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The federal litigation alleging that prenatal acetaminophen exposure can cause autism spectrum disorder and attention-deficit/hyperactivity disorder has been revived following a major appellate ruling on expert testimony.
On July 13, 2026, the U.S. Court of Appeals for the Second Circuit vacated judgments that had dismissed more than 500 lawsuits. The panel concluded that the district court improperly excluded testimony from three of the plaintiffs’ general causation experts.
The decision is a major procedural victory for plaintiffs, but it is not a finding that Tylenol causes autism or ADHD. Instead, it allows key experts to present their scientific interpretations and moves the litigation into a new phase focused on the strength and application of that evidence.
Where the Tylenol Autism Lawsuits Stand
Plaintiffs allege that prenatal use of acetaminophen, the active ingredient in Tylenol, can disrupt fetal brain development and increase the risk of autism and ADHD.
Many federal cases were consolidated into Multidistrict Litigation No. 3043 in the Southern District of New York. The litigation initially focused on general causation: whether acetaminophen is capable of causing the alleged conditions. Specific causation asks whether it caused an individual plaintiff’s condition.
The Initial Expert Exclusions
In late 2023, the district court excluded the plaintiffs’ general causation experts under Federal Rule of Evidence 702.
The court concluded that the experts had not reliably demonstrated that prenatal acetaminophen exposure could cause autism or ADHD. It criticized certain experts for selectively interpreting studies, inadequately addressing confounding variables, and reaching conclusions beyond those of the researchers they cited.
Without admissible general causation testimony, the plaintiffs could not prove an essential element of their claims. The court entered judgment for the defendants and dismissed hundreds of cases.
The Second Circuit has now changed that trajectory.
What the Second Circuit Decided
The Second Circuit concluded that the district court improperly excluded testimony from:
- Andrea Baccarelli, MD, PhD, an epidemiologist and public health researcher
- Eric Hollander, M.D., a psychiatrist
- Brandon Pearson, PhD, a toxicologist
The panel found that these experts used methodologies also employed by other scientists in their fields. Scientific disagreement did not necessarily make their opinions inadmissible.
The appellate court found that some criticisms concerned the weight of the testimony, meaning how persuasive it is, rather than whether the factfinder may hear it.
The court upheld two exclusions where experts had not adequately explained how they applied their methods or connected the research to their conclusions. Using a recognized methodology is not enough. Experts must explain how they evaluated the evidence and reached their opinions.
A Victory on Admissibility, Not Causation
The Second Circuit did not decide whether prenatal acetaminophen exposure causes autism or ADHD, whether additional warnings were required, or whether individual plaintiffs can prove their claims.
The court decided only that three experts should be allowed to present their scientific interpretations.
Plaintiffs now have evidence that may allow their general causation theory to proceed. Defendants can still respond through cross-examination, competing experts, updated research, and summary judgment.
The dispute has moved from whether the plaintiffs’ scientific position may be heard to whether it is persuasive enough to support liability.
The Scientific Challenge: Association vs. Causation
Several epidemiological studies have reported associations between prenatal acetaminophen use and neurodevelopmental conditions. Most of the evidence is observational, meaning it can identify patterns but may not independently establish causation.
Potential confounding variables include:
- Maternal fever, infection, or inflammation
- The reason acetaminophen was taken
- Genetics and family history
- Other medications or prenatal exposures
- Differences in dosage and duration
- Recall and reporting limitations
Because acetaminophen may be taken to treat fever or infection, experts must distinguish the drug’s possible effects from those of the underlying illness. This issue, often called confounding by indication, will remain central.
Medical organizations continue to consider acetaminophen an important option during pregnancy when used appropriately. Kenvue maintains that the evidence does not establish a causal link. The litigation will test where courts draw the line between association and legally sufficient proof.
Where the Litigation May Go Next
The Scientific Record May Be Updated
The literature continued to develop while the cases were on appeal. On remand, the court may permit supplemental reports, depositions, and renewed challenges addressing newer studies. Both sides will need experts who can evaluate the complete body of research.
General Causation Will Remain Contested
Reinstating three experts does not guarantee that plaintiffs will establish general causation.
Defense teams will likely argue that plaintiffs’ experts overstate observational findings or fail to account for confounders. Plaintiffs will argue that reasonable scientific disagreements should be resolved by the factfinder.
The focus may shift to whether each expert applied accepted methods rigorously and consistently.
Specific Causation May Be the Next Battleground
If general causation survives, individual case evidence will become more important. Attorneys may need to examine:
- Timing, frequency, dosage, and duration of acetaminophen use
- The medical reason for its use
- Maternal fever, infection, or inflammation
- Other medications and prenatal exposures
- Genetic and familial risk factors
- Prenatal and pediatric medical histories
- Developmental milestones and diagnostic history
- Alternative explanations for the condition
Even if plaintiffs establish that acetaminophen is generally capable of contributing to a neurodevelopmental condition, each claimant may still need expert testimony linking the exposure to the individual diagnosis.
Settlement Pressure May Increase
Reviving hundreds of cases changes the parties’ leverage and may increase settlement pressure. A global resolution is not necessarily imminent, however.
Additional expert rulings, summary judgment motions, and possibly bellwether cases may be necessary before the parties can assess overall exposure.
What the Decision Means for Plaintiff Attorneys
Plaintiffs have regained an opportunity to present their causation theory, but the ruling should not be treated as validation of every scientific argument.
Their experts will likely need to:
- Explain how supportive and contradictory studies were weighed
- Address confounding by indication
- Distinguish association from causation
- Incorporate newly published research
- Explain any proposed dose, timing, or duration relationship
- Address alternative causes
- Connect general causation to individual medical histories
The decision also highlights the value of multidisciplinary teams spanning epidemiology, toxicology, psychiatry, neurology, pharmacology, genetics, and maternal-fetal medicine.
What the Decision Means for Defense Attorneys
Defendants lost the broad exclusion that previously ended the federal cases, but they retain substantial avenues for challenging the claims.
Defense teams can test whether experts applied their methods consistently, addressed unfavorable findings and confounders, supported dose and duration opinions, and considered alternative explanations.
The ruling does not prevent targeted challenges to unsupported conclusions.
Ongoing monitoring of retained and opposing experts may also become important as experts publish new research, refine their opinions, or testify in related cases.
The Broader Expert Witness Lesson
The decision presents two complementary principles.
Courts should not resolve legitimate scientific disagreements merely by choosing one reasonable interpretation over another, but experts must still explain how they assessed conflicting studies, addressed limitations, and reached their opinions.
These principles may influence other product liability and mass tort cases involving developing or contested science.
The Role of Expert Institute
Attorneys handling Tylenol autism and ADHD claims need experts whose qualifications closely match the opinions they will provide.
Expert Institute connects legal teams with well-vetted experts in areas including epidemiology, developmental toxicology, pediatric neurology, psychiatry, pharmacology, biostatistics, genetics, and maternal-fetal medicine.
Through Expert Radar, attorneys can monitor retained and opposing experts for new publications, prior testimony, legal challenges, public statements, and changes in scientific opinions.
Medical record review may become especially important as the cases move toward specific causation. A detailed review can help reconstruct prenatal exposure, identify why acetaminophen was taken, evaluate alternative causes, and determine whether an individual claim is supported by the record.
Expert Institute can also assist attorneys preparing for Daubert challenges, developing cross-examination strategies, evaluating competing causation theories, and coordinating multidisciplinary expert teams.
The Expert Fight Is Entering a New Phase
The Tylenol autism and ADHD litigation is active again, but the Second Circuit did not settle the scientific question at its center.
Plaintiffs have cleared an important admissibility hurdle. Defendants still have significant opportunities to challenge the weight, reliability, and sufficiency of the evidence. The district court may also need to consider research published while the cases were on appeal.
The litigation is not moving directly from dismissal to resolution. It is entering a more detailed phase centered on updated science, competing experts, individual medical evidence, and the line between scientific uncertainty and legally sufficient proof.
The causation fight did not end. It moved from admissibility toward persuasion.
