A U.S. appeals court on Monday reversed a trial judge’s decision to dismiss lawsuits against the makers of Tylenol, reviving hundreds of cases filed by families who claim that their children developed autism or attention deficit hyperactivity disorder after their mothers took Tylenol during pregnancy.
The judges, all Democratic appointees, ruled that the lower court overstepped by excluding scientific evidence presented by expert witnesses on behalf of the plaintiffs. That expert testimony, the judges argued, was valid evidence about a scientific question that they said was still under dispute.
The decision came less than a year after President Trump and top health advisers warned that taking acetaminophen, the active painkiller in Tylenol, while pregnant could cause autism and A.D.H.D. in children. That link is unproven: While some studies have suggested that there may be a small increase in risk, large trials aiming to account for underlying genetics and other factors have found no evidence that the painkiller can cause neurodevelopmental disorders in children.
Major medical groups, including the American College of Obstetricians and Gynecologists, have consistently stressed that Tylenol is the safest option to treat pain or fever during pregnancy. Untreated fevers can pose risks to both the mother and the fetus, including neurodevelopmental issues.
The ruling acknowledged the national attention on the issue. The appeals “arise against the backdrop of significant debate in the relevant scientific communities,” the judges wrote. “That debate has also become political. But the issue before us is not political.”
At the center of the appeals court decision was the question of whether the scientific evidence presented by the plaintiffs held merit. The primary expert witness for the plaintiffs was Andrea Baccarelli, the dean of the Harvard T.H. Chan School of Public Health, who submitted a report claiming that “substantial evidence” supported a causal link between use of acetaminophen during pregnancy and neurodevelopmental disorders like autism and A.D.H.D. in children, especially when taken frequently at high doses.
In December 2024, U.S. District Judge Denise Cote sided with lawyers for the defendants and ruled that Dr. Baccarelli had “cherry-picked and misrepresented study results” in his testimony and was therefore “unreliable.”
During the appeals court hearing in November 2025, Judge Gerard Lynch argued that the scientific information presented was valid, even if it was under dispute. “Reasonable scientists do appear to disagree,” he said.
Ashley Keller, a lawyer for the main law firm representing the plaintiffs, Keller Postman, said that Monday’s ruling represented “vindication for the scientific evidence our clients have presented from the outset.” He added, “We look forward to presenting that evidence to a jury.”
The defendants in the suits are Kenvue, which has been the maker of Tylenol since it was spun off from Johnson & Johnson in 2023, and major retailers that sell generic versions of acetaminophen.
“The procedural ruling today does not change the fact that credible, independent science shows no proven link between taking acetaminophen and autism or attention deficit hyperactivity disorder,” said Melissa Witt, a spokesperson for Kenvue. “Science matters, and we stand with the many public health and medical professionals who have reviewed the science on this topic and agree.”
Mrs. Witt said that the company would continue to argue that the expert opinions should not be allowed in the case. “We stand behind the safety of our product and will continue to defend these cases,” she said.
Dr. Baccarelli did not immediately reply to a request for comment.
Because acetaminophen passes into the brain and crosses the placenta during pregnancy, scientists have researched its possible effects on fetal brain development for more than a decade.
But it has been difficult to draw firm conclusions about the impacts of Tylenol use on fetal development in part because no randomized, controlled clinical trials — the gold standard in medical research — have been conducted to answer the question.
Large observational studies cannot entirely account for the underlying factors driving a possible association in the data. And because acetaminophen is considered the safest painkiller for use during pregnancy and is available over the counter, it is difficult for scientists to collect data on how much women are using and when it is being used.
In August 2025, Dr. Baccarelli, along with collaborators at the Icahn School of Medicine at Mount Sinai, published a review of 46 existing studies that concluded that there was an association between Tylenol use during pregnancy and autism and A.D.H.D. That article was cited by Mr. Trump’s health advisers, who also consulted with Dr. Baccarelli directly, in their recommendation to women to avoid the painkiller during pregnancy except in cases of high fever.
But many scientists argued that this review, which did not include any new data, did not appropriately account for underlying factors. Another review published in the British medical journal The Lancet in January of this year gave more weight to studies that attempted 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. That study found no evidence for a link between acetaminophen and autism.
In June, a new sibling-controlled study of more than 700,000 mothers and children in Hong Kong also found no evidence of an association between acetaminophen use during pregnancy and autism or A.D.H.D. in children.
On Monday, the judges cautioned that their ruling should not be interpreted as evidence of a causal relationship between acetaminophen use in pregnancy and neurodevelopmental disorders in children.
“We are not deciding whether there is a general causal relationship between acetaminophen and ADHD and/or ASD,” the judges wrote. “We are also not deciding whether the manufacturers of acetaminophen must warn consumers about any alleged risk posed by such a potential causal relationship. And we are certainly not deciding the approach that policymakers concerned with protecting public health should take to regulating the use of acetaminophen.”
Rather, they continued, “We are called upon to decide how closely a trial court may scrutinize a qualified expert’s conclusions when that expert follows methodologies that are generally accepted in their field.”
