A decades-old debate about fluoride has gained new momentum since early November, when Robert F. Kennedy Jr., President-elect Trump’s nominee for secretary of the Department of Health and Human Services, called it an “industrial waste,” and said the administration would advise water systems across the country to remove it once Trump takes office.
Florida Surgeon General Joseph Ladapo followed suit, issuing an advisory for Florida communities to stop fluoridating their water supplies, citing controversial studies that link fluoride with a range of conditions, even though research findings about these associations are far from conclusive as we explain in this research-based piece.
Since then towns and cities across the nation, and at least one in Canada, are considering the removal of fluoride from their waters.
Meanwhile, advocacy groups like the Fluoride Action Network, which lobbies against water fluoridation, have had some success with their efforts. In September, a federal judge in California ordered the Environmental Protection Agency to strengthen the regulations for fluoride in drinking water, siding with advocacy groups that had raised concerns about the effects of the mineral on children’s health.
But many public health officials say that years of research show that low levels of fluoride are safe and help prevent dental cavities, particularly in children.
Major health organizations recommend water fluoridation as a way to prevent dental cavities and find it safe. The American Dental Association has reaffirmed its staunch support of water fluoridation. So has the Florida Chapter of the American Academy of Pediatrics. The International Association for Dental Research also supports water fluoridation and finds it safe, as does the U.S. Centers for Disease Control and Prevention.
“The U.S. Public Health Service; the United Kingdom’s National Institute for Health Research, Centre for Reviews and Dissemination, at the University of York; and the National Health and Medical Research Council, Australia have all conducted scientific reviews by expert panels and concluded that community water fluoridation is a safe and effective way to promote good oral health and prevent decay,” according to a May 2024 CDC Scientific Statement on Community Water Fluoridation.
To be sure, the prevalence of dental cavities has decreased in recent decades, thanks to the widespread use of fluoride toothpaste and fluoride mouthwash since the 1970s. As a result, the benefits of water fluoridation may be less pronounced than they were when the programs were implemented in 1945.
But despite arguments to remove fluoride from community water supplies, studies also haven’t found that low levels of fluoride in water are harmful, or that the practice of water fluoridation should be stopped.
To help inform news stories and conversations about this topic, we scoured dozens of research studies and articles and organized them as answers to 15 commonly asked questions.
We’re including hyperlinks to the journal articles we cite. Some of the journals may be behind a paywall, so be sure to check out our 2023 tip sheet on how to access academic research for free.
This piece is a companion to a research roundup.
1. What is fluoride?
Fluoride is a naturally occurring mineral found in soil and water, generally in low concentrations. It is a compound of fluorine, one of the elements on the periodic table.
2. How does fluoride work on teeth?
The outer layer of the tooth is called the enamel. It is made up of mineral crystals, including calcium and phosphate.
Fluoride prevents tooth decay in three ways: It prevents the loss of minerals from tooth enamel; it improves remineralization, a natural process in which minerals like calcium and phosphate are added back into the tooth enamel with the help of saliva; and it prevents bacterial activity, such as metabolism of bacteria and acid production, in dental plaque. (Dental plaque is a soft, sticky, colorless film of bacteria and other microorganisms that forms on the surfaces of teeth, gums and mouth tissue.)
“The science is simple: Fluoride strengthens tooth enamel, the protective outer layer of teeth, by promoting remineralization,” writes Dr. Amal Noureldin, clinical professor of cariology at Texas A&M University, in a December 2024 article in The Conversation. “It also makes teeth more resistant to the acids produced by bacteria in the mouth. This helps prevent cavities, a problem that remains widespread even in modern societies.”
3. What is water fluoridation?
Almost all water contains some fluoride naturally, but the amount is typically too low to prevent cavities.
Community water fluoridation is the process of adjusting the amount of fluoride in drinking water to the recommended national level of 0.7 milligrams of fluoride per liter of water, according to the U.S. Centers for Disease Control and Prevention. That’s equivalent to about three drops of water in a 55-gallon barrel, the CDC says.
In 1999, the CDC named fluoridation of drinking water as one of 10 great public health interventions of the 20th century, because of the dramatic decline in dental cavities since the process was implemented in 1945.
Increasing the proportion of people whose water systems have the recommended amount of fluoride is one of the health policy objectives of Healthy People 2030. Healthy People is a federal initiative that provides 10-year public health goals.
4. What’s the typical fluoride intake in the U.S.?
Among children aged 6 months to 14 years, drinking water accounts for 40% to 70% of total fluoride intake, according to a 2015 federal study published in the journal Public Health Reports. For adults, drinking water provides 60% of total fluoride intake. Accidentally swallowing toothpaste accounts for about 20% of total fluoride intake in very young children 1 to 3 years old. Other major contributors to total daily fluoride intake are commercial beverages and solid foods.
Most kinds of toothpaste sold in the U.S. contain fluoride in the form of sodium fluoride or monofluorophosphate, at about 1,100 to 1,100 mg/L, or 1.3 mg in a quarter teaspoon, according to the NIH’s Office of Dietary Supplements.
Fluoridated water keeps a low level of fluoride (0.7 mg/L) in the mouth all day. Fluoride toothpaste provides a much higher concentration at important times of day such as bedtime, according to the CDC. The agency recommends small amounts of fluoride toothpaste for children younger than 6 and advises parents to talk to their doctor or dentist.
Other types of fluoride products include mouthwash, varnish, gel and supplements. Gels used by dentists are typically applied one to four times a year and can lead to ingestions of 1.3 to 31.2 mg of fluoride each time.
The National Institutes of Health’s Office of Dietary Supplements has a fact sheet for recommended fluoride intake by age. The American Dental Association reviews topical and systemic fluoride supplements on its website.
It’s difficult to assess people’s dietary fluoride intake globally because fluoride concentrations in water vary across regions and countries, as noted in a 2024 Cochrane review.
5. What is the history of water fluoridation in the U.S.?
Dr. Frederick S. McKay, a dentist in Colorado Springs, Colorado, was the first person to link fluoride in public water with fewer cavities in 1901. He had noticed an unusual permanent dark stain, or “mottled enamel,” on many of his patients’ teeth. He also noticed that these patients seemed less susceptible to tooth decay.
In 1931, the Dental Hygiene Unit at the National Institutes of Health was established and the primary responsibility of its director, Dr. H. Trendley Dean, was to investigate the association between fluoride and “mottled enamel.” Dean replaced the term “mottled enamel” with “fluorosis,” a term that continues to be used today.
In 1942, after extensive observational surveys, Dean created the Fluorosis Index to classify the condition, ranging from very mild to severe, depending on how much of the tooth was covered by the stains and the color of the stains.
Dean also compared the prevalence of fluorosis with the prevalence of dental cavities among children in 26 states and noticed that children with fluorosis had fewer cavities. Indeed, children who lived in cities with more fluoride in community water supplies had fewer dental cavities.
The next step was to determine what level of fluoride in the water supply was optimal so that enough fluoride could be added to community water supplies to help prevent cavities.
In 1945, several cities in Michigan, New York, Illinois, and Ontario, Canada, began testing the effect of negligible levels of fluoride in community water supply. The amount ranged from 1 to 1.2 milligrams per liter. Grand Rapids, Michigan, was the first city in the world to implement water fluoridation.
The study of this initial fluoridation went on for 15 years, showing that dental cavities were reduced by 50% to 70% among children who lived in communities with fluoridated water.
In 1962, federal health officials in the U.S. created the optimum range of fluoride concentration in the water supply, from 0.7 to 1.2 mg/L, with the lower concentration recommended for warmer climates, where water consumption is higher, and higher concentration for colder climates.
In 2015, the U.S. Public Health Service — a collection of agencies within the Department of Health and Human Services — updated the recommendation, setting 0.7 mg/L as the optimal fluoride concentration in community water supplies.
Here’s the CDC’s visual timeline of water fluoridation in the U.S.
6. Who makes decisions about water fluoridation?
A community water system supplies water to the same population year-round and serves at least 25 people at their homes or at least 15 homes, such as mobile homes and subdivisions, according to the CDC.
Community water fluoridation is not enforced at the federal level. The decision of whether and how much to fluoridate public drinking water is made by state and local officials.
The federal government has set two different levels for fluoride in the water. One is the maximum allowable amount to prevent toxicity, and the other is the optimal level, to help prevent dental cavities.
The Environmental Protection Agency regulates drinking water contaminants, including fluoride, which can occur naturally at toxic levels. In 1986, the agency set the maximum allowable amount of fluoride in drinking water at 4 milligrams of fluoride per liter of water.
Community water systems that exceed the 4 mg/L level must notify people who are served by that system as soon as possible, but no later than 30 days after the violation.
EPA also has a non-enforceable secondary standard for fluoride of 2.0 mg/L, which is recommended to protect children against tooth discoloration or pitting.
Meanwhile, the Department of Health and Human Services recommends optimal water fluoridation levels to help prevent dental cavities. This level is 0.7 mg/L.
The recommended level of 0.7 mg/L is not an enforceable standard.
State and local governments decide whether to implement water fluoridation. Some states have laws that require public water systems of a certain size to provide fluoridated water, according to the CDC.
The CDC provides funding to support infrastructure in states for water fluoridation, monitoring and quality of fluoridation and to develop resources for training and public awareness. The agency also provides technical assistance to state fluoridation programs.
A Nov. 13, 2024, KFF piece delves deeper into the federal role in water fluoridation. The Washington Post has created an interactive map of states that mandate fluoride in drinking water.
7. What about private wells?
Private wells are not regulated by the federal government or most state governments. More than 23 million households in the U.S. rely on private wells as a source of drinking water, according to the Environmental Protection Agency.
A 2009 study of well water by the U.S. Geological Survey finds that one in five wells had contaminants at concentrations above the benchmarks for drinking water.
Here’s an interactive map of private wells in the U.S.
8. How effective is water fluoridation?
Years of research have found that water fluoridation prevents dental cavities.
In the decades following the implementation of water fluoridation, the reduction of cavities in children was significant. However, recent studies show that the effect of water fluoridation has lessened since the mid-1970s with the widespread use of fluoride toothpaste and fluoridated mouthwash.
An October 2024 review of 157 studies, published in the Cochrane Database of Systematic Reviews, finds that water fluoridation leads to only a slight improvement — 3 to 4 percentation points — in the proportion of children without cavities. Another large study in England, published in January 2024, also finds “very small positive health effects” to water fluoridation.
In comparison, older studies show a greater benefit.
A 2015 Cochrane review of 155 studies finds water fluoridation reduced the average number of decayed, missing, or filled children’s primary teeth by 35%. It reduced the average number of decayed, missing or filled permanent teeth by 26%.
A 2018 study, published in the Journal of Dental Research finds that children who lived in counties where 75% or more of the water supplies were fluoridated had 39% fewer cavities in their baby teeth than those who lived in less fluoridated communities. Older children had 12% fewer cavities in their permanent teeth.
A 1990 study published in the Journal of Dental Research, which looked at trends between 1979 and 1987, finds that children’s dental cavities decreased by 36% over the same period. And by 1987, about half of the children didn’t have any cavities in their permanent teeth.
The study also finds that children who had always been exposed to fluoridated community water had 18% fewer cavities than those who had never lived in fluoridated communities.
Water fluoridation has also been associated with a 20% to 40% reduction of enamel cavities in adults, according to a 1989 review published in the Journal of Public Health Dentistry.
A 2007 meta-analysis of 20 studies finds water fluoridation was linked with a 27% reduction in dental cavities among adults.
It’s important to note the studies don’t draw a cause-and-effect conclusion between the reduction in cavities and water fluoridation. However, they emphasize that water fluoridation has played a role in the decline.
“At present, there are many unanswered questions regarding the reasons for the continued downward trends in dental caries in U.S. children, but there can be little doubt that for this decline to be maintained, fluoride-based prevention must be continued,” write the authors of the 1990 study.
In 2015, on the 70th anniversary of U.S. community water fluoridation, U.S. Surgeon General Vivek Murthy wrote in the journal Public Health Reports, “Community water fluoridation is one of the most practical, cost-effective, equitable, and safe measures communities can take to prevent tooth decay and improve oral health.”
9. How many people have access to fluoridated water?
By 1969, 44% of the U.S. population was receiving fluoridated water or about 88,475,684 people.
By 1985, this percentage increased to 55%, or 130,172,334 people. By 2000, this percentage was 57%, or 161,924,080 people, according to a 2015 study published in the journal Public Health Reports.
According to the latest available data from the CDC, in 2022, more than 209 million people, or 72.3% of the U.S. population served by public water supplies, had access to water with fluoride levels that prevent tooth decay.
The CDC’s Water Fluoridation Statistics page also breaks down the numbers by state. You can find historical fluoridation statistics here.
Although the proportion of the U.S. population drinking fluoridated water increased rapidly between 1945 and the 1970s, the rate has slowed down since.
The authors of a 1999 article in the CDC’s Morbidity & Mortality Weekly Report attribute the trend to several factors. The public, some scientists and policymakers no longer view dental cavities as a public health problem or don’t believe that fluoridation is necessary or effective.
Also, as evident today, nearly 25 years since the MMWR article was published, adopting water fluoridation can require a political process, which can make implementing public health measures difficult.
Other factors include unsubstantiated claims about adverse health effects of fluoridation, and cost, especially in small communities with limited budgets, according to the 1999 study.
10. How much fluoride is in my community’s water?
The CDC’s My Water’s Fluoride is a great source of information to find out whether a community water system in the U.S. is fluorinated. It also includes information about community fluoride levels. In cases where that information is not provided, you can check with your local government.
The site also includes State Fluoridation Reports, which include average fluoride levels by month.
A 2023 study, published in the Journal of Exposure Science & Environmental Epidemiology, finds that between 2006 and 2011 about 4.5% of community water systems in the U.S. had an average fluoride concentration exceeding 1.5 mg/L, while 15.4% had levels higher than 0.7 mg/L. These trends were especially true for water systems that served semi-urban Hispanic communities and communities in the Southwest U.S., researchers find.
11. Is water fluoridation cost-effective?
Water fluoridation can save individuals and communities money by preventing tooth decay, older studies have shown.
For communities of 1,000 or more people, the return on investment is $20 for every dollar spent on water fluoridation, according to a 2016 study published in the journal Health Affairs. The return on investment increases and the community size increases, the study finds.
A 2005 study of community water fluoridation programs in Colorado, including 172 public water systems that served 1,000 people or more, finds that one year of exposure to fluoridated water resulted in average savings of $60 per person.
A 2015 federal study finds that the annual per-person cost varied by the size of the water system ranging from $0.50 in communities of $20,000 to $3.70 for communities of 5,000, updated to 2010 dollars using the Consumer Price Index — only a fraction of the cost of one dental filling.
Tooth decay is one of the most common chronic diseases among children in the U.S. About a quarter of children living below the federal poverty level have untreated tooth decay, according to the CDC.
Nearly half of children between ages two and 19 had untreated or treated dental cavities in one or more of their primary or permanent teeth, according to the CDC’s National Center for Health Statistics.
12. How does fluoridation help address health disparities?
Children living in families with low incomes were twice as likely to have decay in their primary teeth compared with children in families with high incomes, according to the CDC’s Oral Health in America report, published in 2021. In addition, Black and Mexican-American children are more likely to have tooth decay compared with their white counterparts, according to the report.
Some studies suggest the potential for water fluoridation to reduce inequalities, while more recent studies have found the impact to be inconclusive.
A 1999 study on 5-year-old children in England, published in the International Journal of Epidemiology, finds that water fluoridation not only reduced dental cavities but also lessened social inequalities in dental health.
A February 2020 study of 2,075 adolescents in North Carolina, published in the International Journal of Environmental Research and Public Health, finds no effect of community water fluoridation on racial disparities. However, researchers find that in communities that had no water fluoridation, the parents’ education made a difference. Children whose parents had lower education levels had a higher risk of dental cavities.
A 2017 study, published in the Journal of Public Health Dentistry, finds that income-based disparities in dental cavities among U.S. children and adolescents remained largely unchanged between 1988 and 2014, reflecting the limited impact of public health initiatives targeting these inequalities.
In addition to the lack of research on the impact of fluoridation on dental disparities, “there have been other note-worthy changes during the period that may have exacerbated disparities in dental caries, such as the increased prevalence of obesity, greater consumption of bottled water and increasing public mistrust in the safety of tap water,” the authors write.
13. What happens when fluoride is removed from the water supply?
Only a few studies have explored the after-effects of fluoride removal, and they show an increase in the number of cavities among children once water fluoridation stops. Meanwhile, there is no definitive data from Oregon, which is the third-least-fluoridated state in the U.S.
Calgary, Canada, which took out fluoride from the city water in 2011, is putting it back in, following a rise in dental cavities among children, reported The Sunday Times in early December.
In Juneau, Alaska’s capital city, which removed fluoride in 2007, the prevalence of dental cavity procedures increased, according to a 2018 study published in BMC Oral Health. For instance, children younger than 7 who lived in areas without fluoridated water needed about 34% more dental procedures than children in areas with fluoridated water. That’s the difference of 2.68 dental procedures versus 2.01 procedures.
A 1989 review of research, published in the Journal of Public Health Dentistry, aims to find out whether water fluoridation is still effective or necessary given the continued decline of dental cavities in children.
The review highlights the case of Antigo, Wisconsin, which began water fluoridation in 1949 and stopped in 1960, only to start again in 1965.
The prevalence of cavities “increased remarkably (about 100 percent) between 1960 and 1966,” according to the review. “Similarly, in Wick, Scotland, which started water fluoridation in 1969 but stopped it in 1979, the caries prevalence in 5- to 6-year-old children increased by 27 percent between 1979 and 1984, despite a national decline in caries and increased availability of” dental products like toothpaste with fluoride.
“The data reviewed here and elsewhere (5,872) document both the past and present effectiveness of water fluoridation in consistently lowering the level of caries prevalence,” the review concludes.
The October 2024 study finds that the benefits of fluoridated water may be smaller than it was before the widespread use of fluoride toothpaste. At the same time, there’s not enough evidence to decide what happens if communities stop fluoridating their water supply or how it could affect disparities. The review found no reliable studies on whether fluoridation had an impact on adult dental cavities.
14. What happens if you consume too much fluoride?
Excessive fluoride intake typically occurs through drinking groundwater that has high levels of fluoride, and when high-fluoride water is used in cooking or irrigation of crops.
Excessive fluoride intake has been associated with dental fluorosis, which leaves white streaks or spots on teeth in its mild form. In severe form, dental fluorosis can cause brown or gray discoloration of teeth.
It can also lead to a bone disease called skeletal fluorosis. Consuming high levels of fluoride can lead to its accumulation in bones over time, changing the structure and density of the bones.
Excessive levels of fluoride have also been associated with lower IQ in children in some studies, although this association warrants additional research.
The odds of developing dental fluorosis with a water fluoride concentration of 0.7 mg/L is about 12%. Those odds increase by 2.9% as fluoride levels increase by 1 mg/L. The odds of developing dental fluorosis is 72% when the water fluoride concentration is 4 mg/L.
15. Is fluoride in water safe?
Since the 1950s, opponents of water fluoridation have shared unsubstantiated claims linking it with an increased risk for cancer, Down syndrome, heart disease, bone loss, low IQ, Alzheimer’s disease, and allergic reactions, among others.
There’s yet to be any credible evidence to support an association between fluoridation and these conditions.
The main documented risk of community water fluoridation is dental fluorosis, a cosmetic change in dental enamel, mostly in the form of visible white markings on teeth.
As Dr. Katelyn Jetelina, an epidemiologist and the author of the popular Substack Your Local Epidemiologist, puts it: “The dose makes the poison.”
“Very high levels of fluoride can be dangerous,” she writes. “Fluoride toxicity first impacts the skeletal bones, beginning at an exposure of 5 mg/kg of body weight per day. If we do the math, the average child (40 kg) must drink 286 liters of fluoridated water daily to reach toxicity. At that point, they would die from water overconsumption.”
Here’s what research says about the link between fluoride and various conditions:
Thyroid function: Jetelina points to a meta-analysis of 27 studies, published in the journal Environmental Research, that finds thyroid hormone levels began to increase when water fluoride levels were about 3.5 times higher than the 0.7 mg/L that’s added to water systems.
Children’s IQ: Exposure to high levels of fluoride over a long period of time has been linked to lower IQ scores in children, according to research mostly conducted outside the U.S.
A systematic review and meta-analysis of 74 studies, published in January 2025 in JAMA, finds a drop in children’s IQ points as exposure to fluoride increases. The authors write that “There were limited data and uncertainty in the dose-response association between fluoride exposure and children’s IQ when fluoride exposure was estimated by drinking water alone at concentrations less than 1.5 mg/L.” They call for additional research. All studies included in the systematic review were conducted outside the U.S.
An August 2024 systematic review of published scientific literature from non-U.S. countries, including Canada, China, India, Iran, Pakistan and Mexico, finds that higher levels of fluoride exposure, including water containing more than 1.5 milligram of fluoride per liter of water, are associated with lower IQ in children. The 1.5 mg/L is more than twice the optimal 0.7 mg/L standard in the U.S.
A report by the U.S. HHS’ National Toxicology Program notes there wasn’t enough data to show if the 0.7 mg/L level of fluoride has a negative effect on children’s IQ.
“An association indicates a connection between fluoride and lower IQ; it does not prove a cause and effect,” according to the report. “Many substances are healthy and beneficial when taken in small doses but may cause harm at high doses. More research is needed to better understand if there are health risks associated with low fluoride exposures.”
A 2023 meta-analysis published in the journal Public Health finds that community water fluoridation at the current recommended levels does not negatively impact children’s IQ. However, further research is needed to better understand fluoride’s effects in areas with high exposure levels and to address methodological weaknesses in existing studies, according to the authors.
A 2022 long-term study of 15,793 Australian schoolchildren, published in the Journal of Dental Research, finds that fluoridated water doesn’t affect the emotional and behavioral development of the children. The study was conducted between 2012 and 2014 at baseline. Its follow-up was from 2019 to 2020.
A 2019 study of 161 children in Mexico between May and December 2017 also finds no evidence that fluoridated water affects children’s IQ.
But a California judge ruled in September that the current optimal level of fluoride in drinking water — 0.7 milligrams of fluoride per liter of water — “poses an unreasonable risk of reduced IQ in children.”
“Unless EPA appeals the ruling, it must initiate a rule-making to further evaluate the risk,” according to an E&E News by POLITICO article written by Miranda Willson on Nov. 18. The article adds that a nationwide push to get rid of fluoride through EPA policies would probably take more than four years.
In response to the California district court ruling, the American Dental Association issued a statement, noting that the ruling “provides no scientific basis for the ADA to change its endorsement of community water fluoridation as safe and beneficial to oral health.”
The association also urged its members and the public “to be cautious of ‘pseudo-scientific information.’ This information is not always based on research conducted according to impartial and evidence-based scientific methodology; and the conclusions drawn from research are not always scientifically justifiable or without bias.”
Pregnant women: A study published in May 2024 in JAMA Network Open, has linked the consumption of fluoride by pregnant moms to neurological conditions in their children by age 3 or 4. Another study in Canada, published in 2019, also shows an association; however, that study has received criticism for its methodology.
Dr. Christine Till, the author of the 2019 study, is among the researchers who maintain that there’s enough evidence to suggest that pregnant women should try to limit their fluoride intake, according to a recent article in The New York Times.
Meanwhile, a scoping review of 95 studies published in February 2023 in the journal Advances in Nutrition, which looks at the relationship between iodine and fluoride in pregnancy and maternal thyroid function and a child’s neurodevelopment, draws no specific conclusion and recommends more research to develop future recommendations for iodine intake and fluoride exposure for pregnant women.
Chronic kidney disease: A 2007 review, published in the journal Nephrology Dialysis Transplantation, finds no association between water fluoridation and chronic kidney disease.
“The poor evidence quality and deficient methodological rigor of the identified studies means that no definitive conclusions regarding the association between consumption of optimally fluoridated community water and CKD can be made,” the authors write.
Cancer: The American Cancer Society provides a comprehensive review of research on the link between fluoride and cancer, which together show “that there is no strong evidence of a link between water fluoridation and cancer,” according to the organization. “However, several of the reviews noted that further studies are needed to help clarify the possible link.”
A systematic review of 14 studies, published in the journal Bone in November 2024, finds no clear link between fluoride and primary bone cancers. The authors note that most of the studies, however, were low quality and there’s a need for more robust studies.
A 2000 systematic review of 214 studies, published in the BMJ, also found no clear evidence of potential negative effects of water fluoridation, aside from dental fluorosis.
“As always, the dose makes the poison, and the benefits of adding the small, recommended amount of fluoride to public water supplies have been shown to far outweigh any risks at the community level,” writes Dr. Katrine Wallace, an epidemiologist and adjunct assistant professor at the University of Illinois Chicago, in a MedPage Today opinion piece. “The challenge ahead will be quieting the disinformation and encouraging continued use of a proven public health measure.”
Correction: An earlier version of this piece included incorrect numbers summarizing the 2024 Cochrane review. We regret the error and have corrected our mistake, thanks to an email from an astute reader.
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