An appeals court on Tuesday revived hundreds of lawsuits alleging that children developed autism or attention-deficit/hyperactivity disorder after their mothers took Tylenol during pregnancy, overturning part of a federal judge’s decision that had dismissed the cases.

The 2nd U.S. Circuit Court of Appeals ruled that U.S. District Judge Denise Cote, an appointee of former President Bill Clinton, erred by excluding testimony from three key expert witnesses for the plaintiffs, finding that the judge withheld expert evidence.

Cote had dismissed the lawsuits in December 2024 after concluding that the plaintiffs’ experts relied on unreliable scientific evidence to support claims that prenatal exposure to acetaminophen, Tylenol’s active ingredient, causes autism spectrum disorder or ADHD.

In a unanimous opinion, the three-judge appeals panel, all Democratic appointees, said the district court exceeded its role by weighing competing scientific conclusions rather than determining whether the experts used reliable, generally accepted methodologies.

“These appeals concern what qualifies as admissible epidemiological testimony in support of a general causal relationship,” the judges wrote. “We are not deciding whether there is a general causal relationship between acetaminophen and ADHD and/or ASD. We are also not deciding whether the manufacturers of acetaminophen must warn consumers about any alleged risk posed by such a potential causal relationship.”

Instead, the court said the case centers on “the rules of evidence” and what the standard trial courts should apply when determining whether expert testimony is admissible in areas of active scientific debate.

The appeals court reinstated testimony from three of the plaintiffs’ experts: Andrea Baccarelli, dean of the Harvard T.H. Chan School of Public Health; Eric Hollander, a psychiatry professor at Albert Einstein College of Medicine; and Brandon Pearson, a toxicologist at Columbia University.

The ruling comes during renewed political scrutiny of acetaminophen use during pregnancy. In September, President Donald Trump and Health and Human Services Secretary Robert F. Kennedy Jr. urged pregnant women to avoid taking the pain reliever whenever possible.

“Don’t take Tylenol. Don’t take it,” Trump said. “If you just can’t, I mean, fight like hell not to take it.”

The administration’s comments drew criticism from medical organizations. Dr. Steven Fleischman, president of the American College of Obstetricians and Gynecologists, said suggestions that acetaminophen causes autism “dangerously simplify the many and complex causes of neurologic challenges in children.”

“Suggestions that acetaminophen use in pregnancy causes autism are not only highly concerning to clinicians but also irresponsible when considering the harmful and confusing message they send to pregnant patients, including those who may need to rely on this beneficial medicine during pregnancy,” Fleischman said.

The scientific evidence remains contested. Some observational studies have found a small association between prenatal acetaminophen use and neurodevelopmental disorders, while larger studies that account for genetic and other confounding factors have found no evidence that the medication causes autism or ADHD.

Baccarelli, the plaintiffs’ lead expert, had previously testified that “substantial evidence” supports a causal relationship between frequent, high-dose acetaminophen use during pregnancy and neurodevelopmental disorders in children. Cote then rejected that testimony in 2024, writing that he had “cherry-picked and misrepresented study results” and was therefore unreliable.

During oral arguments before the appeals court last year, Judge Gerard Lynch noted that “reasonable scientists do appear to disagree,” suggesting that disagreements within the scientific community should not automatically bar expert testimony.