When claims are made that one of the most widely used medications during pregnancy may cause autism, the public understandably pays attention. Scientists, healthcare professionals, and even judges, however, must ask: What does the evidence actually show?
That question is now before the U.S. Court of Appeals for the 2nd Circuit in New York.
The court is reviewing whether to revive more than 500 lawsuits brought by parents alleging that prenatal acetaminophen exposure caused their children’s autism or attention deficit hyperactivity disorder.
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In 2023, U.S. District Judge Denise Cote correctly excluded key expert testimony supporting those claims, concluding that they failed to meet the reliability requirements under Federal Rule of Evidence 702.
Rule 702 requires judges to ensure that expert testimony presented to juries is grounded in sufficient data, reliable methodology, and a sound application of those methods to the facts of the case. In other words, courts must act as gatekeepers to prevent speculative or methodologically unsound scientific claims from being treated as fact.
Now, plaintiffs are asking the appellate court to reverse that ruling.
The 2nd Circuit’s decision will test judges’ authority to prevent questionable scientific claims from reaching a jury before they satisfy established reliability standards.
This judicial gatekeeping role is essential. When expert opinions are admitted without rigorous scrutiny, litigation can amplify hypotheses that have not yet met the standards of the broader scientific community.
One reason the plaintiffs’ expert testimony was dismissed is the problem of confounding variables. Neurodevelopmental conditions such as autism and ADHD have strong genetic components. Disentangling whether a condition stems from a medication exposure or from shared familial and hereditary factors is extraordinarily complex. Critics argue that some of the underlying acetaminophen studies cited by the plaintiffs’ witnesses did not sufficiently control for these inherited influences, raising the possibility that correlation was mistaken for causation.
Additionally, the expert analyses combined autism spectrum disorder and ADHD outcomes despite their distinct diagnostic criteria. Putting autism and ADHD into the same analysis can make a weak relationship look stronger on paper, even though the conditions aren’t the same, so they do not necessarily have the same causes.
Many of the observational studies relied heavily on retrospective, self-reported medication use, sometimes years after pregnancy. Such designs introduce the potential for recall bias. In addition, large, well-controlled studies that have not found a causal relationship were not given comparable weight in the experts’ conclusions.
None of this means the question should not continue to be studied. But when the vast majority of high-quality studies find no causation between prenatal acetaminophen use and neurodevelopmental conditions, careful gatekeeping must be exercised.
This case’s potential impact on public health underscores the importance of gatekeeping.
Acetaminophen is one of the most widely used medications in the world and remains the only generally recommended pain and fever reliever during pregnancy when used as directed. That guidance reflects decades of regulatory review and clinical experience.
Several studies have reported an increased likelihood of birth defects and other adverse outcomes from untreated maternal fever. Sustained elevated maternal temperature is not benign. Physicians routinely advise pregnant patients to treat significant fevers promptly because unmanaged fever itself can pose risks to fetal development.
Discouraging acetaminophen use without causal evidence of autism is incredibly risky. If pregnant women avoid acetaminophen based on unproven claims, they may turn to alternatives such as ibuprofen or aspirin, which carry known risks later in pregnancy, including possible complications affecting fetal kidney development.
By upholding judges’ gatekeeping responsibility under Rule 702, the 2nd Circuit can reaffirm that scientific claims must meet established thresholds before being presented as reliable proof in court.
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If those standards weaken, unproven claims could influence public behavior in ways that put mothers and babies at risk.
The 2nd Circuit now has an opportunity to reinforce a simple but essential principle. In both law and public health, scientific standards matter.
Tom Giovanetti is the president of the Institute for Policy Innovation.