Most widely used herbicide in the world is now linked to leukemia and tumors

A two year carcinogenicity study in rats reports increased benign and malignant tumor rates after continuous exposure to glyphosate or glyphosate-based herbicides

The exposure levels studied in this research are considered low or acceptable, including 0.5, 5, and 50 milligrams per kilogram of body weight per day, with exposure beginning before birth and continuing through two years.

Glyphosate is the most widely used herbicide worldwide, spread across farms, parks, and roadsides. 

Understanding herbicides – the basics

Glyphosate is the main ingredient in many popular herbicides, including the well-known product Roundup. Farmers, gardeners, and even city maintenance crews use it to kill unwanted plants, especially weeds that compete with crops.

Glyphosate works by blocking a specific enzyme that plants need to grow, which means the plant eventually withers and dies.

Since animals and humans don’t have this enzyme, glyphosate was originally considered relatively safe for people. That’s one reason it became so widely used across agriculture and landscaping over the last few decades.

Some weeds have developed resistance, meaning they no longer die when sprayed, which forces farmers to use more herbicide or stronger mixes.

Researchers have also debated whether glyphosate might harm soil health, reduce biodiversity, or pose long-term risks to people, though studies don’t all agree.

Studying the dangers of glyphosate

The study was led by the Cesare Maltoni Cancer Research Center at the Ramazzini Institute (CMCRC) in Italy, a research center with more than fifty years of experience studying the health effects of chemicals.

The institute coordinated the project and collaborated with universities and public health organizations in Europe and the United States.

The design started exposure in pregnancy and continued through 104 weeks in Sprague-Dawley rats. That approach captures sensitive windows of development that adult only studies can miss.

Animals received either pure glyphosate or two commercial formulations, Roundup BioFlow in the European Union and RangerPro in the United States, via drinking water at 0.5, 5, or 50 milligrams per kilogram per day.

These doses align with the acceptable daily intake and the no observed adverse effect level used by European regulators.

The researchers examined dozens of organs using standardized pathology and peer review. Survival, body weight, and feed and water intake looked similar to controls, so tumor differences were not explained by general sickness.

Glyphosate and leukemia

Leukemia stood out, with a significant dose-related trend and no cases in concurrent controls.

The study reported organ specific increases spanning skin, liver, thyroid, nervous system, ovaries, male mammary gland, adrenal glands, kidneys, urinary bladder, bone, and endocrine pancreas.

“Early life onset and mortality were observed for multiple tumors,” wrote Panzacchi and colleagues.

About 40 percent of leukemia deaths in treated animals occurred before 52 weeks of age, a strikingly early onset for this strain in long term bioassays. 

Patterns differed by product, suggesting that co-formulants may change toxicity compared with glyphosate alone.

That point is consistent with broader evidence showing that formulated mixtures can act differently than the nominal active ingredient.

Human vs. animal studies

In 2015 the International Research on Cancer (IARC) concluded there is “sufficient evidence of carcinogenicity in experimental animals” for glyphosate.

The Working Group reviewed animal, human, and mechanistic data, citing signals that include genotoxicity and oxidative stress.

In the U.S. Agricultural Health Study, the highest exposure group showed an elevated, though imprecise, risk of acute myeloid leukemia among pesticide applicators, even as most site specific associations were null.

That pattern keeps attention on blood cancers while acknowledging the limits of exposure reconstruction in large cohorts.

Where regulators stand

Europe’s food safety authority issued a 2023 peer review supporting renewal of glyphosate while noting data gaps in areas such as risks to some aquatic organisms and full disclosure of formulation ingredients.

Regulatory opinions can diverge because agencies weigh animal, human, and mechanistic evidence differently.

In 2022 the U.S., the Environmental Protection Agency (EPA) withdrew its interim decision after a federal appeals court vacated the human health portion and ordered the agency to redo parts of its analysis.

That reset left open questions about hazard classification and risk assumptions that future updates will need to address.

What to watch next

The prenatal start to exposure in this work matters, because early development can be more vulnerable than adulthood.

Separate research in a California birth cohort links higher childhood biomarkers of glyphosate exposure to increased odds of elevated liver enzymes and metabolic syndrome by late adolescence.

A practical question now is how low dose, long duration exposures interact with formulation ingredients across different organs.

The answer will influence how acceptable daily intake values are interpreted against animal bioassay signals and how risk managers handle glyphosate versus formulated products in food and non food uses.

The study is published in Environmental Health.

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