Glyphosate on Trial: Landmark Rat Study Adds New Fire to Cancer Debate

Courtesy: Unsplash ph.Loren King

A sweeping new long-term study led by the Ramazzini Institute has reignited controversy over glyphosate, the world’s most widely used herbicide, linking low-dose exposures to multiple types of cancer in rats—including leukemia—at levels regulators currently deem safe for humans. Published on June 10 in the journal Environmental Health, the study provides some of the most extensive animal data to date, challenging the chemical’s safety profile and is likely to intensify ongoing litigation, policy debates, and public health concerns worldwide.

Conducted by an international team of researchers from institutions in the United States and Europe—including Boston College, King’s College London, George Mason University, and the Italian National Institute of Health—the study forms part of the Global Glyphosate Study, a broader initiative aimed at reassessing regulatory assumptions about the chemical’s long-term safety.

The researchers administered glyphosate and two glyphosate-based herbicide (GBH) formulations—Roundup BioFlow (EU) and Ranger Pro (US), both made by Bayer—to over 1,000 Sprague-Dawley rats via drinking water over a two-year period, starting prenatally. Rats were exposed to three dosage levels: 0.5, 5, and 50 mg/kg body weight per day. These span the European Union’s acceptable daily intake (0.5 mg/kg) and the “no observed adverse effect level” (NOAEL) of 50 mg/kg.

Across all treated groups, researchers observed statistically significant increases in both benign and malignant tumors in organs such as the liver, thyroid, ovaries, pancreas, adrenal glands, and most notably, haemolymphoreticular tissues—where an uptick in leukemia cases was recorded. In a particularly alarming finding, nearly half of leukemia-related deaths in the exposed groups occurred in rats under one year old, the rodent equivalent of 35–40 human years.

“Our study provides solid and independent scientific evidence of the carcinogenicity of glyphosate and glyphosate-based herbicides,” said lead investigator Daniele Mandrioli of the Ramazzini Institute, adding that the findings reinforce the 2015 classification by the International Agency for Research on Cancer (IARC), which labeled glyphosate a “probable human carcinogen”.

Science Meets Skepticism—and Litigation

Bayer, which inherited glyphosate and the Roundup product line through its 2018 acquisition of Monsanto, swiftly pushed back on the study’s findings. In a statement to AgFunderNews, the agrochemical giant dismissed the research as flawed, citing “serious methodological issues” and accusing the Ramazzini Institute of a “long history of making misleading claims.” Bayer also pointed out that U.S. regulators, including the EPA, have previously rejected Ramazzini Institute studies in other regulatory contexts due to concerns over data quality and diagnostic consistency.

Despite these rebuttals, the timing of the study couldn't be more critical. Bayer is currently embroiled in over 67,000 lawsuits in the U.S. alleging that its glyphosate-based herbicides caused users to develop non-Hodgkin lymphoma. While the company has settled nearly 100,000 cases for around $11 billion, high-stakes trials continue—including a recent Missouri case that upheld a $611 million verdict in favor of plaintiffs. Some legal experts speculate that if the legal tide doesn’t shift, Bayer may consider placing its U.S. glyphosate operations into bankruptcy protection.

A Global Regulatory Tangle

The new study lands in the middle of a fierce regulatory divide. While the European Commission reauthorized glyphosate’s use for another ten years in 2023, some EU member states, like France and Germany, are working to restrict or phase out its use domestically. In the U.S., the Environmental Protection Agency (EPA) continues to maintain that glyphosate is “unlikely to be a human carcinogen” when used as directed, though a 2022 court ruling forced the agency to withdraw and revisit its interim registration decision. The EPA is currently reevaluating the risks associated with glyphosate, particularly in light of environmental and endangered species protection laws.

Scientists involved in the Ramazzini study argue that rodent bioassays—especially those involving Sprague-Dawley rats, despite their known tendency toward spontaneous tumors—remain one of the best tools for predicting human carcinogenicity. “Many of the cancers linked to substances like asbestos, benzene, and tobacco were first detected through long-term rat studies,” said Mandrioli. The study’s co-authors also highlight that the most severe effects were observed at doses equivalent to, or below, what regulators currently deem non-harmful.

A Broader Toxicological Picture

The glyphosate controversy doesn’t end at cancer. Recent peer-reviewed studies have flagged a broader spectrum of concerns, including metabolic disorders, reproductive toxicity, microbiome disruption, and neurodevelopmental harm, even at low exposure levels. A 2022 study showed neurotoxic effects at doses below regulatory safety thresholds, while a 2023 review linked childhood glyphosate exposure to liver inflammation and metabolic changes.

Philip Landrigan, a public health expert at Boston College who participated in the study, emphasized the broader implications: “The observation that prenatal exposures to glyphosate increase early-life leukemia in rats is a powerful reminder of human infants’ great vulnerability to toxic chemicals.”

What Comes Next?

As the science evolves, so too will the battlegrounds—legal, regulatory, and public. Whether the Ramazzini Institute study leads to policy shifts remains to be seen, but it adds significant weight to concerns long raised by independent scientists, public health advocates, and communities.

For now, Bayer continues to defend the safety of its product. “For more than 50 years, leading health regulators around the world have repeatedly concluded that our glyphosate products can be used safely, and that glyphosate is not carcinogenic,” the company states on its website.

Yet with each new piece of evidence, that assertion is becoming harder to sustain without scrutiny. The global glyphosate debate—spanning scientific integrity, corporate accountability, and environmental justice—shows no signs of fading into the background. In fact, with findings like these, it may be entering its most consequential chapter yet.