A lawsuit claiming that Monsanto Co.’s popular weed killer, Roundup, causes cancer was dealt a blow by a judge’s conclusion that the opinions of the experts testifying against it were “shaky,” a potentially devastating development for the case getting to trial.

In In re: Roundup Products Liability Litigation, U.S. District Judge, Vince Chhabria, of San Francisco weighed in on the toxicity of the world’s most popular herbicide, which is the subject of a heated debate among scientists and regulators worldwide. Any key witness cut from the lineup may shape the outcome of more than 300 lawsuits before the judge that seek to hold Monsanto liable for its failure to warn about the risks of using Roundup.

The San Francisco judge heard from about a dozen witnesses including toxicologists, statisticians and an oncologist, but he took an especially keen interest in a couple of epidemiologists who study how humans contract disease.

“I…have a difficult time understanding how an epidemiologist in the face of all the evidence that we saw and heard last week” can conclude that glyphosate “is in fact causing” non-Hodgkin lymphoma (“NHL”) in human beings, he stated. He proceeded “[t]he evidence that glyphosate is currently causing NHL in human beings” at current exposure levels is “pretty sparse.”

It remains to be seen which witnesses will be allowed to testify at trial on behalf of more than 700 farmers, landscapers, and gardeners claiming that exposure to glyphosate – through skin contact or inhalation – caused their NHL. The judge didn’t say when he’d arrive at a final decision.

Judge Chhabria credited Beate Ritz, a public health professor at the University of California at Los Angeles, for having conducted independent analysis. Still, Chhabria described her conclusion that glyphosate is currently causing NHL in humans “dubious.” He strongly indicated that Ritz may be the only witness he allows to testify for the plaintiffs, and that even she is at risk of being eliminated.

However, Monsanto is not off the hook yet. As a gatekeeper, a judge’s role is to exclude evidence not backed by scientific rigor, and evaluating whether the witnesses are qualified as experts to present their conclusions to a jury. Judge Chhabria said his role is to decide whether the testimony is “in the range of reasonableness,” not whether glyphosate causes cancer.

After the hearings, Judge Chhabria stated that epidemiology is a “loosey-goosey” and “highly subjective field.” But due to constraints with regard to eliminating witnesses, Ritz may be allowed to testify. Maybe Ritz “is operating within the mainstream of the field,” he said. “Maybe that means it’s up for the jury to decide if they buy her presentation.”

All the plaintiffs’ experts, except Ritz, relied on a 2015 determination that glyphosate is a probable human carcinogen by the International Agency for Research on Cancer (“IARC”), an arm of the World Health Organization, Chhabria said. For Judge Chhabria that’s “not enough” to argue that exposure to glyphosate is more-likely-than-not the cause of the plaintiffs’ cancer.

Brent Wisner, a lawyer for the plaintiffs, urged the judge not to reject witnesses based on their reliance on the IARC report alone and to instead “dissect” and consider the “subset of opinions” within their reports and findings.

Michael Baum, a lead lawyer for the group suing Monsanto, said “the weight of the epidemiology, toxicology and mechanistic science strongly supports” the conclusion that Roundup causes NHL. “Our experts used valid methodologies to arrive at their conclusions,” he said in an emailed statement. “Ultimately, we think courts will agree.”

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