The mineral fluoride, at the right dose, has been shown to reduce the risk of tooth decay. Based on studies demonstrating this in children drinking naturally fluoride-containing water, individual cities in the U.S. began to add fluoride to tap water beginning in 1945.
The Centers for Disease Control and Prevention and multiple expert groups endorse water fluoridation as a safe way to reduce tooth decay, including the American Dental Association, the American Academy of Family Physicians and the American Academy of Pediatrics.
However, a Feb. 4 post from independent presidential candidate Robert F. Kennedy Jr. on X, formerly known as Twitter, made a sweeping claim about fluoride’s effects on the nervous system. “As president. I’m going to order the CDC to take every step necessary to remove neurotoxic fluoride from American drinking water,” the post said.
Kennedy, who has a history of advocating against water fluoridation, accompanied his claim about fluoride’s neurotoxicity with a link to a Law360 article about testimony in a trial that has been unfolding in a San Francisco-based federal district court. The case was brought against the Environmental Protection Agency by nonprofit organizations and other plaintiffs and alleges that fluoridation poses “an unreasonable risk of injury to health” under a version of the Toxic Substances Control Act amended in 2016. The plaintiffs are asking the EPA to disallow adding fluoride to drinking water.
Other popular social media posts have also referenced the trial, claiming that “multiple studies confirm fluoride is a neurotoxin that violates the Toxic Substances Control Act and reduces IQ in kids.”
But the data on water fluoridation and neurotoxicity are less clear-cut than social media posts by Kennedy or others make them out to be.
Some studies — many of them done in areas of the world with naturally high levels of fluoride in their water supplies well above the optimally recommended level — suggest a possible association between greater levels of fluoride exposure during pregnancy or early childhood and reduced IQ in children. But many scientific experts have said the evidence for this association is weak.
The EPA has argued that there isn’t strong or consistent evidence fluoridation at recommended levels lowers IQ — in line with the general sentiment held by the CDC and various expert groups that water fluoridation is safe.
U.S. Regulation of Fluoride in Water
On a federal level in the U.S., the Public Health Service first recommended fluoridation of tap water in 1962. However, the decision on whether to add fluoride to tap water is up to states and municipalities. As of 2020, around 63% of Americans received fluoridated water.
Exposure to fluoride in early childhood is known to cause dental fluorosis, a condition most often characterized by mild discoloration of the teeth. The AAP says that it is safe to mix baby formula with fluoridated tap water, although consuming fluoride isn’t necessary for babies under 6 months old and comes with a small risk of dental fluorosis.
According to the CDC, experts have concluded there isn’t an association between recommended water fluoridation and any other negative health impacts.
Based on evidence of skeletal problems when people are exposed to quite high levels of fluoride over time, the Environmental Protection Agency has set an upper limit of 4 mg per liter for fluoride in tap water from public water systems. However, the agency recommends that fluoride levels in tap water be kept below 2 mg per liter to protect from dental fluorosis. The fluoride level recommended by the Public Health Service to improve dental health is below these limits — at 0.7 mg per liter.
Beyond fluoridated water, sources of fluoride can also include such items as black tea or swallowed toothpaste. It is generally only present in very small amounts in food, although fluoridated salt or milk rather than fluoridated water are used in some non-U.S. countries.
Draft Report Wasn’t Meant to Evaluate Water Fluoridation Safety
In a Feb. 6 post, also on X, Kennedy elaborated on his fluoridation claims, referencing a draft report from the National Toxicology Program that has been a focus of the case against the EPA. A final version of the report has not been published.
“The National Toxicology Program (NTP) has declared, ‘… the data support a consistent inverse association between fluoride exposure and children’s IQ,’” Kennedy’s post said, quoting from an outdated version of a meta-analysis document associated with the report and leaving out some context. A meta-analysis is a type of study in which researchers gather the available data on a topic and combine it to attempt to draw a larger conclusion.
But the NTP report was not meant to establish whether water fluoridation at typical levels was safe and looked at fluoride exposure from any source and at any level. Scientists who reviewed the draft for the NTP expressed concerns that the sentence Kennedy quoted did not make this clear.
The NTP’s reports “are used by other federal agencies as a starting point for further study to determine if there is a risk to humans, and at what exposure level,” a spokesperson from the National Institute of Environmental Health Sciences, which houses the NTP administratively, told us via email. The fluoride report “is not a risk assessment, and therefore, does not determine the safety of fluoride.”
Kennedy also claimed in his post that the NTP report had been “hidden from the public.” The NIEHS spokesperson told us that the report is still being revised and that publication was delayed by the NTP director, who tasked a working group with reviewing the many comments and criticisms of the document.
Multiple groups of experts — from both within and outside the government — reviewed various drafts of the report, saying they had concerns that its conclusions were not properly supported. A recurring area of concern was whether the authors of the NTP report had sufficiently made clear that their overall conclusions on fluoride’s effects on IQ might not apply to the lower levels of fluoride found in properly fluoridated drinking water.
“The authors point to their inclusion of studies with low fluoride levels but provide no interpretation of the evidence at these levels,” wrote the working group assembled to review criticisms of the report. “Rather, the authors provide a single statement in the Abstract that encompasses all studies: ‘The data support a consistent inverse association between fluoride exposure and children’s IQ’. This may overstate the evidence provided by studies with low exposure.”
Evidence on Water Fluoridation and IQ Is Limited
David Savitz, an epidemiologist at Brown University who studies the effects of environmental exposures on reproductive health, led a group of experts that reviewed two early versions of the NTP report. This group was convened by the National Academies of Sciences, Engineering, and Medicine, which provides independent advice on scientific topics to help the government set policies.
Savitz testified in the trial as a witness for the EPA, zeroing in on four long-standing cohort studies looking at prenatal and early-life exposures to fluoride via various sources, including fluoridated water but also fluoridated salt. These studies evaluated fluoride exposures at levels most relevant to the discussion of water fluoridation. They all provided measurements of fluoride in the urine of pregnant women and assessed their children using cognitive tests.
“There is not at this time a consistent indication of there being an association present, let alone a causal association,” Savitz said during Feb. 7 testimony, speaking of IQ and fluoride exposure in the “range of interest.”
The OCC and INMA studies, respectively performed in Denmark and Spain, found no link between increased urinary fluoride levels and reduced cognitive test scores. A study in women in Mexico, called ELEMENT, found an association between increased urinary fluoride levels during pregnancy and reduced cognitive test scores in children.
The MIREC study, of women in Canada, “in my view is mixed,” Savitz said. “In the aggregate results, which is I think where one starts, it’s very limited in indicating a potential adverse effect.” But it did show “notable sex differences,” he said. The study stated that increased fluoride in the urine of pregnant women was associated with reduced IQ scores in boys.
Other researchers have criticized some of the methods and conclusions of the MIREC study, writing, for instance, that it was unclear whether the researchers planned their assessment by sex prior to starting the study. Doing unplanned subgroup analyses can lead to false-positive results, the researchers wrote.
Authors of some other recently published meta-analyses have also discussed the limited evidence on fluoride’s neurotoxicity — particularly for people drinking water with the recommended 0.7 mg of fluoride per liter.
A 2021 meta-analysis published in Scientific Reports found that exposure to high levels of fluoride was associated with lower IQ but did not find a link between exposure to low levels of fluoride and neurological problems. The researchers defined high fluoride exposure as above 2 mg per liter and low exposure as between 0.5 and 1 mg per liter. The researchers ultimately concluded that the quality of the evidence was low overall and did not allow them “to state that fluoride is associated with neurological damage,” even at relatively high doses.
Another meta-analysis, published in 2023 in Environmental Research, did conclude that studies indicated fluoride exposure was associated with lower IQ in children, potentially starting at 1 mg per liter or lower. But the researchers also noted problems with the quality of the studies that had been done, finding that those showing the greatest negative impact of fluoride were at a high risk of bias. Bias occurs when there is some systematic error that leads a study’s findings to be incorrect — such as confounding factors that would make a relationship seem real when it is not. The single study found to be at low risk of bias did not find a negative effect of fluoride on IQ.
Finally, a study published in the journal Public Health in 2023, which only evaluated studies in which people were exposed to levels of fluoride 1.5 mg per liter and lower, did not identify a relationship between fluoride levels and IQ in various analyses. “These meta-analyses show that fluoride exposure relevant to community water fluoridation is not associated with lower IQ scores in children,” the researchers concluded.
The Stakes of Ending Fluoridation
In the case against the EPA, lawyers are not allowed to discuss the benefits of water fluoridation. But amid calls to halt fluoridation, experts told us, a discussion of the potential impacts is warranted.
Lindsay McLaren, a professor of community health sciences at the University of Calgary, looked at what happened after the city of Calgary stopped fluoridating its water in 2011. She also has reviewed other research on the impacts of stopping fluoridation. (Calgary will resume water fluoridation later this year.)
“At least in the settings that have been studied, if you cease community water fluoridation, children’s oral health declines,” McLaren said. This particularly affects children who do not have good access to dental care.
“Tooth decay is not an innocuous problem,” McLaren said. “It causes pain, it can get infected, it can make it so that it hurts to eat, kids might have trouble concentrating in school.” She added that in extreme cases tooth decay can lead young children to need surgery under general anesthesia, which comes with known risks.
“The reason why we put fluoride in water is because it has a demonstrable positive impact on dental health,” Dr. Steven Novella, a neurologist at Yale School of Medicine, told us. In addition to reducing cavities and improving overall dental health, it “has downstream effects as well because bad dental health can cause general health problems, heart disease, etc,” he said. Novella has written about anti-fluoride claims for many years on his blog and on the website Science-Based Medicine, which he founded.
Novella said that while data indicate potential neurotoxicity from fluoride at high doses, fluoridation at recommended levels “hasn’t been shown to be an actual risk in the real world.”
“You have to show that it’s causing an unacceptable risk that’s greater than the benefit at the dose people are actually getting exposed to,” he said, which is not what the data show.
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