Kenvue, the company that makes Tylenol, walked into Monday facing a $40 billion merger and a product it has defended for decades. It walked out facing something else: more than 500 revived lawsuits, a federal appeals court that said the science isn't as settled as the company claims, and investors who shaved nearly 2% off the stock before the afternoon was over.

The 2nd U.S. Circuit Court of Appeals in Manhattan ruled that a lower court judge was wrong to throw out expert testimony from three researchers who linked acetaminophen use during pregnancy to autism and attention deficit hyperactivity disorder in children. Those experts include the dean of Harvard's School of Public Health, a psychiatry professor at the Albert Einstein College of Medicine, and a toxicologist at Columbia University. The appeals court did not say Tylenol causes autism. What it said, in a 64-page decision, is that those researchers used methods other scientists recognize, and that their conclusions "constitute acceptable interpretations of scientific evidence where scientists may, and in fact do, disagree."

That distinction matters enormously for what comes next.

What this doesn't settle

The court was explicit: it is not deciding whether acetaminophen causes autism or ADHD, and it is not telling elected officials what to do about it. The ruling simply sends the lawsuits back to the original district court judge, Denise Cote, for further proceedings. Kenvue says it will again try to show that the plaintiffs' experts are unreliable.

Doctors and major medical societies still consider acetaminophen the preferred way to treat pain and fever during pregnancy. That clinical consensus has not changed.

But the legal landscape has. When Cote dismissed all of these lawsuits in December 2024, the door appeared closed. Now it is open again, and Kenvue has to fight through it.

Why the timing is complicated

Kenvue agreed last November to be acquired by Kimberly-Clark, the maker of Kleenex, for more than $40 billion. That deal is expected to close this year. Kimberly-Clark shares fell 2.7% on Monday, a sign that investors understand the liability doesn't disappear at closing. Whoever owns Kenvue when this litigation resolves will own the outcome.

The political backdrop adds another layer. President Trump and senior health officials suggested a link between acetaminophen and autism last September. That didn't create the lawsuits, which have been moving through courts for years, but it amplified public attention on a question that scientists genuinely disagree about. The appeals court was careful to note it was not weighing in on that political dimension.

The bigger picture for anyone who takes Tylenol

Nothing about Monday's ruling means Tylenol is unsafe to use. Acetaminophen remains one of the most widely studied drugs in the world, and its general safety profile is not in dispute. What is in dispute, in both science and now in court, is whether there is a specific risk tied to sustained use during pregnancy, and at what doses, and for whom.

For pregnant people, that uncertainty is the real story. The medical guidance hasn't shifted, but the litigation revival will likely prompt more conversation between patients and doctors about alternatives and about what "preferred" actually means when the research base is still contested.

For Kenvue, and soon for Kimberly-Clark, the question is financial scale. More than 500 cases are now back in play. Retailers named as defendants, including CVS, Kroger, Target, Walgreens, and Walmart, are also back in the picture. Product liability cases at this volume, with expert testimony from Harvard and Columbia that a federal appeals court has called credible, tend to resolve in one of two ways: expensive settlements or expensive trials.

The science may still be unsettled. The legal bill is starting to add up.