The 3rd U.S. Circuit Court of Appeals in Philadelphia rejected plaintiff David Schaffner's claim that Bayer's Monsanto unit violated state law by failing to put a cancer warning on the label for Roundup.
Schaffner was diagnosed in 2006 with a kind of cancer called non-Hodgkins lymphoma, a common claim for Roundup plaintiffs.
He and his wife Theresa sued Bayer in 2019, in part over how his illness affected their relationship.
Chief Judge Michael Chagares wrote for a unanimous three-judge panel that the Federal Insecticide, Fungicide, and Rodenticide Act requires nationwide uniformity in pesticide labels, and prevented Pennsylvania from adding a cancer warning.
Bayer said the decision conflicts with rulings from federal appeals courts in San Francisco and Atlanta in similar cases.
That may increase the prospect that the U.S. Supreme Court could step in to resolve the split, and potentially reduce Bayer's liabilities.
Chip Becker, a lawyer for the Schaffners, said he was disappointed with the decision, and that federal law should not preempt his clients' failure-to-warn claim. He said the Schaffners are reviewing their legal options.
Bayer said it was pleased with the decision, and the Supreme Court should "settle this important issue of law."
It has maintained that Roundup and its active ingredient glyphosate are safe, and said it "continues to stand fully behind" the brand.
Bayer has faced extensive litigation over Roundup, and has seen its share price fall more than 73% since buying Monsanto for $63 billion in June 2018.
The company settled much of the Roundup litigation for $10.9 billion in 2020, but still faces about 58,000 claims. Another 114,000 claims have been settled or deemed ineligible.
Though Bayer won 14 of 23 Roundup trials through July 23, one victory was overturned on appeal, and the losses saddled it with billions of dollars of damages awards.
The Schaffners settled with Bayer in September 2022, conditioned on Bayer being unable to convince courts that federal law preempted Pennsylvania from requiring a cancer warning.
Chagares said it did, and that this approach "best achieves Congress's stated aim of uniformity in pesticide labeling."
Roundup is among the most widely used weed killers in the United States. Bayer phased out sales for home use last year.
The case is Schaffner et al v Monsanto Corp, 3rd U.S. Circuit Court of Appeals, No. 22-3075.
