Do glyphosate-based weedkillers, such as Roundup, cause cancer?
It’s the one question regulators dodge, courts debate, and scientists keep returning to.
Ever since glyphosate—the active ingredient in Roundup and the world’s most widely used weedkiller—became a household name, one worry has lingered above all others: can it cause cancer?
If you ask regulators, you’ll hear one story: glyphosate is “safe when used as instructed.” If you ask the World Health Organization’s cancer experts, you’ll hear another: glyphosate is “probably carcinogenic to humans.” And if you ask the thousands of people who have taken Bayer (the maker of Roundup) to court, claiming the chemical gave them non-Hodgkin lymphoma, you’ll get yet another answer.
How can there be so much disagreement? Is it politics? Is it science? Is it the way the studies are designed? Or is it simply that no one wants to admit what the evidence is really pointing toward?
The truth is rarely a neat yes or no. Cancer isn’t caused by one single factor—it’s a tangled web of exposures, genetics, lifestyle, and luck. But here’s the catch: when a chemical shows up everywhere—from farm fields to breakfast cereals, from waterways to backyard gardens—people are right to demand clarity.
So what does the latest research actually tell us? And why does it matter who we choose to believe?
What the Science Really Shows
So let’s strip away the headlines and the industry spin for a moment. What happens when we look at the evidence itself?
Scientists have been asking the cancer question for decades. Some studies say yes, some say no, and the result is a confusing patchwork that leaves the public none the wiser. But in 2023, researchers pulled together a massive systematic review of glyphosate and cancer—2,537 papers screened, 175 studies included—that tried to cut through the noise.
Instead of asking only “Does glyphosate cause cancer in humans?”—a question that can take decades to answer definitively—they asked: “Does glyphosate act like a carcinogen in the body?”
That’s a subtle but important shift. Why? Because carcinogens tend to share tell-tale traits. They damage DNA. They create oxidative stress. They interfere with hormones. They disrupt the immune system. They mess with the way cells grow and die.
These are called the 10 Key Characteristics of Carcinogens. Think of them as the biological fingerprints of cancer-causing chemicals.
When glyphosate and glyphosate-based products like Roundup were tested against those characteristics, the evidence lined up alarmingly well. DNA damage? Check. Oxidative stress? Check. Disrupted cell signaling and immune impacts? Check.
The authors concluded that the evidence “strengthens the mechanistic case that glyphosate is a probable human carcinogen.”
In other words: glyphosate is showing the same biological red flags we see in known cancer-causing substances.
What Human Studies Are Telling Us
Mechanisms are one thing. But people don’t live in petri dishes, and what most of us want to know is: are humans actually getting cancer from glyphosate exposure?
This is where the evidence gets tangled. Some of the biggest studies—like the U.S. Agricultural Health Study—tracked thousands of farmers and pesticide applicators over decades. Its most recent update didn’t show a clear overall link between glyphosate and cancer. That sounds reassuring, right?
But here’s the twist: when researchers re-analyzed the same dataset with a focus on non-Hodgkin lymphoma (NHL), the picture started to shift. And when independent scientists pooled multiple studies together in a 2019 meta-analysis pooling multiple human studies, the results were striking: people with the highest exposures to glyphosate had a compellingly higher risk of NHL.
Why non-Hodgkin lymphoma? Because it’s a cancer of the immune system, and glyphosate seems to hit immune pathways hard. That lines up with the mechanistic evidence: DNA damage, oxidative stress, disrupted immune signaling. The lab science and the human findings are telling a consistent story.
Of course, not every study shows the same thing. Exposure levels, recall bias, and study design can all muddy the water. Regulators often lean on the “no overall association” message from certain large cohorts. But what happens if you focus on the right question—not “does glyphosate cause all cancers in all people,” but “does it raise the risk of specific cancers in those most exposed”?
That’s where the signal is hardest to ignore.
And the courts have noticed. In the U.S., juries have awarded billions in damages to plaintiffs who developed non-Hodgkin lymphoma after years of Roundup use. These aren’t just scientific debates in ivory towers—they’re life-altering cases playing out in real time.
Why Are the Answers So Different?
If the science is showing red flags, why are regulators still giving glyphosate the green light? Why does one agency call it “probably carcinogenic” while another says “no cause for concern”?
It comes down to the lens you choose to look through.
The World Health Organization’s cancer agency (IARC) takes a hazard-based approach: can this substance cause cancer under some conditions? Their answer in 2015 was clear—glyphosate is “probably carcinogenic to humans”, based on animal evidence, mechanistic studies, and limited human data.
Meanwhile, regulators like the U.S. EPA or the European Food Safety Authority (EFSA) ask a different question: under approved uses and average exposures, does glyphosate pose an unacceptable risk? That narrower frame leads them to say “no,” or at least “not enough evidence to change course.”
And then there’s the courtroom. Juries in the United States have looked at the same body of evidence—and the lived experiences of plaintiffs with non-Hodgkin lymphoma—and concluded that glyphosate was a likely cause. Their rulings have forced Bayer to pay out billions in damages.
So, we end up with three different arenas—scientific bodies, regulators, and courts—all looking at overlapping evidence but drawing different lines in the sand.
Is that science working as intended? Or is it a sign that economics, liability, and politics play as much of a role as the data itself?
Glyphosate on Paper vs. Roundup in Reality
Here’s something most people don’t realise: when regulators assess glyphosate, they often test the active ingredient in isolation. But that’s not what farmers, gardeners, or councils are spraying. Out in the real world, it’s glyphosate mixed with a cocktail of surfactants and adjuvants designed to make it stick to plants, penetrate their cells, and do its job more effectively.
Take Roundup as the prime example. It’s glyphosate plus extra chemicals that change how the product behaves. Some of these additives make glyphosate more mobile, some help it get into the plant faster, and some might even enhance its toxicity to human cells.
So when a regulator says “glyphosate on its own doesn’t look too bad,” should that reassure us? Or is it a sleight of hand—assessing the ingredient on paper while people are exposed to the formulation in practice?
Independent scientists have raised this exact concern. Several studies show that Roundup formulations are often more toxic to cells than glyphosate alone. That means risk assessments focused only on the active ingredient could be underestimating the danger.
It raises an uncomfortable question: if what’s in the bottle is more harmful than the ingredient being tested, then are our current safety assessments worth the paper they’re written on?
So Where Does That Leave Us?
So, does glyphosate cause cancer? The honest answer is messy—but it isn’t meaningless.
We know glyphosate shows multiple hallmarks of a carcinogen in the lab. We know human studies, especially at higher exposures, point toward non-Hodgkin lymphoma. We know courts have sided with the victims again and again. And we know that real-world products like Roundup are more complex—and often more toxic—than glyphosate alone.
Yet regulators still say: “safe when used properly.” Safe for who? The farm worker spraying fields without full protective gear? The council contractor handling roadside weeds? The parent pulling weeds around the backyard veggie patch? Or the child eating cereal made from crops sprayed just before harvest?
When so much uncertainty remains—and when the stakes are life and death—shouldn’t the burden of proof fall on the chemical, not the people exposed to it?
Maybe the better question isn’t “Does glyphosate definitely cause cancer?” but rather: “With this much evidence stacking up, why are we still taking the chance?”
The science is pointing in one direction. The question now is whether our regulators, our politicians, and our communities are ready to listen.
Resources & References
When it comes to glyphosate and cancer, the debate isn’t short of research. What matters is how you weigh it, and whether you see the red flags as warnings or dismiss them as background noise. Here are some of the key studies and assessments behind the discussion above:
Exposure to Glyphosate-Based Herbicides and Risk for Non-Hodgkin Lymphoma
Zhang L, et al. (2019)
A meta-analysis pooling data from multiple human studies. Found a compelling association between high exposures to glyphosate and non-Hodgkin lymphoma.
Mapping the Key Characteristics of Carcinogens for Glyphosate and Its Formulations
Rana I, Nguyen P, Rigutto G, Louie A, Lee J, Smith M, Zhang L. (2023)
A systematic review published in Chemosphere that screened 2,537 articles, with 175 studies included. Found that glyphosate and its formulations exhibit multiple cancer-linked biological traits, strengthening the case that glyphosate is a probable human carcinogen.
Glyphosate Use and Cancer Incidence in the Agricultural Health Study
Andreotti G, et al. (2018)
Large U.S. cohort study of pesticide applicators. Reported no overall association with glyphosate and cancer, though signals were noted for certain cancers at high exposure levels. Often cited by regulators.
IARC Monograph (2015) – Glyphosate [PDF]
The World Health Organization’s cancer research agency classified glyphosate as “probably carcinogenic to humans (Group 2A)”, based on limited human evidence, sufficient animal evidence, and strong mechanistic data.
Peer Review of the Risk Assessment of the Active Substance Glyphosate (2023)
European Food Safety Authority (EFSA)
EFSA’s conclusion published 6 July 2023 stated that the assessment of glyphosate’s impact on human, animal, and environmental health “did not identify any critical areas of concern,” although it noted data gaps/issues that couldn’t be finalised
U.S. EPA – Withdrawal of Glyphosate Interim Decision (Sept 2022)
Following the Ninth Circuit Court’s June 2022 ruling that vacated EPA’s human-health assessment for glyphosate, the agency withdrew all remaining portions of its interim registration-review decision. EPA is now reworking its evaluation, with a new decision expected in the coming years.
- Court ruling: Natural Resources Defense Council v. EPA (9th Cir. No. 20-70787), opinion issued June 17 2022. Read the opinion [PDF]
- EPA action: Withdrawal of the Glyphosate Interim Registration Review Decision (Sept 21 2022). Read the release
- EPA summary page: Glyphosate overview
Related articles on nomoreglyphosate.nz
EPA Glyphosate Court Ruling
Our summary of the U.S. Ninth Circuit’s 2022 decision that forced EPA to withdraw its interim review of glyphosate. Explains what the court found wrong with EPA’s cancer assessment and why it matters for New Zealand.
Glyphosate & Food Safety Monitoring NZ
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Ghostwritten Science & Glyphosate Debate
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Bayer Verdict NZ & Roundup Warning Labels
Examines why overseas juries have awarded billions in Roundup cancer cases, while New Zealand consumers are still left without health warnings on glyphosate-based weedkillers.
Glyphosate Regulation: Who Really Sets the Standards?
A deep dive into the politics of pesticide regulation—showing how industry pressure, trade interests, and selective science shape the rules. Connects directly to the confusion over why different authorities reach such different conclusions.
MPI’s Missing Glyphosate Data: Can We Trust Their Claims?
Investigates MPI’s track record on glyphosate testing, highlighting missing or incomplete data, shifting explanations, and why that leaves the public in the dark about real exposure levels.
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