A study found no evidence of a negative association between community water fluoridation and either adolescent IQ or cognitive functioning at any point across the life course, using representative data from the state of Wisconsin.
The study replicated and extended prior work by the same research group using data from the Wisconsin Longitudinal Study (WLS), a one-third random sample (n = 10,317) of Wisconsin high school graduates from 1957. Improving on the earlier analysis in two key ways, the investigators modeled effects on adolescent IQ directly—rather than academic achievement—and accounted for geographic mobility across childhood to more precisely characterize the timing and duration of fluoride exposure.
Exposure Classification and Outcomes
Fluoride exposure was inferred from historical fluoridation records as well as data on naturally occurring fluoride levels in untreated well water. The participants were classified into four exposure categories: no exposure (n = 3,614), exposed from birth (n = 2,595), exposed from age 8 years (n = 2,087), and exposed from age 14 years (n = 2,021). Because naturally occurring fluoride contributed to exposure classification, these categories reflected fluoride exposure from drinking water more broadly—not just from community water fluoridation.
Adolescent IQ at age 16 was measured using the standardized Henmon-Nelson test. Cognitive function at 53, 64, and 72 years was assessed using the similarities task of the Wechsler Adult Intelligence Scale (WAIS), and cognition at 80 years was measured using the Modified Telephone Interview for Cognitive Status (TICSm). All cognitive outcome measures were standardized.
Results
Across 45 coefficient estimates spanning combinations of five cognitive outcomes and multiple exposure timings, just two were statistically distinguishable from 0—a number the authors noted is consistent with chance.
This pattern held across three analytic samples: the full WLS cohort and two restricted subgroups limited to participants who lived in the same county at age 18 and earlier in childhood (either age 11 years or age 1 year). These subgroup analyses were designed to increase confidence that exposure classification reflected consistent residence—and thus fluoride exposure—over time.
Descriptive data showed that unadjusted cognitive scores were highest among participants first exposed to fluoride in late adolescence (mean standardized IQ = 0.11). However, this group also had the highest levels of socioeconomic advantage, including higher family income, parental education, and father’s occupational status. After adjusting for individual- and school-level confounders, no statistically significant associations between fluoride exposure and any cognitive outcomes were observed.
Methodologic Considerations
The investigators incorporated several design features intended to reduce confounding and improve the credibility of estimated associations, including adjustment for family socioeconomic background and school-level characteristics, as well as the use of multiple imputation to address missing data.
However, they noted important limitations. The study couldn't directly measure individual fluoride intake—for example, through biomarkers such as urinary fluoride—and didn't include data on dental care or other sources of fluoride exposure (eg, toothpaste or supplements). As a result, exposure was inferred rather than directly observed.
Context
The findings differed from evidence cited in recent state-level decisions to end community water fluoridation in Utah, Florida, and various counties across the United States. The investigators noted that a 2025 systematic review and meta-analysis relied largely on studies that involved fluoride exposures substantially higher than those found in community drinking water, didn't use population-representative samples, and didn't include US-based data.
Lead study author John Robert Warren, of the Institute for Social Research and Data Innovation at the University of Minnesota and colleagues concluded that they “find no evidence that [community water fluoridation] is associated with lower adolescent IQ or cognition later in life.”
The study was supported by the National Institute on Aging, the Vilas Estate Trust, the National Science Foundation, and the Spencer Foundation. The study authors declared no competing interests.
Source: PNAS