The world’s most famous herbicide is under fire again. Striking back against an evaluation by the United States Environmental Protection Agency (EPA) that they considered deeply flawed, several members of a scientific advisory panel on glyphosate teamed up to conduct their own meta-analysis. Their paper, published online February 10, 2019, in the journal Mutation Research, reports a 41% increase in the risk of non-Hodgkin lymphoma (NHL) associated with exposure to glyphosate (meta-relative risk [meta-RR], 1.41; 95% CI, 1.13-1.75).1

Glyphosate continues to generate controversy, as a jury in August 2018 sided with a California groundskeeper in his lawsuit against Monsanto; the groundskeeper claimed the weedkiller caused his cancer. The International Agency for Research on Cancer (IARC) classified glyphosate as “probably carcinogenic to humans” in 2015.

The EPA also recently reevaluated glyphosate’s carcinogenic potential. To help craft their assessment, the agency sought recommendations from a group of experts called the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Scientific Advisory Panel. Ultimately, after reviewing the evidence, the EPA determined that glyphosate is “not likely to be carcinogenic to humans.”2 Some members of the panel disagreed with the EPA’s process, however.

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“Because of their dissatisfaction with EPA’s report, my advisor and 2 other panel members decided to pursue related research to dig further into the issues,” wrote study coauthor Rachel Shaffer, MPH, in a blog post.3 The 3 scientists involved in the study were Luoping Zhang, PhD, of the University of California, Berkeley; Emanuela Taioli, MD, PhD, of the Icahn School of Medicine at Mount Sinai, New York; and Lianne Sheppard, PhD, of the University of Washington, Seattle.

In their meta-analysis, the researchers incorporated data from 5 case-control studies, plus data from the Agricultural Health Study (AHS), a large cohort study conducted over a period of 25 years that collected data from licensed pesticide applicators in Iowa and North Carolina. The current study is a collaborative project of the National Cancer Institute, the National Institute of Environmental Health Sciences, the EPA, and the National Institute for Occupational Safety and Health. The latest update to the AHS was just published in November 2017,4 and those data were included in the new meta-analysis.

“This is the latest in a series of meta-analyses that have been done on this topic,” said epidemiologist Laura Beane Freeman, PhD, who works at the National Cancer Institute.5-6 Dr Beane Freeman is one of the principal investigators on the AHS team. “Each of them make some different choices about how they do the analysis,” she says. All of the studies, she noted, reported a slightly increased risk of NHL associated with glyphosate exposure.

Though there were few new data, this paper differs from previous analyses because the researchers focused on the highest exposure levels whenever possible. Dr Sheppard explained that the aim of the study was to see whether there was any association between exposure and NHL risk, not to calculate a dose-response relationship. “If it’s carcinogenic,” she said, “you’d expect to see that in the most-exposed people.”