roundup weedkiller

Weeding Out the Truth in the Roundup Cancer Case

A massive legal settlement against Monsanto makes it seem like one of their products causes cancer. But the science isn't that simple.


A California jury made prominent international headlines in August when they awarded $289 million US in damages to school groundskeeper Dewayne Johnson in a lawsuit against Monsanto. Johnson alleged that glyphosate, the active ingredient in Monsanto’s Roundup pesticide, caused his terminal cancer, and that the company failed to warn him of the potential dangers of their product.

The weight of the verdict feels definitive. It would be easy to take home the message that Roundup causes cancer. In truth, there is a low standard of proof in civil proceedings like this one, and the scientific experts still haven’t reached a consensus.

“By no means should anybody take that to mean that it’s now been demonstrated that those things cause cancer. That’s not what a court is designed to do,” said biologist Scott Findlay to CBC News.

Findlay, a professor at the University of Ottawa and co-founder of science advocacy group Evidence for Democracy, is a vocal champion for evidence-based policy decisions.

Unlike in a criminal court case, jurors in a civil court case base their decisions on balance of probabilities: based on expert testimony, do you believe that it is more probable than not that Roundup caused Johnson’s cancer?

Jurors were instructed to use their judgement when it comes to deciding what evidence, and even which witnesses, are credible.

The scientific community, on the other hand, is still grappling with that remaining uncertainty. Cases like this underscore the difficulty in reaching a consensus when the studies to prove it would never be done, because it would be a huge ethical violation to deliberately expose people to substances just to see whether they cause cancer.

That leaves epidemiology as the main way scientists look for increased cancer risk. Richard Stevens, professor of medicine at the University of Connecticut, points to the parallels between this and the case for whether cigarettes cause lung cancer.

He notes that there are two main types of study designs. In a cohort study, a large group of people is divided between people who are exposed (like smokers in the case of cigarettes), and those who are not, and researchers follow both groups of participants over time to track who gets sick.

By contrast, in a case-control study, a large group of lung cancer patients would be asked about their smoking history, for comparison against the smoking histories of an equal number of people without lung cancer.

Inevitably, these studies can be confounding as it is difficult to get true measures of exposure, and it can be argued that association doesn’t mean causation.

Ultimately, it took hundreds of these epidemiological studies to solidify the consensus that cigarettes cause lung cancer, and Stevens believes a similar path is ahead for plaintiffs in Roundup cases.

“That does not mean we shouldn’t act on the evidence that we have,” added Findlay. “Somebody has to decide at the end of the day how much does the positive evidence have to outweigh the negative evidence before we’re willing to say okay for the purposes of decision-making it causes cancer.”

In the end, a big part of this case centered around whether Monsanto knew of the potential cancer risks of their product and failed to inform their users. Even in the midst of a debate with no clear answers, the verdict in Johnson’s case demands that corporations be held accountable for being transparent with what they know.

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Karyn Ho is a science animator and engineer who thrives at the interface between science, engineering, medicine, and art. She earned her MScBMC (biomedical communications) and PhD (chemical engineering and biomedical engineering) at the University of Toronto. Karyn is passionate about using cutting edge discoveries to create dynamic stories as a way of supporting innovation, collaboration, education, and informed decision making. By translating knowledge into narratives, her vision is to captivate people, spark their curiosity, and motivate them to share what they learned.