Glyphosate: The World's Most Used Herbicide Under the Microscope

In terms of volume applied to the global environment, glyphosate has no rival. More than 8.6 billion kilograms of glyphosate have been applied worldwide since its introduction in the 1970s — more than any other herbicide in history. It is the active ingredient in Roundup, and the expansion of glyphosate-resistant "Roundup Ready" crop varieties from the 1990s onward drove its use to levels that would have been unimaginable when it was first registered.

The regulatory and scientific controversy surrounding glyphosate is one of the most complex and contested in contemporary environmental health — not because the science is simple and one side is simply lying, but because the evidence is genuinely mixed, the mechanistic questions are unresolved, and the regulatory frameworks of different countries have reached opposite conclusions from reviewing overlapping datasets. Understanding the glyphosate debate requires understanding what IARC concluded, what the EPA and EFSA concluded, why they differ, and what the litigation has revealed about the quality of some of the science underlying regulatory approvals.

In March 2015, IARC's Monograph Working Group on glyphosate — a panel of 17 independent scientists from 11 countries — classified glyphosate as Group 2A: probably carcinogenic to humans.

The classification was based on: • Limited evidence in humans: Several epidemiological studies of agricultural workers found elevated rates of non-Hodgkin's lymphoma associated with glyphosate exposure, with the strongest evidence from the North American Pooled Project, which pooled data from multiple farm worker cohort studies. • Sufficient evidence in animals: Multiple rodent studies showing increased incidence of malignant tumours at multiple sites in both rats and mice. • Strong mechanistic evidence: Glyphosate induces oxidative stress and DNA damage in human and animal cells; disrupts the gut microbiome; and has effects on immune cell populations relevant to lymphoma development.

Group 2A means "probably carcinogenic" — the same category as red meat, very hot beverages, working as a hairdresser, and consumption of biomass fuel. It is a meaningful scientific statement about the strength of evidence, not a claim that glyphosate definitely causes cancer in everyone who uses it.

The EPA reviewed glyphosate and concluded in 2020 that it is "not likely to be carcinogenic to humans." The European Food Safety Authority (EFSA) reviewed glyphosate and similarly concluded that it does not meet the criteria for classification as a carcinogen. Both IARC and the regulatory agencies reviewed much of the same data — yet reached opposite conclusions. Understanding why illuminates important issues about how chemical regulation works.

The data selection difference IARC evaluates published, peer-reviewed evidence. Regulatory agencies like the EPA and EFSA review both published studies and unpublished industry-submitted studies — the raw data packages that manufacturers submit as part of registration requirements. A significant fraction of the industry-submitted studies are not publicly available and cannot be independently verified.

Critics of the regulatory conclusions argue that some industry-submitted studies that found positive carcinogenicity signals were downweighted or excluded from the regulatory analyses in ways that are not transparent.

The litigation revelations Discovery in the Monsanto/Bayer glyphosate litigation — which has resulted in over $10 billion in settlements for non-Hodgkin's lymphoma plaintiffs — produced internal Monsanto documents showing that the company had ghost-written scientific papers, influenced regulatory review processes, and strategically sought to undermine the IARC classification.

These revelations do not resolve the scientific question of whether glyphosate causes cancer in humans. But they do raise legitimate questions about the integrity of some of the industry-submitted evidence that regulatory approvals have relied on.

The glyphosate debate is legitimately contested in ways that some simpler chemical controversies are not — and communicating that complexity honestly is essential for a series committed to science.

What the evidence supports: • IARC's Group 2A classification is defensible based on the published evidence — the animal carcinogenicity evidence is real and the non-Hodgkin's lymphoma epidemiology shows a consistent signal across multiple independent studies • The regulatory agencies' conclusions are also defensible given the full dataset including industry-submitted studies showing no carcinogenicity — if those studies are taken at face value • Glyphosate disrupts the gut microbiome, induces oxidative stress, and has endocrine-disrupting properties at concentrations relevant to human exposure — these are documented effects regardless of the cancer classification question

What remains uncertain: • Whether the non-Hodgkin's lymphoma signal in agricultural workers translates to a cancer risk for the general population at much lower exposure levels • Whether the industry-submitted negative carcinogenicity studies accurately represent the compound's carcinogenic potential • What the long-term health consequences of the global ubiquitous glyphosate exposure in food and drinking water are

The consumer action question For the general population, the primary glyphosate exposure route is dietary — from conventionally grown grains, legumes, and other crops. Choosing organic foods that prohibit glyphosate use eliminates the dietary exposure. This is a precautionary step that the IARC Group 2A classification, combined with the broader mechanistic evidence, provides a reasonable scientific basis for — particularly for people with heightened concern about pesticide exposure.