A US Supreme Court ruling about the scope of preemption under the nation’s pesticide law could significantly expand legal challenges to warning requirements under California's Proposition 65 scheme, legal experts have told Chemical Watch News & Insight.
In its 25 June decision in Monsanto Co v Durnell, the court held that the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) preempts state law failure-to-warn claims that would impose safety warnings conflicting with EPA-approved pesticide labels.
The ruling could have implications for dozens of Prop 65-listed insecticides, fumigants and other biocides that are also covered by FIFRA and could spark new challenges to Prop 65 warnings for cosmetics, over-the-counter (OTC) drugs and other consumer products subject to federal labelling requirements, according to legal experts.
Michael Novak, a partner with Keller & Heckman, said the Supreme Court’s decision "strengthens and broadens the federal preemption provisions" of FIFRA.
The ruling centralises decision-making on pesticide warnings with the EPA, Novak said. Through this lens, he said, "Prop 65 is a warning scheme that is then encroaching on EPA’s expertise" to review pesticide chemicals and approve their labels.
Dennis Raglin, a shareholder with Carlton Fields, agreed that the Supreme Court's decision means certain Prop 65 warnings could be vulnerable to federal preemption.
The ruling suggests preemption could extend to "any regulation that requires an agency be charged with ensuring uniform national effect", if that uniformity would be frustrated or weakened by a contradictory state warning, he said.
As such, the ruling could have effects that go beyond pesticides to other federally regulated products.
Glyphosate legal battle
The Durnell case originated in a Missouri state court in 2019, when John Durnell filed suit against Monsanto – now owned by Bayer – alleging he had used glyphosate-based Roundup products for some 20 years, causing him to develop non-Hodgkin's lymphoma.
Durnell said Monsanto failed to include a cancer warning on the Roundup label, and a jury awarded him $1m on his failure-to-warn claim, a verdict affirmed in 2025 by the state court of appeals.
The US Supreme Court agreed to take up the case to resolve whether FIFRA preempts a state-law labelling requirement that differs from the requirements set by the US EPA.
"The answer is yes," the court said in a 7-2 opinion written by Justice Brett Kavanaugh.
FIFRA "demands ‘uniformity’ and expressly preempts state labelling requirements that are ‘in addition to’ or ‘different from’ federal labelling requirements", he wrote for the court’s majority.
"After EPA approves a pesticide’s label at registration, manufacturers are legally required to use that label – unless and until EPA approves or requires a label change and amends the pesticide’s registration", or they could be subject to civil and criminal penalties, Kavanaugh said.
In the case of Roundup, the EPA has reapproved the Roundup label on multiple occasions, and it does not include a cancer warning. By contrast, Kavanaugh said, Durnell’s claim "would require Monsanto to add a cancer warning to its labels".
Such a supplementary warning would be "‘in addition to or different from’ EPA’s labelling determinations that do not mandate a cancer warning", and is therefore expressly preempted, he said.
Implications for Prop 65
Although the lawsuit centred on a failure-to-warn claim for glyphosate, legal experts said the Supreme Court's decision could reach beyond the Roundup label to other Prop 65-listed pesticides.
"Prop 65 warning requirements for pesticides other than glyphosate certainly could be preempted based on the Durnell decision," said Lisa Burchi, of counsel with Bergeson & Campbell. "Specifically, Prop 65 warnings that exposure to a particular pesticide can cause cancer, birth defects or other reproductive harm would seemingly be in jeopardy when the EPA-approved label does not include any such language," she said.
Burchi noted that the history behind glyphosate is somewhat unique. The EPA has repeatedly concluded that the herbicide is unlikely to cause cancer, and the agency even took "the unusual step" in 2019 of issuing guidance stating that it would not approve labels that included a warning claiming that glyphosate is known to cause cancer.
For other pesticides where the EPA’s assessments are not as clear, "there may be questions in future litigation as to whether the Durnell decision applies" to preempt a state-level requirement, she said.
Another wrinkle in understanding the implications of Prop 65 is the extent to which Durnell could erode a long-standing California precedent protecting Prop 65-listed pesticides from preemption claims.
According to Novak, the Durnell case could pose a threat to a 1991 ruling from the US Court of Appeals for the Ninth Circuit in Chemical Specialties Manufacturers Association v Allenby, which held that Prop 65 warning requirements for pesticides are not preempted by federal law.
The court in that case reasoned that Prop 65 warnings could still be provided off-label at the point of sale. While the case is still binding law in the Ninth Circuit, Novak said, Durnell could give industry a chance to contest Allenby in the future and scale that ruling back.
Implications for other labelling schemes
The implications of the Durnell decision may also go beyond pesticides, experts said, noting that the court’s reasoning could apply wherever a federal statute imposes a uniform label through a similar agency review process.
Other areas could be "ripe for testing the breadth [of] the Durnell decision" and preemption of state-level requirements, Burchi said.
Preemption issues have been raised in other contexts in the past, for products subject to the Federal Food, Drug, and Cosmetic Act (FD&C Act) or consumer items regulated by the Federal Hazardous Substances Act (FHSA), she said.
According to Burchi, the argument for preemption will be stronger where the preemption language and review process under the statute closely parallels that of FIFRA.
Raglin said the key distinguishing factor "is the goal of uniformity in labelling" in the underlying statute.
For example, the California Court of Appeal in 2023 ruled that generic OTC drug makers are exempt from Prop 65, holding that the "duty of sameness" preempted Prop 65. Raglin, who was part of the legal team that secured the appellate ruling, said that the court found that OTC drugs "must use the same language as a brand drug and cannot add a state warning which would defeat the need for uniform patient safety information".
Some level of preemption could also apply to cosmetics regulated under the Modernization of Cosmetics Regulation Act (MoCRA), which was intended to provide uniform and consistent regulation of cosmetics, Raglin said.
The ruling in Durnell specifically noted the labelling and packaging requirements for cosmetics under the FD&C Act as being similar to FIFRA. And in the decision, the justices made it clear "that where federal regulations explicitly prevent states from requiring labelling or warning that is additional to or different from that allowed by regulation, it creates a binding standard", Raglin said.
Ruling could ‘supercharge’ preemption challenges
Ultimately, the Durnell ruling could help "supercharge preemption challenges" to Prop 65, Raglin said, including at the state court level.
The Supreme Court’s decision in Durnell follows a string of recent industry wins in First Amendment cases that successfully blocked Prop 65 warning requirements for substances where there is no strong scientific consensus on toxicity. The cases, all heard in federal courts, turned on the notion that the government cannot compel speech that is not "purely factual and uncontroversial".
Taken together, the Supreme Court decision and the First Amendment cases could give California state courts hearing Prop 65 challenges the legal precedent necessary to find that warnings for other substances are preempted or unconstitutional, Raglin said.
State courts have thus far largely avoided wading into constitutional challenges to Prop 65, but the combination of these cases could change that, he said.
There will be more Prop 65 chemicals "found to be either preempted or in violation of the First Amendment", he predicted.