Does Roundup Cause Cancer? Experts Weigh In on Monsanto's Controversial Weed Killer

Updated | More than 300 lawsuits have been filed on behalf of farmers and others who said that Monsanto's popular weed killer, Roundup, gave them cancer. Could their claims be true? In theory, a scientist ought to know the answer. But how scientists come to their conclusions is being evaluated in a federal courtroom this week.

Some general guidelines govern what kind of science is allowed to be considered in a court of law. Whether or not experts in a lawsuit are working within those guidelines—and, therefore, should be allowed to testify—is evaluated during something called a Daubert hearing. The hearing in this particular case began Monday in San Francisco.

That doesn't mean any decision is being made about what the good science means. "The judge is not deciding this week whose experts are correct," one of the lawyers for the plaintiffs, Timothy Litzenburg, told Newsweek. (It also doesn't mean that the trial could be starting soon; it's impossible to say when the judge, U.S. District Court Judge Vince Chhabria, might issue a ruling based on what he hears this week.)

Experts appearing in courts are expected to rely on scientific evidence based on techniques that are testable, peer-reviewed and widely accepted, according to Cornell Law School's open legal dictionary. Scientists should also be able to describe how accurate the technique is and agree on how it's done.

All this was supposed to be evaluated back in December. But Laura Beane Freeman and her team changed those plans. She and her colleagues published a study in the Journal of the National Cancer Institute stating that among the 54,000 farmers studied, glyphosate did not appear to create an increased risk of almost any cancer.

Only one particular type of cancer, acute myeloid leukemia (AML), appeared to be linked to glyphosate. However, the link wasn't considered "statistically significant." David Spiegelhalter, a statistician at the University of Cambridge, noted in a statement given to the U.K.'s Science Media Centre that the link was "no more than one would expect by chance when looking at 22 different cancer types."

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Bottles of Monsanto's Roundup pesticide in a gardening store in France. In March 2015, the U.N.'s International Agency for Research on Cancer classified the active ingredient in Roundup, glyphosate, as "probably carcinogenic to humans." PHILIPPE HUGUEN/AFP/Getty Images

And, Beane Freeman notes, that study was the first to ever look at AML and glyphosate exposure. Any link, even a weak one, should be confirmed by further studies.

That study was published on November 9. By November 10, Chhabria had heard about it. In a pretrial motion specifically citing the study, he asked the lawyers in the case if they believed the Daubert hearing should be postponed.

Clearly, someone thought it should be.

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Farmer Nicolas Denieul fills his agricultural sprayer with roundup, a glyphosate-based herbicide made by Monsanto, in France on December 29, 2017. JEAN-FRANCOIS MONIER/AFP/Getty Images

Searching for a conclusion from scientific authorities turns up a bit of a mess. Beane Freeman works for the NCI. She isn't testifying, but people who used to work at the institute are among those who are on behalf of the farmers. The International Agency for Research on Cancer, part of the World Health Organization, said in 2015 that glyphosate probably could cause cancer in humans, though the evidence is still not conclusive. But a committee that included the World Health Organization said something a little different—specifically, that glyphosate on people's foods probably doesn't cause cancer. The California Environmental Protection Agency, meanwhile, has glyphosate listed on its roster of chemicals that can cause cancer.

While those assessments all relied on data and studies, few involved humans. "There are relatively few epidemiological studies of glyphosate and human cancer risk," Beane Freeman said. "Doing a study to evaluate this in a human population is actually a pretty challenging study to do." Measuring the amount of the chemical that a person has been exposed to is one of those challenges, she noted; gathering enough people to pick up on an increased risk of rare cancers is another. And, of course, even studies that can find enough people with enough good information will have limitations, she said.

"There is no perfect study."

This article has been updated with the correct spelling of Beane Freeman's name.