Full Second Circuit Should Affirm District Court’s Exclusion of Plaintiffs’ Expert Testimony In Tylenol Autism/ADHD Litigation

The plaintiffs in the “Tylenol” multidistrict litigation allege that acetaminophen—the active ingredient in Tylenol—causes autism spectrum disorder and attention deficit hyperactivity disorder (ADHD).  A New York federal district judge found that the plaintiffs’ causation testimony was unreliable and thus inadmissible under Federal Rule of Evidence 702. Rather than deferring to the district court’s role as gatekeeper, a Second Circuit panel vacated the district court’s ruling as to certain experts and deemed parts of their testimony admissible.

Tylenol’s manufacturer, Kenvue Brands, is seeking rehearing en banc. ALF, the nation’s foremost advocate for sound science in judicial and regulatory proceedings, has submitted an amicus brief (along with a motion for leave to file) in support of the rehearing petition. The amicus brief was drafted for ALF by Eric G. Lasker and Shannon N. Proctor of Hollingsworth LLP, attorneys with extensive expertise and experience on the purpose and application of amended Rule 702.

Issue Areas:

Sound Science

Read the Amicus Brief:
See News Release:
Question(s) Presented:

Whether the district court erred in applying Federal Rule of Evidence 702, as amended, and ruling that Plaintiffs’ causation experts failed to reliably show that prenatal exposure to acetaminophen can cause autism spectrum disorder or attention deficit hyperactivity disorder.


ALF’s Amicus Brief:

ALF’s amicus brief explains that Federal Rule of Evidence 702 was amended in 2023 to correct recurring misapplication of the rule and clarify district judges’ gatekeeping duty to exclude expert testimony that is unreliable. More specifically, under Rule 702, the reliability of an expert’s application of scientific methods, including on subjects such as general causation, must be assessed by the judge and excluded if unreliable rather than being sent to a jury for a determination of the weight it should be afforded.

The brief then discusses why and how the Second Circuit panel  erred by essentially ignoring Rule 702 and criticizing the district judge for fulfilling his duty as expert testimony gatekeeper.

Equally important, the panel erred by concluding that the excluded testimony is reliable because the expert’s methodology is the same as methodologies used by scientists outside the courtroom. The brief explains that the panel’s “overreliance on the mere existence of any similar practice of science outside the courtroom as proxy for in-court reliability is fundamentally flawed.” Using purported scientific debate or the practices of other scientists in this manner would effectively gut Rule 702, thereby undermining adjudication based on sound science.

Contact:

Email ALF Executive Vice President & General Counsel Lawrence Ebner

Date Originally Posted: September 17, 2026

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