New study adds to evidence that glyphosate weed killer can cause cancer

A new long-term animal study of the widely used weed killer glyphosate find fresh evidence that the herbicide, introduced by Monsanto in the 1970s, causes multiple types of cancer, and may do so at doses considered safe by regulators.

The results of the two-year study, which were published June 10 in the journal Environmental Health, add to an ongoing global debate over the safety of the pesticide, which is commonly used by farmers to kill weeds in fields and pastures. The chemical is also used widely to manage weeds on golf courses, in parks and playgrounds, and in forestry management.

“Our study provides solid and independent scientific evidence of the carcinogenicity of glyphosate and glyphosate-based herbicides,” said Daniele Mandrioli, director of the Cesare Maltoni Cancer Research Center of the Ramazzini Institute in Italy. Mandrioli is the principal investigator for the study.

Weighing the science

Germany-based Bayer, which bought Monsanto in 2018 and inherited the glyphosate-based Roundup brand and other glyphosate-based herbicides, is currently embroiled in litigation in the United States brought by tens of thousands of people who allege that exposure to the company’s glyphosate herbicides caused them to develop non-Hodgkin lymphoma, a type of blood cancer.

One trial is underway now in Missouri, not far from Monsanto’s former headquarters. The company has already paid out billions of dollars in settlements and jury awards, and the new study comes as Bayer warns that if it cannot put an end to the litigation, it may shut down its glyphosate operations in the US, and possibly place its Monsanto businesses in bankruptcy.

Just as Monsanto always did, Bayer maintains that its glyphosate herbicides are not carcinogenic, and says the weight of scientific research backs that position.

But many studies have found cancer connections, and in 2015, the World Health Organization’s International Agency for Research on Cancer classified glyphosate as a probable human carcinogen.

Other studies have found a range of harmful effects from glyphosate exposure. A 2022 study, for example, found that glyphosate can have neurotoxic effects at doses lower than levels considered safe by regulatory agencies. And a 2023 study linked childhood exposure to glyphosate to liver inflammation and metabolic disorders.

Last month, a review of 15 years of published studies confirmed that human exposures to glyphosate herbicides have been associated with numerous adverse health outcomes that not only include cancers, liver and metabolic problems, but also reproductive and endocrine-system effects, and disruption of the microbiome, among others.

In response to the study, Bayer issued a statement accusing the Ramazzini Institute of having “a long history of making misleading claims about the safety of various products.”

“The US [Environmental Protection Agency] determined past Ramazzini studies did not meet the criteria of scientific quality for consideration in the registration review process,” Bayer said. “In addition, the EPA has retracted risk assessments that relied on Ramazzini Institute data regarding other substances and {the European Food Safety Authority] has publicly expressed its frustrations with the institute’s lack of transparency after it claimed to find adverse health effects caused by artificial sweeteners.”

Dosing drinking water

In the new study from Italy, the researchers looked at the impacts of glyphosate alone as well as the impacts of two types of commercial glyphosate-based formulations that are used in Europe and the United States. They administered the weed killers to rats via drinking water beginning in prenatal life, at doses of 0.5, 5, and 50 mg/kg body weight per day for two years and compared them to control groups that did not receive the pesticide doses More than 1,000 rats were part of the study.

The European Union’s acceptable daily intake for glyphosate is set at 0.5 mg/kg body weight/day, and the EU’s “no-observed adverse effect level” (NOAEL) is 50 mg/kg body weight/day. In all three treated groups, increased incidences of benign and malignant tumors were seen in multiple tissue sites compared to rats in a control group that did not receive any of the pesticide. The researchers said the study results indicate the glyphosate herbicides can cause a range of cancers, including leukemia.

“Our results indicate that, while glyphosate alone is capable of causing a number of benign and malignant tumors, [glyphosate-based herbicide] co-formulants may enhance the carcinogenicity of glyphosate, particularly in the case of leukemia,” the study states.

Most of the tumors that developed are considered rare in Sprague Dawley rats, the type used in the study, the scientists said. They noted that roughly 40% of the leukemia deaths seen in the treated groups occurred early in the animals’ lives, though increased early deaths were also seen in connection with other types of tumors. Long-term studies on rats are commonly used to predict is a substance is a human carcinogen. Cancer links to asbestos, benzene and many other substances were first detected in rodent studies.

The research is part of a “Global Glyphosate Study” led by the Cesare Maltoni Cancer Research Center of the Ramazzini Institute in Italy and involves scientists from Boston College, George Mason University, King’s College London, Icahn School of Medicine at Mount Sinai, Scientific Centre of Monaco, University of Bologna, the Institute of Agricultural Biology and Biotechnology of the Italian National Research Council the Italian National Institute of Health, and the National Food Safety Committee of the Italian Ministry of Health.

In 2022, the group published prior findings showing adverse effects of glyphosate at doses that are currently considered safe.

“The findings from this carefully conducted study, and especially the observation that prenatal exposures of infant rats to glyphosate during pregnancy increase incidence and mortality from early-life leukemia, is a powerful reminder of human infants’ great vulnerability to toxic chemicals,” Philip Landrigan, who participated in the study and directs the Program for Global Public Health and the Common Good at Boston College, said in a statement.

(Featured image by Getty Images for Unsplash +.)