Proposition 65 Takes Another Hit: The Monsanto Preemption Ruling

The Monsanto preemption ruling combines with four recent successful First Amendment challenges to create a potent two-prong attack on Proposition 65.

The Monsanto Ruling

The U.S. Supreme Court issued its ruling last week in the much-anticipated Monsanto Co. v. Durnell case. In a 7–2 decision, the court held that states are preempted from requiring a cancer warning on glyphosate-based herbicides where the Environmental Protection Agency (EPA) reviewed the chemical and determined that no warning was required under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Glyphosate is the main ingredient in Roundup, manufactured by Bayer, Monsanto’s parent company.

The Key Is "Uniformity"

FIFRA requires pesticide manufacturers to register their products with the EPA, which decides whether the label must carry a warning. FIFRA’s “uniformity” provision is unambiguous: states may not “impose or continue in effect any requirements for labeling or packaging in addition to or different from those required under” 7 U.S.C. § 136v(b). The EPA has consistently approved Roundup’s label without a cancer warning because it has concluded glyphosate is not likely carcinogenic in humans — a deliberate federal judgment. A state jury that says Monsanto should have warned anyway isn’t just second-guessing Monsanto. It’s second-guessing the EPA.

Many Prop 65 Chemicals Subject to FIFRA Are Vulnerable to a Preemption Challenge

There are dozens of pesticides, insecticides, and fumigants (FIFRA covers all three) on the Prop 65 list that the EPA has reviewed and found pose no human health risk for either of Prop 65’s endpoints — cancer or reproductive harm. Each is now a candidate for a preemption challenge.

Take malathion. In the 1970s, a fruit fly infestation threatened California agriculture, and the state sprayed the insecticide from low-flying helicopters over the crops, and over everyone else. California listed malathion as a known carcinogen in 2016. But the EPA reviewed malathion’s safety profile as recently as 2024 and found no human health risk. No grounds for a warning are now grounds for a preemption challenge.

What About Prop 65 Chemicals Not Covered by FIFRA?

Monsanto’s preemption analysis isn’t limited to pesticides. The court pointed to comparable “uniformity” labeling regimes governing other industries, including over-the-counter drugs, meat and poultry products, cosmetics, and food nutrition labeling — any scheme where an agency must ensure a uniform national standard that a state warning would frustrate or weaken. This matters for Prop 65, which already yields to federal law where “an exposure [is one] for which federal law governs warning in a manner that preempts state authority.” So Prop 65 preemption isn’t limited to express preemption — it extends wherever a federal labeling scheme occupies the field. For any Prop 65 chemical, ask: Is it regulated by a federal agency, under what statute, and does that statute include a uniformity provision like FIFRA’s? Where the answer points to exclusive federal authority, Monsanto just handed defendants a stronger weapon.

What About FDA-Regulated Chemicals?

FDA preemption is narrower, but it has already succeeded in California — for the same reason: uniformity. Prop 65 exempts prescription drugs but says nothing about OTC drugs. In a published ruling holding Prop 65 preempted for a generic OTC medication, the court found the “duty of sameness” preempted Prop 65: a generic drug must match its brand label, and a state warning would defeat that uniformity. And earlier this year, the Los Angeles Superior Court granted a motion for summary judgment on the same issue for a brand-name OTC drug, agreeing that the FDA’s exclusive authority over uniform drug labeling preempts a conflicting Prop 65 warning. Both rulings track Monsanto: when Congress vests an agency with authority to ensure uniform labeling, states can’t frustrate it with their own warnings. Prop 65 must yield.

Successful First Amendment Attacks on Prop 65

Monsanto strengthens the preemption defense, but it’s only half of a two-pronged attack. Over the last three years, federal courts have permanently enjoined California from enforcing Prop 65 cancer warnings for four chemicals on First Amendment grounds: the government can compel commercial speech — like a warning — only if it is “purely factual and uncontroversial.” When the International Agency for Research on Cancer (IARC) says “carcinogenic,” but the EPA and others disagree, a warning declaring a chemical “known to cause cancer” isn’t factual — it’s misleading. And compelling a misleading statement violates the First Amendment.

The four chemicals no longer enforceable under Prop 65 on First Amendment grounds:

• Glyphosate — National Association of Wheat Growers v. Bonta, 85 F.4th 1263 (9th Cir. 2023)
• Acrylamide (in food) — California Chamber of Commerce v. Bonta, 781 F. Supp. 3d 1071 (E.D. Cal. 2025)
• Titanium Dioxide (in cosmetics and personal care products) — The Personal Care Products Council v. Bonta, 799 F. Supp. 3d 1075 (E.D. Cal. 2025)
• Cocamide DEA — Personal Care Products Council v. Bonta, No. 2:26-cv-00682 (E.D. Cal. 2026)

Four chemicals. Four courts. Three years. And those are only the ones challenged so far. Hundreds more Prop 65 chemicals were listed on thin or disputed evidence that neither the EPA nor the FDA found posed a risk to human health.

What It All Means

Gazing into the crystal ball (which, naturally, carries a Prop 65 warning sticker): Prop 65 is facing the most serious legal challenges in its 40-plus-year history — First Amendment challenges in federal court, preemption challenges at the Supreme Court, and now the coast is clear to launch both at once for an expanding list of chemicals. Expect more challenges built on this two-pronged attack.

California’s state courts have largely sat out constitutional challenges to Prop 65, leaving that fight to federal courts. Monsanto, combined with the four First Amendment injunctions, may finally give state courts the cover to follow suit — and the Office of Environmental Health Hazard Assessment (OEHHA), the agency that lists Prop 65 chemicals, now has to ask whether each listing is constitutional. No pressure.

Dennis Raglin leads Carlton Fields’ Proposition 65 and Consumer Chemical Regulations group. For more information or to discuss how these developments may affect your products or compliance program, contact Dennis at [email protected].

This alert is provided for informational purposes only and does not constitute legal advice.