A U.S. federal appeals court judge ruled Monday that California cannot require companies like agrochemical giant Bayer to include a cancer warning on their glyphosate-based products, despite the World Health Organization's 2015 classification of the weed-killer as a probable human carcinogen.

U.S. District Judge William Shubb of the Eastern District of California wrote in his opinion (pdf) that mandating a cancer warning label on glyphosate products would violate companies' First Amendment rights by compelling them to echo a finding that Shubb characterized as "not purely factual and uncontroversial."

Bayer—which acquired the notorious agrochemical company Monsanto in 2018—has maintained that glyphosate is not carcinogenic even as it faces tens of thousands of lawsuits from plaintiffs alleging that the weed-killer Roundup caused their cancer.

Since last year, Bayer has been ordered by juries to pay out tens of millions of dollars in damages to cancer patients. Plaintiffs have accused the company of manipulating glyphosate research and failing to warn the public of Roundup's carcinogenic effects.

Shubb took Bayer's side in his ruling Monday, contending that "there is insufficient evidence to show" that glyphosate causes cancer in humans.

"California has options available to inform consumers of its determination that glyphosate is a carcinogen, without burdening the free speech of businesses, including advertising campaigns or posting information on the internet," Shubb wrote.

The ruling stems from California's 2017 decision to add glyphosate to its Proposition 65 list of chemicals "known to cause cancer." Approved by voters in 1986, Prop 65 requires businesses to "provide warnings to Californians about significant exposures to chemicals that cause cancer, birth defects, or other reproductive harm."