Fluoride – it's added to the water systems of more than two thirds of Americans. It's in dental products from toothpaste to mouth wash. It's been the subject of long-running conspiracy theories.

And now, Robert F. Kennedy Jr. says Donald Trump could try to take it out of water supplies nationwide if elected president.

Kennedy, who says he is in the running for a position in a second Trump administration, centered false claims about vaccines and other conspiracy theories in his failed presidential bid.

Kennedy also claims − without proof − that fluoride in U.S. water systems has been linked to a host of medical conditions.

"On January 20, the Trump White House will advise all U.S​. water systems to remove fluoride from public water," Kennedy said in a Saturday post to X. "Fluoride is an industrial waste associated with arthritis, bone fractures, bone cancer, IQ loss, neurodevelopmental disorders, and thyroid disease."

U.S. health experts strongly disagree.

The Centers for Disease Control calls the practice of adding fluoride to tap water systems one of the 10 greatest public health achievements of the last century. Major medical groups, such as the American Dental Association and the American Academy of Pediatrics, have also endorsed water fluoridation at recommended levels as a way to boost oral health.

The naturally occurring chemical has long represented a boogeyman for conspiracy theorists – since before Sterling Hayden's famous rant against "communists" adding fluoride to the water in his role as a crazed general in Stanley Kubrick's Cold War satire "Dr. Strangelove."

But what does the science really show about adding fluoride to tap water?

What is fluoride?

Fluoride is a natural mineral. It is found in soil, air, food and water sources across the planet, although in different amounts depending on the place, according to the National Institute of Dental and Craniofacial Research.

Fluoride strengthens teeth by hardening their outer surface, or enamel. It's a common ingredient in oral health and teeth cleaning products such as toothpastes, mouth rinses and varnishes used by dentists.

Why is fluoride adding to drinking water?

The American Dental Association says adding fluoride to water streams is a "safe, beneficial, and cost-effective" public health measure, citing studies that it cuts cavities in children and adults by 25%.

"Decades of research and practical experience indicate that fluoride is safe and beneficial to oral health," Linda Edgar, president of the American Dental Association, said in a statement released in August.

The American Association of Pediatrics also maintains that fluoride in drinking water and toothpaste is a safe means of boosting children's oral health.

Charlotte W. Lewis, a member of the AAP's oral health section, said in a statement in September that she is confident in the "safety of optimally fluoridated water in the U.S."

"Water fluoridation is a public health policy based on a solid foundation of evidence," she said.

Water issues: Asheville residents still without clean water two weeks after Helene

Is fluoride dangerous?

Several studies published over the last year have raised concerns about the effects of fluoride in large quantities and given ammunition to groups who are pushing to ban public water fluoridation in some communities.

A report released in August by the U.S. Department of Health and Human Services found that drinking water containing more than 1.5 milligrams per liter was "consistently associated with lower IQ in children."

But the report, which evaluated studies conducted in Canada, China, India, Iran, Pakistan and Mexico, noted that the fluoride levels found to cause such risks were more than twice as high as the amount U.S. officials recommend be added to public water systems. Fluoride levels that high are mostly found in other countries and a tiny minority of U.S. communities.

The report also did not specify how much IQ levels dipped or whether adults could also be affected.

A study released in May also found that children in the womb in "optimally fluoridated" areas in the U.S. could increase the risk of neurobehavioral issues after birth.

The Department of Health and Human Services said the data was "insufficient" to determine whether the current recommended level of 0.7 milligrams of fluoride per liter of water also had an effect on IQ, and that "more research is needed" to understand whether lower levels also have health effects.

Health position? RFK Jr. said Trump 'promised' him control of health agencies. A Trump aide refuted that.

How long has fluoride been added to tap water?

Grand Rapids, Michigan, became the first city to have fluoride added to its water systems in 1945. Over the next 15 years, scientists monitored the tooth decay rate in almost 30,000 children in the city, and they found that the rate of dental cavities dropped more than 60% in 11 years, according to the NIDCR.

As of 2022, the community water systems of more than 209 million Americans, 72% of the population, are fluoridated, according to the CDC. Another 11.6 million people drink from water systems that naturally contain a high enough level of the chemical to have the dental health benefits pointed to by dentists.

The U.S. Public Health Service currently recommends a fluoride level of 0.7 milligrams per liter of water to maximize oral health benefits while minimizing the risk of fluorosis, a condition triggered by an excessive fluoride intake usually seen in young children.

Naturally occurring fluoride is higher in some parts of the world than others. Fluoride was measured at 8 milligrams per liter in the groundwater of some villages in China, while some Canadian waters contain levels of less than 0.05 milligrams per liter, according to a World Health Organization report.

Are state water systems required to add fluoride?

No, state water systems are not federally mandated to fluoridate their water.

State and local governments implement their own fluoridation levels, sometimes determined by voters at the ballot box, according to the CDC.

Some states lawmakers have introduced recent measures to change fluoridation rules.

A Kentucky law that would make fluoridation optional failed to pass earlier this year but could be reintroduced in 2025. In New Jersey, where fluoride is not required to be added to water utilities, lawmakers introduced a bill to mandate it.