No Link Between Acetaminophen in Pregnancy and Autism, a New Study Finds

The review looked at more than three dozen studies and found no evidence that acetaminophen increased the risk of neurodevelopmental disorders in children.

A scientific review of 43 studies on acetaminophen use during pregnancy concluded that there was no evidence that the painkiller increased the risk of autism or other neurodevelopmental disorders.

“We found no clinically important increase in the risk of autism, A.D.H.D. or intellectual disability,” Dr. Asma Khalil, a professor of obstetrics and maternal fetal medicine at St. George’s Hospital, University of London, and the lead author of the report, said at a news briefing. The study was published on Friday in the British medical journal The Lancet.

Acetaminophen, the active ingredient in Tylenol, remains “the first-line treatment that we would recommend if the pregnant women have pain or fever in pregnancy,” Dr. Khalil said.

Studies that have examined a possible link between acetaminophen in pregnancy and a risk of neurodevelopmental disorders have produced conflicting data, with some finding no connection and others finding small increases in risk.

The new review comes after President Trump told pregnant women during a news conference in September to “tough it out” and “fight like hell” not to take Tylenol, because he said the painkiller could cause autism in children. The message was delivered as part of a broader campaign by Health Secretary Robert F. Kennedy Jr. to try to identify the causes behind rising autism rates among children in the United States, zeroing in on the unproven risks of acetaminophen and long-discredited theories that vaccines cause autism.

Medical groups worldwide, including the American College of Obstetricians and Gynecologists, quickly disputed the president’s statements. They argued that doctors already advised their pregnant patients to use acetaminophen judiciously, and cautioned that untreated fevers during pregnancy could cause health problems for the mother and the baby.

Because acetaminophen passes into the brain and also crosses the placenta during pregnancy, scientists have been researching its possible effects on fetal brain development for more than a decade.

It has been difficult to draw firm conclusions, in part because of limitations on how scientists can study the question. No randomized, controlled clinical trials — the gold standard in medical research — have been conducted, because of ethical issues regarding research on pregnant women.

Another barrier is just how ubiquitous acetaminophen is. Other painkillers are known to cause serious harms during pregnancy, so acetaminophen has long been recommended as the first-line treatment. And because it is available over the counter, it is difficult for scientists to track how much women are using it and when.

A scientific review, published last August by researchers at Harvard and the Icahn School of Medicine at Mount Sinai, and cited by Mr. Trump’s health advisers in September, concluded that there was evidence for a link between acetaminophen and autism.

But some of the studies included in that review did not account for underlying factors that might be driving the connection, Dr. Khalil said. For example, women typically take acetaminophen because of health issues during their pregnancies, including infections and fevers, and those health problems themselves can increase the risk of neurodevelopmental disorders.

The new review excluded any studies that did not account for such possible confounding factors. And it gave more weight to studies that tried to account for the role of genetics, comparing siblings born to the same mother. Genetics is known to be a major contributor to autism risk.

“These were all attempts, really, to derive the evidence from the good-quality studies,” Dr. Khalil said. The sibling studies were the most rigorously designed, she said, because they take into account shared genetic factors and shared family environment.

One major sibling study published in 2024 examined electronic health records from nearly 2.5 million children in Sweden, finding a small association between women who used acetaminophen and the incidence of autism, A.D.H.D. and intellectual disability. But when the researchers did a subsequent analysis comparing siblings, in which one was exposed to acetaminophen in the womb and one was not, they found no link.

Experts praised the new review for helping to better examine the different types of studies that had been done to date. But some also cautioned that questions still remained unanswered. For example, the review did not take into account how frequently pregnant women used acetaminophen, or at what doses.

Eivind Ystrom, a senior researcher at the Norwegian Institute of Public Health, said he supported the conclusion of the new review “given our current knowledge.” But, he added, “at the same time, we should do more and better studies.” Dr. Ystrom conducted a study in 2021 suggesting that pregnant women who take acetaminophen for more than 29 days may be at higher risk of having children with A.D.H.D.

The new scientific review comes amid continuing legal battles over the potential link between acetaminophen and neurodevelopmental disorders. Hundreds of lawsuits have been filed in recent years against the makers of Tylenol and its generic versions by families who claimed that their children developed autism or A.D.H.D. after the mothers took the painkiller during pregnancy. The largest group of cases, filed in federal court, was dismissed in 2023 by a judge who cited the lack of credible scientific evidence. That decision is under appeal.

And in October, Ken Paxton, the Republican attorney general of Texas, sued Johnson & Johnson, which manufactured the drug for decades, and Kenvue, a spinoff company that has sold it since 2023, claiming that the companies hid the risks of the drug on brain development of children.

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