Glyphosate is the world’s most widely used herbicide, a chemical that kills plants by blocking their ability to produce essential proteins. You’ve likely encountered it under the brand name Roundup, though it’s now the active ingredient in hundreds of commercial weed killers. It’s also one of the most debated chemicals in modern agriculture, with major regulatory agencies disagreeing about whether it poses a cancer risk to humans.
How Glyphosate Works
Glyphosate kills plants by shutting down a specific enzyme they need to build aromatic amino acids, the building blocks for proteins and other vital compounds. This enzyme sits on what scientists call the shikimate pathway, a metabolic process found in plants, bacteria, and fungi but not in mammals. When glyphosate enters a plant, it mimics one of the enzyme’s natural substrates and lodges itself into the binding site, effectively jamming the machinery. Without the ability to synthesize these amino acids, the plant starves at a molecular level and dies.
This mechanism is part of why glyphosate became so popular. Because mammals don’t have the shikimate pathway, the chemical was long considered to have a wide safety margin for humans and animals compared to older herbicides. It’s also nonselective, meaning it kills virtually any green plant it touches, which made it useful for clearing fields, roadsides, and garden beds alike.
From Lab Curiosity to Global Staple
Glyphosate was first synthesized in 1950 by a Swiss chemist working for a pharmaceutical company, but nobody could find a medical use for it and it was shelved. Two decades later, in 1970, researchers at Monsanto resynthesized the compound and tested it in a greenhouse, where its powerful herbicidal activity became clear. It was patented in 1971 and hit the market in 1974 under the trade name Roundup.
The real explosion in glyphosate use came in the mid-1990s, when Monsanto introduced genetically engineered crops designed to survive glyphosate applications. Farmers could spray their entire fields to kill weeds without harming corn, soybeans, or cotton. By 2018, roughly 80 percent of domestic corn and cotton planted in the United States were genetically engineered stacked-trait seeds, most of which included glyphosate tolerance. Soybeans followed a similar trajectory. This shift made glyphosate the backbone of weed management across American agriculture.
The Cancer Debate
The biggest controversy around glyphosate centers on whether it causes cancer. The two most influential bodies in this debate have reached opposite conclusions.
In March 2015, the International Agency for Research on Cancer (IARC), part of the World Health Organization, classified glyphosate as “probably carcinogenic to humans” (Group 2A). That determination was based on limited evidence of cancer in humans from real-world exposure data, combined with sufficient evidence of cancer in laboratory animals exposed to pure glyphosate. The specific concern is non-Hodgkin lymphoma. A 2019 meta-analysis pooling data from multiple studies found that people with the highest cumulative exposure to glyphosate-based herbicides had a 41 percent increased risk of developing non-Hodgkin lymphoma compared to unexposed individuals.
The U.S. Environmental Protection Agency sees it differently. In its 2020 interim decision, the EPA concluded that glyphosate is “not likely to be carcinogenic to humans” and identified no human health risks of concern from exposure at current levels. However, a federal court later required the agency to revisit and better explain its evaluation of glyphosate’s carcinogenic potential. The EPA’s underlying scientific position has not changed, but the review process remains ongoing. In Europe, regulators renewed glyphosate’s approval in late 2023 for another 10 years, keeping it legal for use until December 2033.
This split largely comes down to methodology. IARC evaluates whether a substance is capable of causing cancer under any circumstances, while the EPA and European regulators assess whether it poses a risk at the exposure levels people actually encounter. Both approaches are scientifically legitimate, which is why the disagreement persists.
Glyphosate Residues in Food
Because glyphosate is sprayed on crops so widely, trace amounts show up in food. The FDA monitors this through its annual pesticide residue program. In fiscal year 2023, the agency tested over 3,500 human food samples from domestic and international sources. Among domestic samples, 97.2 percent were compliant with EPA tolerance levels, meaning residues fell below the safety thresholds set by regulators. Import samples had an 86.5 percent compliance rate.
For animal-derived foods like milk, eggs, honey, and game meat, the numbers were even more reassuring: out of 95 samples tested, none contained violative pesticide residues, and 87.4 percent had no detectable pesticide residues at all. These figures cover all pesticides, not just glyphosate, but they reflect the general picture of where residue levels stand relative to regulatory limits.
Effects on Soil and Ecosystems
Glyphosate doesn’t just affect the target weeds. Field studies have documented negative effects on soil fungal communities, particularly at high application rates or after repeated long-term use. Researchers have observed reduced fungal biomass and lower species richness among cultivable fungi in treated soils. There’s sometimes a brief spike in fungal activity shortly after application, but the longer-term trend points toward diminished diversity. Since soil fungi play critical roles in nutrient cycling and plant health, these changes can ripple through farm ecosystems.
The EPA’s own 2020 review, while finding no human health risks of concern, did flag potential ecological risks from glyphosate use. This includes effects on non-target plants, aquatic organisms, and the broader food web that depends on the vegetation glyphosate eliminates.
The Weed Resistance Problem
One of the most concrete consequences of heavy glyphosate reliance is the evolution of resistant weeds. As of the latest global surveys, 38 weed species have evolved resistance to glyphosate across 37 countries, showing up in 34 different crops and six non-crop settings. When farmers use the same herbicide repeatedly over years, they create intense selective pressure: any weed with a natural mutation that lets it survive will thrive and spread.
Resistant weeds force farmers into using additional herbicides, tillage, or more complex management strategies, often increasing costs and environmental impact. It’s a textbook example of how overreliance on a single tool in agriculture can undermine its own effectiveness over time.