The Supreme Court appeared divided on Monday during arguments in a dispute that could determine the fate of thousands of lawsuits that claim a widely used weedkiller causes cancer.
The case is the latest turn in a yearslong legal battle over safety concerns with the weedkiller Roundup. Developed by Monsanto in the 1970s, the herbicide is the focus of the lawsuits, which allege that it causes non-Hodgkin lymphoma.
The dispute before the justices stems from a 2019 lawsuit brought by a Missouri gardener, John Durnell, who used Roundup for years. Mr. Durnell claimed in his suit, filed in state court, that glyphosate, a chemical in the weedkiller, caused him to develop cancer.
During Monday’s oral argument, several of the justices expressed skepticism of arguments by Bayer, the German conglomerate that acquired Monsanto in 2018, and the Trump administration, which joined the case in support of the company, that federal rules requiring herbicide safety labeling should bar people from filing state-level claims that accuse companies of failing to warn consumers of dangers.
Chief Justice John G. Roberts Jr. asked a lawyer for the Trump administration whether states would be blocked from taking action if new scientific information raised alarms about a product.
“The states cannot do anything?” the chief justice asked.
Sarah M. Harris, a principal deputy solicitor general, responded that it would be problematic to have each state “jumping the gun” to come to its own conclusion about whether a product caused cancer.
Mr. Durnell’s lawyer argued that the federal Environmental Protection Agency had overreached its regulatory power, a problem that could be rectified by giving a role to the courts, including at the state level. Several of the justices appeared skeptical about that argument too, questioning whether such a ruling would lead to a patchwork of safety regulations throughout the country.
In 2023, a Missouri jury awarded Mr. Durnell $1.25 million in damages. If the court finds for the company, that verdict would be erased and other claims against the company also endangered. If the court finds for Mr. Durnell, not only would his verdict be upheld but it would also clear the way for a flood of other lawsuits to proceed in courts throughout the country.
A ruling is expected by the end of June or early July.
The central issue in the case involves a collision between state and federal power. Bayer has argued that Mr. Durnell’s victory in Missouri is barred by a federal law, the Federal Insecticide, Fungicide and Rodenticide Act. That statute requires herbicides to be registered with the E.P.A. before they can be sold. Companies must submit scientific and safety data to the agency, along with proposed label information, including safety warnings.
The E.P.A. has previously determined that Roundup is safe to use. Bayer argued that the Missouri court ruling conflicted with that federal determination. Paul D. Clement, who represented Bayer, asserted that the federal agency’s decision invalidates the state court decision.
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Bayer has also said it cannot add warnings to Roundup in any state because they would contradict the E.P.A.’s findings.
In a brief to the justices, lawyers for Bayer wrote that the E.P.A. had “repeatedly determined” that glyphosate does not cause cancer, asserting that the company would not be able to “unilaterally alter the label’s precautionary warnings to include a cancer warning.”
The company appealed the jury verdict in the Missouri case and, after the Missouri Court of Appeals upheld the trial court, asked the justices to weigh in.
Early this year, President Trump issued an executive order that cited the Defense Production Act to guarantee production of glyphosate-based herbicides, appearing to elevate Roundup to a national security priority.
Lawyers for Mr. Durnell argued that nothing in the federal law blocked Bayer from adding a cancer warning to its labels and that such a change would not require E.P.A. approval.
Mr. Durnell was represented by Ashley C. Keller and Jonathan F. Mitchell, a prominent conservative appellate lawyer who represented Mr. Trump in his successful 2024 bid to remain on the Colorado ballot. Both lawyers joined the Supreme Court case only in recent months.
Another lawyer, David C. Frederick, had initially filed briefs on Mr. Durnell’s behalf. In the weeks before the argument, Mr. Frederick asked the justices if he could participate in the argument on behalf of more than 11,500 other people whose claims would be affected by the court’s decision. The justices denied the request.
