The Food and Drug Administration on Friday moved to limit the use of fluoride supplements used to strengthen children's teeth, the latest action by Health Secretary Robert F. Kennedy Jr. and his deputies against a chemical that is a mainstay of dental care.

The FDA said that the products are no longer recommended for children younger than 3 and those who are older but don't face serious risks of tooth decay. Previously, the products have been prescribed for children as young as six months.

The action stopped short of FDA statements in May suggesting regulators would seek the removal of the products from the market. Instead, the agency sent letters to four companies warning them not to market their products outside the new limits.

Fluoride tablets and lozenges are sometimes recommended for children and teens at increased risk of tooth decay or cavities because of low fluoride in their local drinking water. Companies also sell drops for babies.

The FDA released a new scientific analysis Friday, concluding that fluoride supplements have limited benefits for children's teeth and may be linked to emerging safety concerns, including gut issues, weight gain and cognition.

“For the same reason fluoride may work to kill bacteria on teeth, it may also alter the gut microbiome, which may have broader health implications,” the agency said in a statement.

The agency also sent a form letter to dentists and other health providers warning about the risks of the products.

Those claims have been disputed by the American Dental Association, which has said there are no significant health problems associated with fluoride when used at the levels prescribed by dentists. The supplements can cause spotting or discoloration of teeth due to the extra fluoride, a downside the FDA also noted.

Dentists have warned that restricting fluoride supplements may result in more cavities and dental problems in rural communities, which are less likely to have fluoridated water. Kennedy is also seeking to end the practice of adding fluoride to drinking water throughout the U.S.

Fluoride strengthens teeth and reduces cavities by replacing minerals lost during normal wear and tear, according to the Centers for Disease Control and Prevention. In 1962, the agency set guidelines for how much should be added to water.

Kennedy, a former environmental lawyer, has called fluoride a “dangerous neurotoxin” tied to a range of health dangers.

The FDA regulates most dental products, including fluoride-containing toothpastes, supplements, mouthwashes and rinses. The agency’s actions don't affect toothpastes, mouthwash or fluoride treatments used by adults or those offered in dentists’ offices.