On November 7, 2023, the U.S. Court of Appeals for the Ninth Circuit granted a motion filed by several agricultural groups and unions, enjoining California Attorney General Rob Bonta from requiring companies covered by Proposition 65 (Prop 65) to warn users of glyphosate-based products about its carcinogenic effects.

Adopted in 1986 by the State of California, Prop 65, or the Drinking Water Safety and Toxic Substances Act, requires certain companies to indicate in a "clear and reasonable" manner that the consumer is being "voluntarily and intentionally" exposed to a hazardous chemical substance by using the product in question. In the case of substances recognized as carcinogens by the State of California, companies subject to this obligation must indicate that the substance is "known by the State to have carcinogenic effects". This indication can take the form, for example, of a yellow triangle affixed to the packaging with the word "WARNING" accompanied by a sentence explaining that the substance is carcinogenic, or more simply the word "CANCER".

In 2015, the International Agency for Research on Cancer (IARC) identified glyphosate, a herbicide substance used notably in Roundup products marketed by the American company Monsanto, as "potentially carcinogenic". Following this, the California Office of Environmental Health Hazard Assessment (OEHHA) submitted all glyphosate-containing products to Prop 65.

On November 15, 2017, several groups representing the interests of American agricultural producers challenged this decision in the Eastern District Court of California. They accused OEHHA of infringing on their constitutionally enshrined freedom of expression by requiring them to market products as carcinogenic when they were convinced otherwise. On December 6, 2017, the plaintiffs filed a request for a preliminary injunction to obtain a stay of the decision to subject glyphosate to the requirements of Prop 65.

On February 26, 2018, the District Court granted this request. In its reasons for decision, the Court notes in particular that the formulation that "glyphosate is a product known by the State to be carcinogenic" is not sufficiently demonstrated in the facts and is not without controversy. Indeed, while the IARC has recognized the substance as a potential carcinogen, the Environmental Protection Agency (EPA) and other health agencies around the world have not.

On March 26, 2018, the Attorney General of the State of California asked the District Court to amend its preliminary injunction and presented alternative phrasings for the warnings. In particular, he proposed to indicate on the labal that only the State of California recognizes glyphosate as a carcinogen, based on the work of the IARC, but that this is not the case for the EPA and other regulatory agencies. The Attorney General's request is rejected by the District Court in June 2018.

In September 2019, both parties filed motions for summary judgment before the District Court. Plaintiffs sought a permanent injunction prohibiting enforcement of the Prop 65 warning requirement with respect to glyphosate while the California Attorney General moved to dismiss. In June 2020, the District Court finally granted the plaintiff groups' request, and rejected that of the Attorney General. The Attorney General appealed this decision.

In dismissing the appeal, the Court of Appeal for the Ninth District noted that, unless motivated by "purely factual and uncontroversial" considerations, any restriction on corporate freedom of expression must satisfy an "intermediate scrutiny test", under which the restrictive measure is considered lawful only if its purpose is to protect an important government interest, and the means used to that end are substantially related to that interest.

In this case, the Court of Appeal held that the position of the IARC and the OEHHA as to the carcinogenic nature of glyphosate is not the subject of a scientific consensus and is in the minority, so that the restriction on freedom of expression in this case is not based on purely factual and uncontroversial considerations. On the other hand, if protecting the health of Californian citizens does indeed constitute a legitimate interest satisfying the criteria of the "intermediate scrutiny test", the Californian State had other means of protecting this interest without impacting on the companies' freedom of expression.