When two well-respected agencies look at the same chemical and reach opposite conclusions, the obvious question is: who is right? In Roundup litigation, the more useful question is: why did they reach different conclusions? The answer comes down to how each agency weighs three kinds of evidence — and to whose hand has been on the scale.
The IARC Classification: Group 2A "Probably Carcinogenic"
In March 2015, the International Agency for Research on Cancer — the cancer research arm of the World Health Organization — convened a working group of 17 scientists from 11 countries to evaluate the carcinogenicity of glyphosate. Their conclusion: glyphosate is a Group 2A carcinogen, meaning it is "probably carcinogenic to humans."
IARC's Group 2A classification rests on three pillars of evidence:
- Limited evidence in humans (epidemiology) of an association between glyphosate exposure and Non-Hodgkin Lymphoma in particular.
- Sufficient evidence in animals (in vivo carcinogenicity studies) showing glyphosate causes cancer in laboratory animals.
- Strong mechanistic evidence — glyphosate has been shown to induce oxidative stress and genotoxicity (DNA damage) in human cells, which is the biological mechanism by which carcinogens act.
The IARC monograph (Volume 112) is publicly available and was peer-reviewed. The working group's deliberations and conflict-of-interest disclosures are part of the public record.
The EPA's Position: "Not Likely To Be Carcinogenic"
The U.S. Environmental Protection Agency, by contrast, has concluded that glyphosate is "not likely to be carcinogenic to humans" at doses relevant to human exposure. This is the conclusion Monsanto and Bayer point to most often when defending Roundup.
The EPA's analysis draws heavily on registrant-submitted data — meaning data submitted by manufacturers, including Monsanto, as part of the pesticide registration process — and on a different weight-of-evidence framework that relies more on chronic rodent bioassays at high doses than IARC's emphasis on the full mechanistic and epidemiological picture.
Side-By-Side: How The Frameworks Differ
| Question | IARC | U.S. EPA |
|---|---|---|
| What is the agency's mandate? | Identify cancer hazards (whether a substance can cause cancer under any circumstances) | Assess regulatory risk (whether real-world exposures pose unacceptable risk) |
| What evidence does it weigh? | Published peer-reviewed studies; epidemiology + animal + mechanistic; only public data | Heavy reliance on industry-submitted regulatory studies, including unpublished data |
| How transparent is the process? | Working-group composition and conflicts of interest disclosed publicly; deliberations published | Some industry data treated as confidential business information; less complete public record |
| Conclusion on glyphosate | Group 2A — probably carcinogenic to humans | Not likely to be carcinogenic to humans at relevant exposures |
The Mechanistic Evidence: Why I Find IARC More Persuasive
I am a physician and a lawyer. When I read the underlying studies, I find IARC's reasoning more persuasive on three specific points.
1. The genotoxicity signal is consistent.
Glyphosate produces oxidative stress and DNA strand breaks across multiple cell types and species, including human peripheral blood lymphocytes — the precursor cells of Non-Hodgkin Lymphoma. That is the mechanistic chain you would expect to find under a substance that drives B-cell lymphomas. The evidence is reproduced across independent laboratories.
2. The animal evidence shows tumors.
Multiple long-term carcinogenicity studies in rodents have shown statistically significant increases in tumors associated with glyphosate exposure. The IARC working group concluded the animal evidence was sufficient on its own. The EPA reached a different conclusion in part by reweighting which studies counted and at what doses.
3. The agricultural cohort epidemiology is consistent in direction.
Multiple peer-reviewed cohort and case-control studies of farmers, applicators, and other heavily exposed workers have found elevated rates of Non-Hodgkin Lymphoma associated with cumulative glyphosate exposure. Effect sizes in the most-exposed workers cluster around a meaningful increased risk in pooled analyses, including the 2019 Zhang et al. meta-analysis published in Mutation Research.
The Monsanto Papers In The EPA Picture
Internal Monsanto documents released through Roundup litigation — the so-called Monsanto Papers — reveal sustained and at times improper coordination between Monsanto employees and EPA officials reviewing glyphosate. Internal communications discuss how a particular EPA official could "kill" or delay a separate federal toxicology review of glyphosate. The same documents reflect Monsanto's longstanding effort to position the EPA's "not likely to be carcinogenic" conclusion as the definitive word on glyphosate, while simultaneously working to ensure that conclusion remained EPA's position.
This does not mean every EPA scientist working on glyphosate was compromised. It does mean that when Monsanto says, "the EPA agrees with us," the EPA's agreement is not as independent as it appears at first glance. Juries are entitled to weigh that.
What This Means For Your Roundup Case
The IARC vs. EPA disagreement is not just a regulatory footnote. It affects Roundup litigation in three concrete ways.
- Causation evidence. The IARC monograph — together with the underlying mechanistic, animal, and epidemiological studies — provides the scientific spine of every Roundup case. The EPA's conclusion does not foreclose a jury's right to weigh the science independently.
- Cross-examination of defense experts. Defense experts often anchor their testimony on the EPA's conclusion. The Monsanto Papers' showing of EPA-Monsanto coordination provides a direct line of cross-examination on whether that conclusion was independently reached.
- Federal preemption defense. Monsanto has argued that the EPA's approval of Roundup's labeling preempts state failure-to-warn claims. The U.S. Supreme Court declined to take up that defense, leaving plaintiffs free to pursue state-law claims grounded in the full evidentiary record — including IARC's classification.
Bottom Line
Two agencies looked at glyphosate and reached different conclusions. The IARC concluded glyphosate is probably carcinogenic to humans, basing its analysis on transparent, peer-reviewed science. The EPA concluded glyphosate is not likely carcinogenic, basing its analysis on a framework that gives substantial weight to industry-submitted data and that has documented influence from Monsanto itself. In a Roundup case, the science we rely on most heavily is the science IARC relied on — because it is the science a jury can reproduce, examine, and trust.
If you or a family member used Roundup and were diagnosed with Non-Hodgkin Lymphoma, B-cell Lymphoma, Chronic Lymphocytic Leukemia, Hairy Cell Leukemia, or Multiple Myeloma, I read the medical record personally. The free case review is exactly that: free. There is no obligation, and there is no fee unless we recover for you.