The Supreme Court was split Monday amid a dispute that could clarify thousands of lawsuits alleging a widely used herbicide causes cancer.
The case is the latest in a years-long legal battle over safety with the herbicide group Roundup. Developed by Monsanto in the 1970s, the weed killer is the focus of lawsuits, alleging it causes non-Hodgkin lymphoma.
The dispute before the judges stems from a 2019 lawsuit brought by a Missouri farmer, John Durnell, who used Roundup for years. Mr. Durnell said in his lawsuit, which he filed in state court, that glyphosate, a chemical that kills weeds, caused him to develop cancer.
During Monday’s argument, several justices expressed doubt about arguments by Bayer, the German conglomerate that acquired Monsanto in 2018, and the Trump administration, which joined the case to support the company, that federal rules requiring herbicide safety labeling should prevent people from filing federal lawsuits accusing the companies of failing to warn consumers.
Chief Justice John G. Roberts Jr. asked a Trump administration lawyer whether states would be prevented from taking action if new scientific information raises alarms about the product.
“Are the districts unable to do anything?” asked the grand jury.
Sarah M. Harris, deputy attorney general, responded that it would be problematic for each state to “jump the gun” to reach its own conclusion about whether a product causes cancer.
The lawyer of Mr. Durnell argued that the Federal Environmental Protection Agency had overstepped its regulatory authority, a problem that could be remedied by giving the courts a role, including at the federal level. Several justices appeared to question that argument as well, questioning whether such a decision would lead to uniform safety laws across the country.
In 2023, a Missouri judge awarded Mr. Durnell for $1.25 million in damages. If the court finds for the company, that judgment will be voided and other claims against the company will also be in jeopardy. If the court finds Mr. Durnell, not only will his decision be upheld but it will pave the way for other cases to continue in courts across the country.
A decision is expected in late June or early July.
The main issue in the case involves the conflict between the power of the state and the state. Bayer argued that Mr. Durnell in Missouri is prohibited by federal law, the Federal Insecticide, Fungicide and Rodenticide Act. That law requires herbicides to be registered with the EPA before they can be sold. Companies must submit scientific and safety data to the agency, as well as proposed label information, including safety warnings.
The EPA has previously determined that Roundup is safe for use. Bayer argued that the Missouri court’s decision conflicts with that state ruling. Paul D. Clement, who represented Bayer, argued that the federal agency’s decision invalidated the state court’s decision.
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Bayer also said it would not add warnings to Roundup under any circumstances because they would contradict the EPA’s findings.
In a brief to the judges, Bayer’s lawyers wrote that the EPA has “re-determined” that glyphosate does not cause cancer, arguing that the company cannot “independently change the label warnings to include a cancer warning.”
The company appealed the jury’s decision in the Missouri case and, after the Missouri Court of Appeals upheld the verdict, asked the justices to consider it.
At the beginning of this year, President Trump issued an executive order that cited the Defense Production Act to ensure the production of glyphosate-based pesticides, which seems to raise Roundup a national security priority.
The lawyers of Mr. Durnell argued that nothing in federal law prevented Bayer from adding a cancer warning to its labels and that such a change would not require EPA approval.
Mr. Durnell was represented by Ashley C. Keller and Jonathan F. Mitchell, a senior appellate attorney who represented Mr. Both lawyers joined the Supreme Court case in recent months.
Another attorney, David C. Frederick, had initially filed briefs on behalf of Mr. Durnell. In the weeks leading up to the dispute, Mr. Frederick asked the judges if he could participate in the debate on behalf of more than 11,500 other people who would be affected by the court’s decision. The judges rejected the request.
