The Trump administration on Monday drew an unproven link between autism and pregnant women’s use of acetaminophen, the active ingredient in one of the world’s most common over-the-counter pain relievers, Kenvue‘s Tylenol.

President Donald Trump said the Food and Drug Administration will issue a physician’s notice about the risk of patients using acetaminophen during pregnancy unless they have a fever. The agency will also start the process of changing the safety label for acetaminophen on Tylenol and similar products.

The moves clash with a bulk of scientific literature suggesting no causal link between autism and exposure to acetaminophen in the womb. 

Many over-the-counter drugs contain acetaminophen, but Tylenol is widely considered the safest treatment to take during pregnancy to relieve pain and fever, as long as patients use the recommended dose. 

“Taking Tylenol is not good,” Trump said during a press conference on Monday. “They are strongly recommending that women limit Tylenol use during pregnancy unless medically necessary. That’s for instance, in cases of extremely high fever, that you feel you can’t tough it out, you can’t do it.”

The Health and Human Services Department will encourage clinicians to “exercise their best judgment” around the use of acetaminophen during pregnancy by “prescribing the lowest effective dose with the shortest necessary duration and only when treatment is required,” the department’s Secretary Robert F. Kennedy Jr. said during the press briefing.

HHS will launch a nationwide campaign to inform patients about the alleged risk, Kennedy said.

In a statement Monday, Kenvue said it believes in “independent, sound science” that shows taking acetaminophen does not cause autism, and “we strongly disagree with any suggestion otherwise and are deeply concerned with the health risk this poses for expecting mothers.” Without the drug as an option, women may have to experience conditions like fever that are potentially harmful to both them and their babies, or use riskier alternatives, Kenvue said.

Untreated fever and pain during pregnancy can carry risks for both mother and infant, such as miscarriage, birth defects and high blood pressure, according to the Society for Maternal-Fetal Medicine.

FDA Commissioner Marty Makary suggested that treating a fever “can prolong the duration of illness in a young kid,” citing a study from Johns Hopkins, without further details.

“Maybe that’s because a fever is a body’s natural way of ridding an infection,” Makary said.

Trump, several times during the briefing, said “there’s no downside” to not taking Tylenol during pregnancy or in a baby’s early life.

The agency is specifically updating leucovorin’s label for cerebral folate deficiency, which HHS said has been associated with autism. The change will allow children with autism to be treated with the drug, with continued use if kids show language, social or adaptive gains, according to an HHS release.

But HHS said leucovorin is not a cure for autism and may only lead to improvements in speech-related deficits for a subset of children with the disorder. State Medicaid programs will be able to cover the drug for autism after the label update. The NIH will also start trials to confirm the effects of leucovorin on the disorder, including studies into the medicine’s safety.

While promising, it is important to note that leucovorin is not a cure for ASD and may only lead to improvements in speech-related deficits for a subset of children with ASD. Furthermore, this treatment must be administered under close medical supervision and in conjunction with other non-pharmacological approaches for children with ASD (e.g., behavioral therapy). 

Leucovorin is a form of folate, a B vitamin, that is typically prescribed to counteract some medications’ side effects, including chemotherapy, and to treat vitamin B9 deficiency. Some early placebo-controlled clinical trials have shown that oral leucovorin, also known as folinic acid, has the potential to improve symptoms in children with autism spectrum disorder.

The Food and Drug Administration early Monday published a notice saying it is approving a version of leucovorin that was previously made by GSK, the Wall Street Journal reported. But as of Monday afternoon, the Federal Register’s website said it has received an “agency letter” asking to withdraw the notice.

In a statement, a GSK spokesperson said it does not intend to market leucovorin. The company marketed the drug from 1983 to 1999 under the name Wellcovorin, but the product was withdrawn from the market and has since been available as a generic drug. The spokesperson said label changes made to Wellcovorin will help allow generics already on the market to add this new approval for autism to their labeling.

Acetaminophen is the latest widely used and accepted treatment that Robert F. Kennedy Jr. has undermined at the helm of the Health and Human Services department, which oversees federal health agencies that regulate drugs and other therapies. Kennedy has also taken steps to change vaccine policy in the U.S., and has amplified false claims about safe and effective shots that use mRNA technology.

Kennedy has made autism a key focus of HHS, pledging in April that the agency will “know what has caused the autism epidemic” by September and eliminate exposures. He also said that month that the agency has launched a “massive testing and research effort” involving hundreds of scientists worldwide that will determine the cause.

Much of the scientific community agrees that autism results from a complex mix of genetic and environmental factors, making it unlikely that rising rates of the disorder are due to a single cause.

Kennedy said HHS expects several announcements over the coming years that inform parents about the underlying cause of autism and “potential paths for prevention and reversal.” He acknowledged that autism is a complex disorder caused by a combination of factors and said HHS is continuing to investigate other factors, such as vaccines.

“One area that we are closely examining … some 40% to 70% of mothers with autism believes that her child was injured by a vaccine,” Kennedy said. “President Trump believes that we should be listening to these mothers instead of gaslighting like prior administrations.”

Extensive research has debunked longstanding concerns that vaccines are linked to autism, a claim that Kennedy and other immunization critics have pushed for several years.

The Washington Post on Friday reported that Trump administration officials have been reviewing previous research that suggests a link between the use of acetaminophen during pregnancy and an increased risk of autism. The Post said that includes an August review by Mount Sinai and Harvard researchers on 46 earlier studies that suggest a link between prenatal exposure to the drug and increased risks of neurodevelopmental disorders, such as autism or attention deficit hyperactivity disorder, or ADHD. 

The review found the association is strongest when acetaminophen is taken for four weeks or longer, Dr. Andrea Baccarelli, one of its authors and dean of the faculty at the Harvard T.H. Chan School of Public Health, said in a statement. The review was funded by a grant from the National Institutes of Health.

“This biological evidence lends support to the possibility of a causal relationship between prenatal acetaminophen exposure and neurodevelopmental disorders, including autism,” Baccarelli said, adding that further research is needed to “confirm the association and determine causality.”

He said based on existing evidence, he believes that “caution about acetaminophen use during pregnancy—especially heavy or prolonged use—is warranted.” But Baccarelli said acetaminophen remains a critical tool for pregnant women and their physicians, as the drug is the only approved medication for pain and fever relief during pregnancy.

He said he and his colleagues recommend a “balanced approach based on the precautionary principle:” patients who need fever or pain reduction during pregnancy should take the lowest effective dose of the drug for the shortest possible duration, after consulting their physician about their individual risks and benefits of doing so. Baccarelli said he discussed that recommendation and the review’s findings with NIH Director Jay Bhattacharya and Kennedy in recent weeks.

The findings of the studies reviewed by the researchers are at odds with other robust studies, including one published last year in the Journal of the American Medical Association that found acetaminophen use during pregnancy was not linked to autism, ADHD or intellectual disability. Researchers analyzed health records of 2.5 million children in Sweden.

When researchers looked at the general population initially, there was a very small increased risk of the disorder in children whose mothers took the drug while pregnant. But researchers found no link after comparing siblings within the same family: one exposed to acetaminophen during pregnancy, and the other not.

As of Monday before the announcement, the FDA website said the agency had not found “clear evidence” that appropriate use of acetaminophen during pregnancy causes “adverse pregnancy, birth, neurobehavioral, or developmental outcomes.” The American College of Obstetricians and Gynecologists maintains that acetaminophen is safe during pregnancy when taken as directed and after consulting a health-care provider. 

Some parents have brought lawsuits claiming that they gave birth to children with autism after using the painkiller.

But a federal judge in Manhattan ruled in 2023 that some of those lawsuits lacked scientific evidence and later ended the litigation in 2024.

— CNBC’s Angelica Peebles contributed to this report