Trump promotes unproven theory about Tylenol and autism. What does science say?

A day after claiming his administration has “found an answer to autism,” President Trump announced new efforts Monday to warn Americans that use of Tylenol and other acetaminophen-based painkillers during pregnancy could be linked to the neurological condition — and to encourage the use of leucovorin, a lesser-known cancer and anemia drug, to treat it.

But both theories are unproven and Trump has provided no new evidence to support his administration’s new recommendations.

“I’ve always had very strong feelings about autism and how it happened and where it came from,” the president emphasized. “We understood a lot more than a lot of people who studied it.”

Since returning to the Oval Office in January, Trump has repeatedly pledged to tackle America’s rising autism rate. In April, Secretary of Health and Human Services Robert F. Kennedy Jr., who has long promoted debunked theories about the disorder, said the administration had launched “a massive testing and research effort involving hundreds of scientists from around the world,” promising that “by September we will know what is causing the autism epidemic, and we will be able to track these exposures to eliminate.”

Kennedy did not keep that promise on Monday. Instead, he said the National Institutes of Health would continue investigating “multiple” hypotheses about possible causes and would begin awarding 13 research grants this month, with updates likely next year.

But Trump and Kennedy, along with other administration officials, claimed that prenatal exposure to acetaminophen, the active ingredient in Tylenol and one of the most commonly used medications worldwide, could increase the risk of autism spectrum disorder (ASD) — and as a result, the Food and Drug Administration has issued a new recommendation that pregnant people should only use it for high fever.

Officials also highlighted research showing that folinic acid (a form of vitamin B9), also called leucovorin — a decades-old drug often prescribed to counteract the toxic effects of a certain cancer drug — could help improve communication and cognition in at least some people with autism.

During Monday’s announcement, Kennedy continued his efforts to link childhood vaccines to autism — a claim that has been thoroughly debunked. He called ASD a “complex disorder” and emphasized that there would be “no taboo areas” in future research. “One area we are looking closely at is vaccines,” Kennedy said. “It will take time for scientists to take an honest look at this issue. We will be uncompromising and relentless in our search for answers.”

The rest of Monday’s announcement was not based on similarly discredited science. But experts don’t consider it “an answer to autism” either.

What we know about Tylenol and autism

Recent studies have come to conflicting conclusions about acetaminophen. In August, the journal BMC Environmental Health published a review of existing research – including six studies looking at the link between prenatal paracetamol use and the risk of ASD in children – which claimed to find ‘strong evidence of a link’ between the drug and the disorder.

The article was co-authored by Dr. Andrea Baccarelli, the dean of Harvard’s TH Chan School of Public Health, and it ultimately recommended “prudent acetaminophen use – lowest effective dose, shortest duration – under medical supervision, tailored to individual risk-benefit assessments.”

Yet a large 2024 study that looked at nearly 2.5 million people born in Sweden between 1995 and 2019 concluded that “paracetamol use during pregnancy was not associated with the risk of autism in children.”

Why the difference? Like other researchers, the Swedish team found an increased prevalence of autism among the offspring of people who took paracetamol during pregnancy. But according to their study, the risk was only slightly higher – 0.09 percentage points to be precise – and it disappeared when they focused on sibling cases where the parent took acetaminophen during one pregnancy and not the other.

“This suggests that what initially appeared to be an increased risk of autism from acetaminophen during pregnancy may have been a result of other risk factors,” Scientific American recently explained — namely “the fever or underlying infections that Tylenol was used to treat.” (A 2014 study of more than 2 million people found that if a pregnant person is hospitalized with an infection, the chance of their child developing autism increases by about 30%.)

“The conditions associated with people taking acetaminophen during pregnancy are far more dangerous than any theoretical risks and can cause serious morbidity and mortality to the pregnant person and the fetus,” the American College of Obstetricians and Gynecologists said in a statement.

What we know about leucovorin and autism

In the meantime, leucovorin has shown promise as a possible treatment for autism, but it is far too early to draw definitive conclusions about its efficacy.

Scientists have long known that a folic acid deficiency during pregnancy can increase the risk of neural tube defects. (The neural tube eventually develops into the brain and spinal cord.) In 2004, a study found that some children with autism-like symptoms have a condition that makes it harder for their bodies to transport folic acid to their brains. As a result, researchers in Arizona, France, China, India and Iran have conducted small, randomized controlled trials of folinic acid as a treatment for autism – that is, as a way to deliver folic acid more effectively – and all have found modest improvements in receptive and expressive language.

Yet only a few dozen children have participated in each of these studies, and larger trials of leucovorin have been slow to start because its original patents have expired (leaving pharmaceutical companies with little incentive to fund further research).

Controversial claims

Monday’s announcement will likely prove controversial in the autism community. The number of ASD diagnoses has increased by about 300% over the past two decades – a shift that Trump attributed mainly to environmental factors.

“There is something artificial,” he claimed on Monday. “They’re taking something.”

In contrast, half a century of research shows that ASD is “a complex neurodevelopmental disorder that results from a constellation of genetic factors and environmental influences,” as Scientific American puts it, and most public health officials attribute the rising rates to a broader definition of the disorder — along with increased screening and awareness — rather than to some kind of toxin.

So while the promise of special causes and miracle cures might attract attention, experts warn that getting ahead of existing science could have a counterproductive effect on families.

“A press statement talking about a possible connection will cause a lot of fear,” Dr. Debra Houry, former chief physician at the Centers for Disease Control and Prevention, told reporters Monday morning. “If there’s not the science to back this up, we’re going to see changes in practice, concerned mothers, all kinds of things, and that’s not appropriate.”