The U.S. Circuit Court of Appeals for the Second Circuit recently ruled that the Manhattan district court improperly excluded the expert testimony of three out of five plaintiffs’ general causation experts in cases alleging that Tylenol/acetaminophen caused autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) in children exposed prenatally, reviving more than 500 lawsuits against the manufacturers and retailers of Tylenol/acetaminophen, including Kenvue (the current maker of Tylenol), Walmart, Walgreens, CVS and Kroger.
Limits of the Decision
The decision is careful to emphasize its own limits, making clear the Court did not find that acetaminophen causes ASD or ADHD.
“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. And we are certainly not deciding the approach that policymakers concerned with protecting public health should take to regulating the use of acetaminophen.”
Limited Revival of Expert Testimony
The first issue on appeal was the admissibility of epidemiological testimony in support of general causation. More specifically, the core issue is the scope of Rule 702/Daubert gatekeeping in a Multidistrict Litigation (MDL) setting, including when district courts may exclude epidemiological Bradford Hill and related analyses. The qualifications of plaintiffs’ experts were not in dispute. Rather, the appeal concerned how closely a trial court may scrutinize a qualified expert’s conclusions before allowing the testimony to reach a jury under Rule 702 and Daubert.
The district court excluded expert testimony that did not reflect a reliable application of epidemiological methodologies and improperly cherry-picked among studies. The appellate court disagreed as to three experts, finding their opinions sufficiently supported even though the studies they relied on did not prove causation but, in the court’s view, could still serve as “a reliable link in a chain supporting a particular Bradford Hill factor.”
Some doctors, including Dr. Andrea Baccarelli, the Dean of Harvard University’s School of Public Health, testified at the district court and submitted a report indicating a possible association between prenatal acetaminophen exposure and neurodevelopmental disorders, though he emphasized the need for further research. That testimony was excluded and the lawsuits were dismissed in December of 2024, but the appellate court disagreed, saying the District Court Judge Denise Cote had overstepped the court’s authority when presented with scientific discourse of opposing opinions.
The court wrote, “[t]hose concededly qualified experts offered opinions that comport with methodologies applied by other scientists in their fields, and constitute acceptable interpretations of scientific evidence where scientists may, and in fact do, disagree on the ultimate answer to the causal question that they are assessing.”
Impossibility Preemption Rejected
The second issue on appeal was federal preemption. Defendants asked the Court to reverse the district court’s refusal to dismiss the failure-to-warn claims on that ground. However, the appellate court affirmed, finding impossibility preemption inapplicable at this stage because manufacturers could add “supplemental warnings when additional, pregnancy-related risks become known.”
The appellate court reasoned that the federal Pregnancy Warning Regulation requires manufacturers to include the applicable FDA-directed pregnancy warning, but does not bar them from adding additional warnings if new pregnancy-related risks become known. The court also emphasized manufacturers remain responsible for ensuring their labels are accurate and adequate.
Further, the court noted “Defendants-Appellees have not shown that simultaneous compliance with the federal Pregnancy Warning Regulation and state law warning requirements is impossible.”
Notably, the preemption ruling should be understood as a pleading-stage decision, not a merits determination that preemption is unavailable as a matter of law. At this early stage, the court determined only whether simultaneous compliance with federal and state warning requirements was impossible on the face of the pleadings. That is a materially different inquiry from summary judgment, where defendants may rely on a developed regulatory and factual record to show what labeling changes were legally available, when they were available, and whether FDA action or the applicable OTC monograph framework would have prohibited the unilateral labeling changes plaintiffs contend should have been made.
Accordingly, although the appellate ruling allows the warning claims to move past the pleading stage, significant preemption and causation defenses remain available for summary judgment on a more developed factual record. The timing of the alleged exposures, the evolution of scientific knowledge and the evolution of the federal regulatory framework may therefore favor the defense.
Next Steps and Defense Implications
The appellate court’s decision returns the lawsuits to the district court for further proceedings, but it does not resolve general causation or the merits of plaintiffs’ failure-to-warn claims. Many major medical groups, including the American College of Obstetricians and Gynecologists, claim there is no scientific evidence of a link between acetaminophen and autism, and that acetaminophen is the preferred and safest pain reliever and treatment for high fevers in pregnant women.
By vacating dispositive summary judgments across a large MDL based on Rule 702 gatekeeping issues and rejecting defendants’ OTC labeling preemption argument, the ruling materially expands litigation exposure and creates strong incentives for en banc rehearing and/or a petition for certiorari to the Supreme Court.
Harris Beach Murtha’s Medical and Life Sciences Practice Group frequently handles such cases and our attorneys are analyzing the decision for future use defending these cases.
If you need assistance with a case or have questions, please reach out to attorney Kelly Jones Howell at (212) 912-3652 and khowell@harrisbeachmurtha.com; attorney Marina Plotkin at (212) 313-5409 and mplotkin@harrisbeachmurtha.com; or the Harris Beach Murtha attorney with whom you most frequently work.
This alert is not a substitute for advice of counsel on specific legal issues.
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