Of all the pesticides out there, few have sparked more controversy than glyphosate. The world’s most used weed killer, glyphosate is perhaps better known by the brand name Roundup. It works by killing off weeds and other unwanted flora that get in the way of agricultural crops, many of which have been genetically engineered to be immune to glyphosate’s deadly effects.
The use of chemical herbicides such as glyphosate has long been denigrated by environmental groups and health advocates, including supporters of U.S. secretary of health and human services Robert F. Kennedy, Jr., who in the past argued that glyphosate caused cancer.
Then, on Monday, President Donald Trump issued an executive order to boost glyphosate production—sparking an immediate backlash from many within the “Make America Healthy Again” (MAHA) movement.
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Kennedy, stunning his followers, backed the president’s order. But other MAHA leaders have not been so sure: Casey Means, a wellness influencer and Trump’s pick to be the next surgeon general, said she was concerned about using “toxic” chemicals to grow food crops during a Senate hearing on Wednesday. “We must, as a country, move away from using toxic inputs in our food supply, and we must study these chemicals more to understand their effects,” she said.
We spoke to two experts to understand what the research into glyphosate’s health effects shows and what we know about how it gets into the environment—and our body.
What is glyphosate?
Glyphosate is an “amino acid inhibitor,” which means it stops weeds from growing by blocking their ability to produce amino acids, which are the building blocks of proteins. The herbicide is commonly applied to crops, such as corn and soybeans, that have been genetically modified to withstand its effects.
But glyphosate absolutely can enter our body through our food—grains and legumes are among the crops that are most likely to be contaminated with the weed killer—or from contact with surfaces. People can also inhale it.
Glyphosate was originally manufactured and sold as Roundup by Monsanto, which was acquired by the German pharmaceutical giant Bayer in 2018. Bayer says it has faced almost 200,000 claims over alleged harms of glyphosate exposure, including a high-profile case that the U.S. Supreme Court is set to review this year. And earlier this month Bayer agreed to pay $7.25 billion to settle a class-action lawsuit that alleged glyphosate exposure had played a role in causing non-Hodgkin lymphoma, a form of cancer that attacks the lymph nodes.
A Monsanto spokesperson told Scientific American that the settlement did not contain an admission of liability or wrongdoing.
“Leading regulators worldwide, including the U.S. [Environmental Protection Agency] and [European Union] regulatory bodies, continue to conclude based on an extensive body of science, that glyphosate-based herbicides—critical tools that farmers rely on to produce affordable food and feed the world—can be used safely and are not carcinogenic,” the spokesperson said.
What research is there into glyphosate’s health effects?
Much of the research focus on glyphosate has explored its links to cancer. In 2015 the World Health Organization’s International Agency for Research on Cancer (IARC) classified the weed killer as “probably carcinogenic to humans.” What that means is that there is some evidence to suggest glyphosate poses a cancer risk.
Other public health bodies have disagreed with this assessment. In 2016 the Food and Agriculture Organization of the United Nations and the U.S. EPA determined glyphosate was likely not carcinogenic.
The research is more nuanced. In 2018 researchers at the National Cancer Institute found “no association” between glyphosate exposure and non-Hodgkin lymphoma in farmworkers. But one year later, in 2019, a meta-analysis found a “compelling link” between glyphosate exposure and the cancer. This meta-analysis “was unique because it focused on the most highly exposed groups,” says Lianne Sheppard, senior author of that paper and a professor at the University of Washington, who studies the health effects of chemical exposures.
Glyphosate’s effects on other animals is also under research scrutiny. Studies show glyphosate exposure may disturb honey bees, harming their ability to forage food, and that the weed killer could have damaging effects on plants, birds and mammals, according to the EPA.
Part of the reason for the apparent contradictions between IARC, the EPA and other bodies is that different experts may weigh the merit of certain studies differently, thereby reaching quite distinct conclusions, says Brenda Eskenazi, a professor emerita of public health at the University of California, Berkeley.
Potential conflicts of interest can also weigh on studies: Last year, citing “ethical concerns,” the academic journal Regulatory Toxicology and Pharmacology retracted an influential 2000 study backed by Monsanto that had concluded glyphosate was not carcinogenic.
Sheppard, who served on the EPA panel that reviewed glyphosate’s potential to cause cancer in 2016, says that scientific evidence for the herbicides’s possible effect on human health since then “has strengthened for cancer and other end points.”
Why don’t we know more about what glyphosate does?
Studying glyphosate is tough: While animal and human cell studies have shown a connection between exposure to the weed killer and health effects such as cancer, endocrine disruption, oxidative stress, and more, human studies are much harder to do.
Glyphosate has a short half-life in the body—by one estimate, as little as 5.5 to 10 hours—so trying to estimate the effects of glyphosate by looking at levels in someone’s urine, for example, offers only a snapshot of that moment in time and doesn’t reveal much about their long-term exposure, Eskenazi says. Long-term studies, which might involve collecting and analyzing urine samples taken from participants over a period of time, are logistically difficult and expensive. Scientists may be able to look at geographical data to estimate long-term glyphosate exposure instead, but that’s still an imprecise measure.
More research may be on the way. Eskenazi says studies that investigate groups, such as pregnant women and fetuses, who may be most vulnerable to glyphosate exposure and research into whether glyphosate might affect human fertility and reproduction would be particularly useful.
“We’re just at the beginning of studying glyphosate, but we absolutely must study it, given it is the most commonly used herbicide in the world,” she says. “Even a small, tiny effect, if it’s real, can have a huge public health impact because so many people would be exposed.”