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A study published in Nature Medicine looks at pesticide exposure and early-onset colon and rectal cancer risk.
Prof Oliver Jones, Professor of Chemistry, RMIT University, said:
“The first version of this paper was a preprint in 2024 (https://assets-eu.researchsquare.com/files/rs-4528579/v1/2475e2b1-67b8-4ffd-a1b6-92fd95acba0f.pdf), although the author lists of the preprint and the final paper differ.”While the authors have taken an interesting approach here, there are a few points to keep in mind when assessing the results.
“First, exposure was estimated using county-level pesticide use data. This assumes that local agricultural use equated to exposure for all people in that county, which it may well not have. We don’t know how much picloram individuals were actually exposed to.
“Secondly, what has been reported is an association, not a direct relationship; it does not prove causation. One might find a statistical association between cancer and any number of factors (as indeed the authors did), but that does not automatically prove that one caused the other. It just gives you an idea of what might be worth exploring further. It is also unclear how picloram could cause cancer. Picloram mimics the action of a plant growth hormone called auxin. This leads to uncontrolled, disorganised growth and ultimately the death of the plant. Auxin mimics have no effects in animals, and picloram has shown low toxicity in animal tests. The US Environmental Protection Agency and the International Agency for Research on Cancer (IARC) say there is a lack of evidence of picloram being carcinogenic in humans.
“Picloram is a professional-use herbicide typically used fro indistriual weed control in places like railway sidings or motorway verges. It is not generally sold for home garden use. So the general publiuc are unlikely to be directly exposed to it.”
“Cancer occurrence rates for young people remain low overall. Reasons for the general increase in cancer detection in younger people are not clear, but are likely a reflection of other risk factors, such as lifestyle (poor diet, less exercise, etc.), as well as more widespread testing for the disease. For example, the screening age for colorectal cancer in the US was reduced to 45 in 2021. So, in short, I don’t think people need to worry about the results in this paper.”
Dr Robin Mesnage, Visiting Research Fellow, King’s College London (KCL), said:
“I enjoyed reading the study; it is innovative work with careful analyses. The scientific value is clear. Using DNA methylation as a proxy for past exposures is an interesting and innovative approach. The authors also attempt to replicate their findings across different cohorts and datasets, which is essential.
“However, it is one thing to conduct a brilliant scientific study, but it is quite another for it to carry sufficient weight to change the risk assessment of pesticides.
“The value for regulatory evaluations is less clear. This study shows associations, not causation. This is a key point. The signal for picloram is interesting, but exposure was not directly measured in the subjects. Therefore, we cannot conclude that picloram causes these cancers.
“Furthermore, we must consider the nature of the substance itself. Picloram has not been found to have carcinogenic properties in regulatory tests. Historically, however, it was often contaminated with carcinogenic substances such as hexachlorobenzene. It is possible that any observed effects were due to this contamination rather than picloram itself, a problem which should have largely been resolved in modern manufacturing. It is always important to remember that real-world exposures involve not just the active ingredient, but also co-formulants and potential contaminants.
“Overall, this is a fascinating and well-done study that raises a plausible hypothesis. Nevertheless, more work is needed, specifically with direct exposure data and longitudinal designs, before making causal claims.”
‘Epigenetic fingerprints link early-onset colon and rectal cancer to pesticide exposure’ by Silvana C. E. Maas et al. was published in Nature Medicine at 10:00 UK time on Tuesday 21 April 2026.
DOI: 10.1038/s41591-026-04342-5
Declared interests
Prof Oliver Jones: I conduct research on environmental pollutants. I have in the past received research funding from various water utilities and the Environment Protection Authority Victoria (though not for research on herbicides).
Dr Robin Mesnage: “I have served as an expert in some litigations related to the pesticide glyphosate. Otherwise there is absolutely no COI related to anything related to picloram.”