Why do we add fluoride to drinking water, and is it safe?

More than 200 million people in the United States live in areas where fluoride is added to tap water. According to the most recent data available, 24 other countries around the world — including the United Kingdom, Brazil and Canada — add fluoride to their drinking water supplies, and many other jurisdictions end up ingesting fluoride via naturally occurring deposits in their water sources.

Water fluoridation has been lauded as one of the top public health achievements of the 20th century by the U.S. Centers for Disease Control and Prevention (CDC). But the practice has long been controversial, with vocal opponents arguing that the water fluoridation has adverse health or cognitive impacts.

So why do we add fluoride to water in the first place? And what do we know about its safety?

How water fluoridation began

The history of water fluoridation begins in the early 20th century, when a young dentist named Dr. Frederick McKay noticed a strange phenomenon in Colorado Springs. Residents of the town had distinctive brown, mottled stains on their teeth. Further observation revealed that, despite these notable stains, people with the "Colorado brown stain" seemed to have teeth that were unusually resistant to decay. Tooth decay, also called caries or cavities, causes holes to form in teeth.

Researchers traced the cause of the stain to the town's water supply, which contained abnormally high levels of fluoride. This finding prompted dentists to investigate whether it was possible to reap the beneficial, tooth-protecting effects of fluoridated water without experiencing the downsides of staining your teeth.

In 1945, Grand Rapids, Michigan, became the first city in the world to approve a pilot water fluoridation program. Researchers added a small amount of fluoride to the city's water supply and measured rates of tooth decay in the town's nearly 30,000 schoolchildren over the next decade. Their findings revealed that children who were born after water fluoridation began had more than 60% fewer cavities than those born prior to the program.

Related: How does fluoride prevent cavities?

The benefits of fluoridated water

Since 1945, water fluoridation has proliferated as a widespread public health practice around the world, and organizations such as the American Dental Association (ADA) stand firmly in support of water fluoridation. Many studies support the notion that water fluoridation prevents cavities in children.

"Fluoride was found to significantly reduce tooth decay," Neil Maniar, a professor of public health practice at Northeastern University, told Live Science in an email. "Putting [it] in the water supply is a form of passive prevention that can improve oral health."

Although fluoridated toothpaste has many of the same tooth-protecting properties of fluoridated water, experts say the latter comes with certain public health benefits that toothpaste lacks. That's because ingesting fluoride, rather than brushing with it, leads to more constant levels of fluoride in the mouth throughout the day, which is especially beneficial for children with developing teeth. Fluoridated water also provides a baseline level of protection for lower-income people who may not have consistent access to fluoridated toothpaste or to other forms of dental care.

"It is a way for everyone in a community to benefit from fluoride in a cost-effective manner," Maniar said. "While we often take access to a dentist and even something as simple as being able to brush teeth twice a day for granted, this is a challenge in many communities."

That was true in decades past — but some recent research is raising the question of how helpful fluoridated water is for cavity prevention.

A review of over 150 studies showed that, in recent years, water fluoridation may not be as effective at preventing cavities as it was in the 1970s, after fluoridated toothpaste became more widespread. This change may be because fluoridated toothpaste is providing enough benefit that fluoridated water no longer makes as dramatic of a difference.

However, it's important to note that the review did not include any studies from low-income countries, where fluoridated toothpaste may be less readily available. The review also notes that challenges in measuring the effects of fluoridation programs make their potential benefits difficult to quantify over time.

Is fluoridated water safe?

Both the CDC and the ADA maintain that at the levels found in the public water supply — about 0.7 mg per liter — fluoride is beneficial to public dental health and carries few documented risks.

Still, fluoride has been the focus of much safety research, and the topic continues to be a matter of public interest and controversy.

It's well known that excessive fluoride consumption can lead to fluorosis, the condition behind the "Colorado brown stain" observed by McKay in Colorado Springs. Dental fluorosis causes mottled white, yellow or brown spots and stains to appear on the teeth. But it does not compromise oral health.

Consuming even higher levels of fluoride over a long period of time can also lead to a condition called skeletal fluorosis, which compromises the integrity of the bones. This is because excessive deposits of fluoride in the skeleton causes the bones to become hard and brittle, making them more susceptible to fractures.

Both dental fluorosis and skeletal fluorosis generally occur in regions with abnormally high levels of natural fluoride deposits that leach into the water supply. Because skeletal fluorosis requires extremely high levels of fluoride exposure, it's mostly observed in regions with fluoride-contaminated groundwater, but dental fluorosis can occur due to low-level fluoride exposure through artificially fluoridated water.

Initial concerns about fluoridated water focused on whether it could cause cancer. This belief stemmed from the theory that fluoride build-up in the growth plate of the bones could lead to osteosarcoma, a type of bone cancer. While this connection is difficult to evaluate in humans, most studies have not found any strong evidence for such a link.

Does fluoride affect IQ?

More recently, concerns have been raised about the potential neurotoxic effects of fluoride.

A contentious 2024 monograph from the U.S. National Toxicology Program (NTP) looked back at dozens of studies and reported that high levels of fluoride exposure were associated with lower IQ in children. Between kids with high and low exposure levels, the average IQ difference amounted to about one or two points.

However, the levels of fluoride exposure associated with IQ changes were roughly double the levels of fluoride found in water supplies of countries like the U.S. or the U.K. — about 1.5 mg per liter. The authors also noted that, while high fluoride levels are correlated with cognitive differences in children, their results do not show that fluoride is a cause of cognitive changes.

Recently, further doubt has been cast on the studies that find a link between fluoride and decreased IQ. Reviewers at the National Academies of Science, Engineering, and Medicine pointed out major methodological and statistical problems with the NTP's monograph twice before the NTP self-published the report.

A new article that repeated the results of the NTP monograph was later published in January 2025, and researchers then pointed out that about 70% of the studies included in the meta-analysis were at high risk of bias. The association the NTP found between high levels of fluoride and IQ may not be reliable, these scientists concluded, adding that the report provided "no evidence of an adverse effect at the lower water fluoride levels" used by fluoridation programs.

Still, some researchers are continuing to investigate the minimum amount of fluoride that could potentially cause neurological effects. So far, the results are mixed — a few dozen studies have reported a relationship between aspects of cognition, such as executive function or IQ, and the levels of fluoride exposure found in artificially fluoridated water supplies. But many others have failed to replicate this link.

Many studies that tie fluoride to negative effects have been poor quality, meaning they may suffer from issues such as small sample sizes or using inconsistent or inaccurate methods. High-quality studies have been less likely to show a link between fluoride exposure and negative cognitive outcomes.

Making a determination about the safety of fluoride is challenging because when individual studies use varied methods, focus on different populations, or examine separate outcomes, it becomes difficult to compare across those studies. And because different studies find conflicting results, it's not possible to point to a single study that can prove or disprove the safety of fluoride. As a result, many experts say more high-quality research is still needed to draw a conclusion on the safety of fluoride.

"It's a very nuanced issue," Deborah Dewey, a professor of pediatrics and community health sciences at the University of Calgary and the author of a study investigating fluoride exposure and executive function in preschoolers, told Live Science. "I don't think there's a right or wrong answer right now."

Fluoride and oral health equity

Ultimately, Maniar said the conversation about fluoride really needs to be a discussion about improving oral health equity.

"Rather than focusing on fluoridation, we should be talking about the disparities that exist in oral health care, the cost burden of routine oral health care, and the significant gaps that exist related to education on oral health," Maniar said.

"The linkage between oral health and a range of serious health issues is becoming more clear every day," he said, referring to research that connects poor oral health and conditions including heart disease and pneumonia. "It's critical that we deploy all available tools to address oral health in the same manner that we would address physical and mental health."

Editor's note: This story was updated on Jan. 7, 2025 to note the publication of the NTP's meta-analysis and the resulting critiques of the research. The story was first published Dec. 28, 2024.