Does RoundUp cause cancer?

(Quick answer: probably not. See my update at the bottom of this post.)

For many years, environmental activists have been concerned about the herbicide glyphosate, which is the main ingredient in RoundUp®, the world's most widely-used weed killer. Since 1996, global usage of glyphosate has increased 15-fold, in part due to the widespread cultivation of "RoundUp Ready" crops, which are genetically modified to be resistant to RoundUp®. This allows farmers to use the herbicide freely, killing undesirable weeds without harming their crops.

RoundUp®'s manufacturer, Monsanto, has long claimed that glyphosate is safe, and they point to hundreds of studies that support their argument.

Nonetheless, a new study raises the question again.

First let's look briefly at another recent study. A bit more than a year ago, in November 2017, a large study in the Journal of the National Cancer Institute looked at nearly 45,000 glyphosate users (farmers and other agricultural workers who apply glyphosate to crops). These "users" have a much higher exposure to RoundUp® than ordinary people. That study concluded:

"no association was apparent between glyphosate and any solid tumors or lymphoid malignancies overall, including NHL [non-Hodgkin lymphoma]."

They did find, though, that there was a trend–not quite significant–towards an increased risk for one type of leukemia, AML. This trend appeared in users who had the highest exposure to RoundUp®.

In the new study, by a group of scientists from UC Berkeley, Mount Sinai School of Medicine, and the University of Washington, the authors (L. Zhang et al.) decided to focus exclusively on people with the highest exposures to glyphosate. They point out that including people with low exposure, who might have no increased risk of cancer, tends to dilute risk estimates. Statistically speaking, this is undeniably correct, but it also means that their results may only apply to people with high exposures, and not to ordinary consumers.

The punchline from the new study: people with the highest exposure to glyphosate had a 41% higher risk of non-Hodgkin lymphoma.

One caveat to this finding is that it's a meta-analysis, meaning the authors did not collect any new data. Instead, they merged the results from six earlier studies including over 65,000 people, and they focused on those with the highest exposure levels.

Meta-analyses can be prone to cherry-picking; that is, picking the studies that tend to support your hypothesis. However, I couldn't find any sign of that here. The authors include a frank assessment of all the limitations of their study, and they also point out that multiple previous studies had similar findings, although most found smaller increases in relative risk. In the end, they conclude:

"The overall evidence from human, animal, and mechanistic studies presented here supports a compelling link between exposures to GBHs [glyphosate-based herbicides] and increased risk for NHL."

A couple more caveats are important. First, this finding is all about relative risk. Non-Hodgkin lymphoma is one of the most common cancers in the U.S. and Europe, but the lifetime risk for most people, according to the American Cancer Society, is just 1 in 42 (2.4%) for men and 1 in 54 (1.9%) for women. A 41% increase in relative risk increases those numbers to 3.4% (men) and 2.6% (women).

Second, this higher risk only applies to people with very high exposure to glyphosate: primarily people who work in agriculture and apply RoundUp® to crops. Ordinary consumers (including people who eat "Roundup Ready" crops) have a far, far lower exposure, and dozens of studies have failed to show any increased risk of cancer for consumers. For most of us, then, this new study should not cause much concern, but for agricultural workers, it does raise a warning flag.

[Update 18 Feb 7:45pm] After I posted this article, the scientists at the Genetic Literacy Project pointed me to Geoffrey Kabat's piece about the Zhang et al. study. Kabat did a deep dive into the studies that Zhang et al.'s work is based on and uncovered a critical flaw in the study, one that I hadn't found. More than half of the "weight" of the meta-analysis by Zhang, and by far the largest number of cancer cases, come from a single study by Andreotti et al. published in 2018. That study reported risks for 4 different time points: 5, 10, 15, and 20 years. It turns out, as Kabat reports, that only the 20-year period showed any increase in risk of cancer. The relative risks of cancer at 5, 10, and 15 years were actually lower in the group exposed to glyphosate, and yet Zhang et al. didn't mention this fact.

Now, no one thinks that glyphosate lowers the risk of cancer, but Zhang et al. did not report that they had cherry-picked in this way. At a minimum, they should have reported what their findings would be if they used the other time periods. I suspect that they'd have found no increased risk of cancer–but this wouldn't make for such a catchy headline. This omission on their part is a serious flaw, indicating that they (and their results) might have been unscientifically biased.

The bottom line: even in those with very high exposures to glyphosate, the evidence that it causes any type of cancer is very weak. And for ordinary consumers, there's nothing to worry about.