The US Supreme Court heard arguments on Monday in Monsanto v. Durnell, a case that will decide whether Americans can sue pesticide makers under state law when a federal agency has approved the product’s label. The ruling could affect roughly 4,000 pending Roundup cancer claims worth billions of dollars.

Why it matters: If the court sides with Monsanto, cancer victims across the country would lose the ability to bring failure-to-warn lawsuits against pesticide companies whose labels carry EPA approval. Plaintiffs facing a 4 June opt-in deadline for a separate settlement must decide before the ruling arrives.

The plaintiff

John Durnell of Missouri used Roundup for years without gloves or a mask. He was diagnosed with non-Hodgkin’s lymphoma and sued Monsanto, now owned by Bayer, for failing to warn that the product’s active ingredient, glyphosate, could cause cancer. A Missouri jury found the company liable for failure to warn and awarded Durnell $1.25 million.

Monsanto’s argument

Monsanto told the court that the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) pre-empts state failure-to-warn claims. The company argued that because the EPA reviewed and approved Roundup’s label without requiring a cancer warning, allowing state juries to demand a different label would undermine uniform federal standards. The EPA has maintained that glyphosate is “not likely to be carcinogenic to humans.”

Durnell’s argument

Durnell’s legal team argued the case does not require Monsanto to change its federally approved label. Instead, they said, state law holds the company accountable for failing to warn consumers through other means, such as advertising, training materials, and direct communications with users.

The political split

The Trump administration filed a brief supporting Monsanto and was granted oral argument time on the company’s behalf. That position has drawn sharp criticism from the Make America Healthy Again (MAHA) movement, which organised a “People v. Poison” rally outside the courthouse on Monday. MAHA activists have warned of midterm consequences for Republicans who side with the pesticide industry.

What happens next

A ruling is expected by early July. Plaintiffs in a separate Roundup settlement must decide by 4 June whether to opt in, meaning they will make that choice before knowing whether the court will uphold or eliminate their right to sue.