The short answer
California listed glyphosate under Prop 65 in 2017 based on IARC's classification. But federal courts blocked enforcement of the cancer warning, ruling it could be false or misleading given other agencies' findings. It's a striking clash between hazard-based listing and disputed science.
Glyphosate was listed under Prop 65 in 2017 via the IARC-linked route.
IARC calls glyphosate 'probably carcinogenic'; the EPA disagrees.
Federal courts blocked enforcement of the warning on First Amendment grounds.
Courts found a required cancer warning could be false or misleading here.
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The full explanation.
When agencies disagree, courts get involved
The story of glyphosate and Proposition 65 is where the hazard-versus-risk debate stops being academic. It lands in federal court.
Glyphosate is the world's most widely used weedkiller. In 2015, the International Agency for Research on Cancer classified it as "probably carcinogenic to humans" (Group 2A). Through California's IARC-linked listing route, that classification triggered a Prop 65 listing, effective 2017. Listing meant sellers could be required to warn that glyphosate is "known to the State of California to cause cancer."
The scientific disagreement underneath
But there was a problem. Other major agencies disagreed with IARC. The U.S. EPA concluded that glyphosate is "not likely to be carcinogenic to humans" at expected exposures. The European Food Safety Authority reached a similar view. As our page on glyphosate explains, this is a real scientific dispute that is not yet settled. It is also a textbook case of IARC judging hazard while the EPA judged risk.
So California's law was set to require a warning saying, flatly, that glyphosate is known to cause cancer. The EPA had concluded the opposite.
The First Amendment challenge
Agricultural groups, led by the National Association of Wheat Growers, sued. Their argument was not mainly about the science. It was about compelled speech: the government cannot force a business to state a disputed claim as settled fact. Requiring sellers to warn that glyphosate is "known to cause cancer" was false or misleading, they argued, given the weight of scientific opinion. That, they said, violated the First Amendment.
Federal courts agreed. They blocked enforcement of the glyphosate cancer-warning rule. The courts found the required wording was misleading and could not be compelled. Later rulings, including at the appeals level, upheld that reasoning.
What the case reveals
The glyphosate dispute matters beyond glyphosate itself.
- It shows what happens when a hazard-based listing (IARC's) collides with a risk-based conclusion (the EPA's). The clash can end up in court.
- It shows a limit on Prop 65. A warning can be legally blocked if forcing it would state disputed science as certainty.
- It underlines that "known to cause cancer" on a label is a strong claim. Courts look closely at whether it is accurate.
The bottom line
California listed glyphosate under Prop 65 based on IARC's hazard classification. Federal courts then blocked the cancer warning on free-speech grounds, finding it could be false or misleading given other agencies' conclusions. Whatever one thinks of glyphosate's risks, the case is a vivid reminder that hazard classifications and real-world risk conclusions can diverge. Sometimes they part company sharply enough to need a judge.
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Common questions
Why was glyphosate listed under Prop 65?
Because IARC classified it as 'probably carcinogenic to humans' in 2015, which triggered listing through California's IARC-linked (Labor Code) mechanism, effective 2017.
Why did courts block the warning?
Groups challenged the required warning on First Amendment grounds, arguing that forcing sellers to state glyphosate is 'known to cause cancer' was misleading given that the EPA and others concluded it is not likely carcinogenic. Courts agreed the mandated warning could be false or misleading.
Is glyphosate still on the Prop 65 list?
The listing has been the subject of ongoing litigation, but courts have blocked enforcement of the cancer-warning requirement. As a result, the warning is not being enforced.
What does this dispute show?
It highlights the tension when a hazard-based listing (IARC's) conflicts with other agencies' risk-based conclusions (the EPA's), and how that conflict can play out in court.
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Last updated: 2026-08-11Next planned review: 2028-07-05
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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