A new study published Tuesday found that adding fluoride to drinking water was not associated with lower IQ or impaired cognition.
Researchers from universities in Minnesota, Wisconsin and Michigan analyzed data that tracked more than 10,000 people who graduated high school in 1957 from adolescence to old age and found no evidence that exposure to fluoridated water was associated with lower IQ at age 16 or with cognitive decline later in life. These results held even after researchers accounted for factors known to influence cognitive outcomes, such as family income or parental education.
The study builds on an analysis published last year, with refinements to how fluoride exposure was measured over time. While the results found no harmful effects at fluoride levels typically used in U.S. drinking water, other studies — particularly those examining higher exposure levels — have reported associations with lower IQ.
Today, fluoridation is nearly ubiquitous. About 75% of Americans drink fluoridated water, which contains 0.7 milligrams of fluoride per liter of water. But the debate over fluoridation has intensified. Health Secretary Robert F. Kennedy Jr. has advocated to end the practice nationwide and has urged federal agencies to revisit long-standing recommendations. Some critics of fluoridation argue that modern dental hygiene — regular brushing with fluoride toothpaste and routine dental care — renders fluoridation unnecessary. (Last May, Texas Attorney General Ken Paxton launched an investigation into toothpaste makers over fluoride safety concerns.)
The history of fluoride in drinking water
In the early 20th century, physicians and dentists noticed people exposed to high levels of fluoride developed dark brown stains on their teeth. The condition — first called mottled enamel, then later renamed dental fluorosis — was largely cosmetic and not harmful. But in the 1930s, dental surgeon and epidemiologist Trendley Dean observed that people with dental fluorosis also had significantly lower rates of tooth decay.
At the time, dental hygiene was poor; tooth decay was a major public health issue, and the use of dentures was widespread. Dean posited that fluoride might improve oral health and set out to test his theory.
In 1942, Dean published a landmark study that included more than 7,000 children across 21 cities in Colorado, Illinois, Indiana and Ohio, showing that children who lived in areas with higher natural fluoride levels consistently had fewer cavities. The findings helped establish fluoride’s protective effect on teeth. By the 1950s, Dean and others at the U.S. Public Health Service and the American Dental Association advocated for cities with naturally low fluoride levels in their drinking water to intentionally add the mineral.
By 1980, about half of the U.S. population was drinking fluoridated water. But as fluoridation expanded across the country and globally, a number of studies — largely conducted in rural China, where fluoride levels were often much higher — started reporting links between fluoride exposure and lower IQ. Many of these studies did not fully account for other factors that influence cognitive development, such as nutrition, socioeconomic conditions or exposure to other environmental toxins, but they raised early concerns about potential risks of fluoride exposure.
A handful of animal studies published throughout the early 2000s also found that high doses of fluoride — around 20 to 80 milligrams per liter of water — had toxic effects on parts of the brain.
In 2012, researchers at Harvard School of Public Health reviewed 27 prior studies on fluoride and cognition in humans and found that children living in “high-fluoride” areas had significantly lower IQ than those in “low-fluoride” areas. The definition of high- and low-fluoride areas varied by study, but generally, high exposure ranged from 0.8 to 31.6 milligrams of fluoride per liter of water.
Early last year, researchers at the National Institutes of Health reviewed 74 studies that measured fluoride exposure and examined associations with cognition. The researchers found again that higher fluoride levels were associated with lower IQ. However, the strength and consistency of that association depended on exposure level: Links were more consistent at higher concentrations — around 2 to 4 milligrams per liter — while findings were weaker and sometimes disappeared at levels below about 1.5 milligrams per liter.
The debate over fluoridation
Last year, lawmakers in Utah and Florida banned fluoridation. At least 17 other states have introduced legislation to either prohibit the practice outright or make it optional for more communities. Short of state-wide bans, some towns and communities — 62 by one estimate from the Fluoride Action Network — have also voted to end the practice.
Public health experts, however, continue to defend fluoridation as a safe and effective intervention. Organizations, including the Centers for Disease Control and Prevention and the American Dental Association, maintain that fluoride at recommended levels reduces cavities and improves oral health, particularly for children and low-income populations with limited access to dental care. They also note that much of the research linking fluoride to cognitive effects involves exposure levels higher than those typically found in U.S. drinking water.