Monsanto’s Roundup is the most popular weed killer in the world, but doubts about the safety of the active ingredient glyphosate continue to persist with public health groups and government agencies constantly reviewing the dangers of the product.

WHO Classification

One of the most controversial updates to the popular weed killer came in 2015 when an agency of the World Health Organization (WHO) declared that glyphosate probably causes cancer.

The organization’s International Agency for Research on Cancer (IARC) added glyphosate to its Group 2A listing, which is a designation for substances that have limited evidence of cancerous effects in humans and sufficient evidence in cancerous effects in animal studies.

The agency, which comprises 17 reviewers from around the world, came to a unanimous agreement on the classification after basing the conclusion on different studies of people, animals and cells.

“All three lines of evidence sort of said the same thing, which is we ought to be concerned about this,” Aaron Blair, the chairman of the IARC study and a retired epidemiologist, told the New York Times in 2015.

Monsanto, which has spent decades defending its weed killer, shot back at the agency for the classification.

“Based on the overwhelming weight of evidence, Monsanto strongly disagrees with IARC’s classification of glyphosate,” Monsanto wrote on its website. “Importantly, IARC overlooked decades of thorough and science-based analysis by regulatory agencies around the world and selectively interpreted data to arrive at its classification of glyphosate. No regulatory agency in the world considers glyphosate to be a carcinogen.”

Glyphosate Studies Leave Findings Open to Interpretation

A few months after the IARC changed glyphosate’s classification, the United Nations and the WHO announced that glyphosate does not pose a cancer risk. In the May 2016 report, the two organizations found that the herbicide was not associated with toxicity in real-life situations.

Two groups within the WHO coming to different conclusions highlights how the same data can be interpreted differently.

In 1985, the Environmental Protection Agency (EPA) determined that the main ingredient in Monsanto’s Roundup weed killer may cause cancer, but six years later, the agency reversed that determination after questioning a mouse study the first conclusion was based on.

In the 2015 reclassification by the IARC, the same mouse study was used as evidence to conclude the carcinogenic nature of the herbicide.

That study discovered three cases of a rare type of kidney cancer in 50 male mice fed the herbicide at the highest dose. The chairman of the IARC noted that the cancer doesn’t occur except when the rodents are dosed with the herbicide while the EPA said the findings were not statistically significant compared to a control group.

Lawsuits Over Roundup’s Cancer Risk Surface

Despite the conflicting evidence and reports over the last 30 years about the safety of Monsanto’s Roundup weed killer, consumers have begun filing lawsuits against the biochemical agriculture company over claims that it gave them cancer.

A group of more than 30 people is suing or planning to sue Monsanto after developing cancer while being exposed to Roundup at work and home.

In one case, Yolanda Mendoza blames the herbicide for her diagnosis of non-Hodgkin’s Lymphoma. She sprayed her one-acre property once a week with Roundup.

According to attorney Robin L. Greenwald, whose firm Weitz & Luxenberg is representing Mendoza and several others, glyphosate is unsafe and has caused her clients to develop potentially lethal cancer.

“Some people are landscapers, some people are migrant farm workers, some people are farmers,” Greenwald told CBS News. “What everyone has in common is that they all used Roundup and they all have non-Hodgkin’s lymphoma.”

Part of the basis of the lawsuit is the controversial IARC decision to classify glyphosate as probably carcinogenic.

However, many scientists and health organizations continue to defend the safety of the Monsanto’s Roundup.

“IARC focuses on hazard identification and the EPA focuses on risk. Risk is a more involved process in that it involves not only can it cause cancer, but is it likely to cause cancer under specific sorts of conditions,” toxicologist David Eastmond explained to CBS News. “I think that certainly the risk is modest and probably very small.”

In 2015, the European Food Safety Authority convened a group of experts who concluded that glyphosate likely does not cause cancer, and the EPA once again said glyphosate should be classified as “not likely to be carcinogenic to humans” in a September 2016 report.

Lawsuits will likely continue to surface against Monsanto on the safety of Roundup.