Glyphosate has been in the new recently because of the recent court case in which a jury awarded DeWayne Johnson $289 million dollars from Monsanto for glyphosate allegedly causing his lymphoma. This, of course, is used as support for those who claim that glyphosate is unsafe, and also for anti-GMO rhetoric as some GMOs are engineered to be tolerant of glyphosate to facilitate its use as an herbicide.

I wrote about this case last year, but now it is back as Johnson prevailed in appeal. I also wrote about the science behind glyphosate in 2014. There have been some significant studies since then, so in light of the recent attention the herbicide is getting, an update is warranted.

There are two basic safety issues with glyphosate – exposure in the general population, mostly from consuming food that was grown using glyphosate, and agricultural exposure. The first question is easier to answer.

A 2017 review found:

Two complementary exposure assessments, human-biomonitoring and food-residues-monitoring, suggests that actual exposure levels are below these reference values and do not represent a public concern.

This has been the consistent finding of studies for decades. The level of exposure for the general public, even with the increased agricultural use in the last 20 years, is well below safety limits. Why, then, did the IARC designate glyphosate as a probable carcinogen? They famously left out the most updated data to which they had access. Also, their designation is an outlier. Other reviewers did not reach the same conclusion.

The data that was excluded from the WHO’s assessment is the 2018 Agricultural Health Study. This study, which included 50,000 people, found:

In this large, prospective cohort study, no association was apparent between glyphosate and any solid tumors or lymphoid malignancies overall, including NHL and its subtypes. There was some evidence of increased risk of AML among the highest exposed group that requires confirmation.

This large and fairly definitive study, especially when put in the context of all the other studies, puts to bed the notion that there is any significant risk from general exposure to glyphosate. As we always point out – there is no such thing as zero risk. But we can use evidence to set probable limits on how large a remaining risk might be. The health risk of general exposure to glyphosate is anywhere from nothing to negligible – it has vanished into the background risk of just being alive.

But this study also brings us to the second consideration – the risk of agricultural workers who might be exposed to much higher levels than the general population. Toxicity is always about dose, and so it is plausible while there is no risk to the general population, there is a risk from much higher exposure.

As an aside, this is why the frequent challenge to those simply pointing out the science on the safety of glyphosate to drink a glass of the stuff, “if you think it’s so safe,” is absurd. There are countless things that we consume and are exposed to all the time that I would not drink an entire glass of. That is not the threshold for safety.

There have been studies that show a small association between agricultural exposure to glyphosate and NHL (Non-Hodgkin Lymphoma). This association shows up in a recent meta-analysis, but only for the highest exposure group. They found a relative risk increase of 41%, but this translates to an absolute risk increase of 8 per 100,000 or 0.008.

This study has also been criticized for the methods of meta-analysis used. For example, they had to determine how to combine very different data that uses different definitions of exposure. Some scientists have argued that the data does not support the conclusions.

A recent study not included in that meta-analysis, however, which was a large analysis looking at many pesticides and lymphoma outcomes, found:

During follow-up, 2430 NHL cases were diagnosed in 316 270 farmers accruing 3 574 815 person-years under risk. Most meta-HRs suggested no association. Moderately elevated meta-HRs were seen for: NHL and ever use of terbufos (meta-HR = 1.18, 95% CI: 1.00-1.39); chronic lymphocytic leukaemia/small lymphocytic lymphoma and deltamethrin (1.48, 1.06-2.07); and diffuse large B-cell lymphoma and glyphosate (1.36, 1.00-1.85); as well as inverse associations of NHL with the broader groups of organochlorine insecticides (0.86, 0.74-0.99) and phenoxy herbicides (0.81, 0.67-0.98), but not with active ingredients within these groups, after adjusting for exposure to other pesticides.

This evidence is fairly weak. The confidence interval for glyphosate includes 1.00. Also, there was only a risk found for B-cell lymphoma, not NHL. Inconsistencies like that suggest the association may not be real.

At present from all existing evidence I think the best conclusion is that there may be a small correlation between the highest levels of agricultural exposure of glyphosate and some types of lymphoma, but the existing evidence is not of sufficient quality and consistency for a final determination. It does warrant further study, but also indicates that any potential risk is likely small. In the meantime it certainly makes sense for agricultural workers who deal with pesticides of any type to practice good safety – using protection suits and breathing masks, for example.

There is another element of the DeWayne Johnson case that should be mentioned. The delay between his exposure and diagnosis was too short, it is argued, to have been that actual cause of his NHL. The legal case itself is complex, and I do not intend to comment on the legal issues here. My only intent is to the put the science into context.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.