A California federal court has temporarily enjoined the State of California from requiring Proposition 65 cancer warnings on chemicals containing glyphosate, forcing the state to acknowledge and reconcile "the heavy weight of evidence in the record that glyphosate is not in fact known to cause cancer," making cancer warnings "factually inaccurate and controversial," "misleading to the ordinary consumer" and in violation of the First Amendment.  

The Proposition 65 warning requirement was to take effect in July 2018. The court took its decision in light of a study released in December 2017 by the US. Environmental Protection Agency and a long-term study within the larger Agricultural Health Study of US agricultural workers, both of which found no evidence of glyphosate carcinogenicity.

Proposition 65 prohibits any person in the course of doing business from knowing and intentionally exposing anyone to the listed chemicals without a prior "clear and reasonable" warning.  On July 7, 2017, glyphosate was listed as a chemical known to the State of California to cause cancer. The attendant "warning requirement" that glyphosate "causes cancer" is set to take effect on July 7, 2018. 

In the underlying case, National Ass'n of Wheat Growers, et al. v. Zeise, a group of grower associations and entities that sell or use glyphosate-based herbicides have brought a lawsuit against California's Office of Environmental Health Assessment and the Attorney General, seeking to enjoin the warning requirement and listing of glyphosate under Proposition 65. Plaintiffs called glyphosate a "vital tool" for growers to provide safe, affordable foods, and it is widely used in pesticides and herbicides, a multibillion-dollar industry. The plaintiffs claim the listing and the warning requirement violate the First Amendment by compelling them to make false, misleading, and highly controversial statements about their glyphosate-based products

The federal court in the agriculture-heavy Eastern District of California granted plaintiffs' request to enjoin Proposition 65's warning requirement for glyphosate (but denied its request to enjoin adding glyphosate to the Proposition 65 list of chemicals). The court observed that "where California seeks to compel businesses to provide cancer warnings, the warnings must be factually accurate and not misleading."

Based on the "heavy weight of evidence" that glyphosate is not known to cause cancer, the court determined the warning would be factually inaccurate and controversial.

While the court refused to enjoin the listing of the chemical under Proposition 65 entirely, since the listing itself neither restricts nor compels speech, the listing will likely have little impact without the attendant warning requirement. 

This outcome will likely put wind in the sails of industry groups and individual companies challenging the science behind other Proposition 65 listings, which are sometimes based on scientific studies controverted in other places. The opinion can be found here.