmental neurotoxins and a health sciences professor at Simon Fraser University in Vancouver.

“We can no longer afford to sweep this under the carpet,” said Lanphear, who wrote an editorial accompanying the study but was not involved directly in the analysis.

The evidence is now “quite convincing that fluoride is toxic down to the levels that are found routinely in fluoridated communities,” he said, adding that this conclusion stuck out even though the authors “were, as government scientists, very cautious.”

Steven Levy, a dental researcher with the University of Iowa, who also wrote an editorial that accompanies the study, disagreed, writing that there is “no evidence” of adverse effects at levels used in water fluoridation. He advocated for continuation of the practice in his editorial.

Robert F. Kennedy Jr., incoming President Donald Trump’s nominee to oversee the US Department of Health and Human Services, has said that fluoridation is associated with a range of health problems in addition to IQ loss, and said the Trump Administration will advise water systems to remove fluoride from public water.

Voters in some parts of the country have also registered opposition to the practice, including recent rejections of ballot measures in Oregon cities that would have put fluoride in drinking water, and since the NTP report and California court cases, dozens of cities have stopped fluoridating their water supplies.

Worldwide studies

The meta-analysis combed through a complex array of studies conducted worldwide, some of which measured fluoride in water, or in maternal or children’s urine, and others of which attempted to establish a dose-response relationship between fluoride and IQ scores. The paper concluded some of the studies carried a risk of bias, which has a specific scientific meaning relating to the study design, transparency, and likely ability to correctly interpret data.

The meta-analysis found that among 12 low “risk-of-bias studies,” or best quality, there was a decline equivalent to 2.8 IQ points associated with fluoride exposure, comparing groups exposed to higher and lower levels of the substance. Across all 59 studies, there was a 7 point IQ difference between the high and lower fluoride groups.

Most of the data support the conclusion that the link between fluoride and IQ decrements appears to be linear. There is some uncertainty and a lack of data at lower levels of fluoride exposure associated with water fluoridation, however.

A majority of the studies examined situations where people are exposed to naturally-occurring levels of fluoride in water, such as in China, in excess of 1.5 ppm, the safety limit established by the World Health Organization.

The paper found that the link between fluoride ingestion and lowered IQ held for urinary fluoride concentrations below 1.5 ppm. This is within the range of levels commonly found in people where water fluoridation is practiced, including the United States and Canada.

The conclusions elaborate upon data from the August NTP report, which concluded that “there is moderate confidence in the scientific evidence that showed an association between higher levels of fluoride and lower IQ in children” but that there were “insufficient data to determine if the low fluoride level of 0.7 mg/L currently recommended for US community water supplies has a negative effect on children’s IQ.”

The data do not conclusively show there is a lower level of exposure where the link between fluoride exposure and IQ impairment disappears, Lanphear said.

“Especially given the consistent linear relationship between individual fluoride exposures and reduced IQ, people should understand that there is no safe level of fluoride exposure when it comes to effects on the brains of developing children,” added Kathleen Thiessen, a researcher with the Oak Ridge Center for Risk Analysis in Tennessee who testified as an expert witness in the California court case.

Urinary fluoride measures are considered superior to solely measuring water levels since it accounts for all sources of fluoride, which is found in high levels in tea, for example, and some other foods, explained Philippe Grandjean, a long-time Harvard researcher and physician who studies neurotoxic substances including lead and mercury.

The paper did not find a statistically significant link with IQ loss in studies that only measure fluoride levels in water — but not in the urine — below 1.5 ppm.

The finding “does not exonerate fluoride as a neurotoxic chemical,” Lanphear said, since this “doesn’t give you the total exposure in many cases.” Between 40% to 70% of people’s fluoride exposure comes from consuming fluoridated water.

Grandjean agreed, noting that only measuring water fluoride does not establish any kind of dosing, and thus such studies “only provide weak evidence.”

The 13 analyzed studies that quantified individual exposures from all sources found an IQ score decrease of 1.63 points per 1 ppm increase in urinary fluoride.

Small declines in IQ can have enormous individual and societal impact. If the average IQ declined by 5 points in the US, for example, the number of people defined as being “intellectually disabled” would double.

The evidence has convinced Grandjean that the risk posed by fluoridation is “unreasonable” and the practice should be stopped.

Still effective?

There is surprisingly little data that water fluoridation is currently effective at reducing cavities. A 2024 Cochrane meta-analysis, the gold standard for assessing evidence-based health interventions, found that studies conducted after 1975 show the practice may reduce levels of decayed, missing or filled teeth by 3% to 4%, but that the practice may also provide “no benefit.”

The same review also found there was insufficient data to determine if ceasing fluoridation caused cavities to increase in any given area. It noted that the practice may be linked with dental fluorosis, a staining of the teeth caused by the substance. “With a fluoride level of 0.7 ppm, approximately 12% of participants had fluorosis of aesthetic concern, and approximately 40% had fluorosis of any level,” the review noted.

Fluoridation began in the 1940s and 1950s and it was linked with significant reductions in cavities in its early days — though similar declines have been observed in countries worldwide that don’t add fluoride to the water. In the last 20 years or so, it has become clear that fluoride primarily acts topically, Grandjean said. Fluoride toothpaste, which should not be swallowed, is effective at reducing cavities.

In 2011, in response to a lawsuit by the Fluoride Action Network, Environmental Working Group, and Beyond Pesticides, the Department of Health and Human Services recommended that water utilities reduce the amount of fluoride added to tap water to 0.7 ppm; previously it had recommended a range between 0.7 and 1.2 ppm. That change took effect in 2015.

“What we’re seeing is an evolution of science,” Lanphear said, with a growing body of research suggesting even low levels of fluoride might present a risk to the brain, and a lack of conclusive proof since 1975 that fluoridation meaningfully reduces cavities.

“The more people look at the evidence, the more it will become obvious that we need alternative ways to protect children against tooth decay — water fluoridation is not a good way to do that,” Lanphear said.

(Featured image by Lia Bekyan for Unsplash+)