The most widely used herbicide in the United States is "probably carcinogenic" to humans, according to the World Heath Organization's International Agency for Research on Cancer (IARC).

Glyphosate is the main ingredient in agrochemical giant Monsanto's herbicide Roundup.

The designation is a dramatic departure from the conclusions of other regulatory agencies around the world, including the US Environmental Protection Agency (EPA), which have repeatedly found glyphosate safe to use.

"We see this as a very loud wakeup call to our country and especially our public agencies," Marcia Ishii-Eiteman, a scientist with the Pesticide Action Network, told VICE News. "Now is really the time to shift away from our kind of addiction to glyphosate, to Roundup, in our agricultural systems."

In March, IARC convened 17 scientists from 11 countries to review the most current science on five different pesticides, including glyphosate. Later that month, the group released a preliminary report designating glyphosate as probably carcinogenic.

The panel evaluated hundreds of studies looking at the effects of glyphosate on workers who had applied the herbicide and on lab animals dosed with it. They also looked at lab studies examining how glyphosate affects cells and and moves through the body.

Animals dosed with glyphosate developed tumors, including cancers of the pancreas and kidney, that are rare in rats and mice, said IARC panel chair Aaron Blair.

Some, but not all, of the human studies showed increased risks of cancer after exposure. The panel also found evidence that glyphosate exposure damaged human chromosomes.

"These are things that tend to happen when you find that something causes cancer," Blair told VICE News. "So these three lines of data point us in the direction of, 'Yeah, we ought to be worried.'"

Under IARC's classification scheme, a substance is deemed carcinogenic if there is strong evidence that it causes cancer in humans. Because the animal studies showed clear evidence that glyphosate caused cancer, but human studies did not, the panel said glyphosate is "probably carcinogenic," which reflects the uncertainty of the chemicals links to cancer in humans.

"It's a conservative approach they have taken," Ola Landgren, a myeloma researcher at Memorial Sloan Kettering Cancer Center whose work was cited in IARC's ruling, told VICE News. "I think that their conclusion is very reasonable."

IARC's decision is likely to spur new research into glyphosate's safety, Blair said.

Monsanto spokesperson Charla Lord said the company is forming a panel of its own to review the agency's decision.

"It's important that the monograph be reviewed carefully to attempt to understand how IARC reached a conclusion that is so at odds with those of scientists and regulators around the world who've conducted extensive research on glyphosate and found it to show no evidence of carcinogenicity," Lord told VICE News.

Since Monsanto first developed Roundup in the 1970s, its use has spread as companies develop crops that are genetically engineered to resist the herbicide. In July, the US Department of Agriculture (USDA) approved a strand of genetically engineered cotton that's resistant to Enlist Duo, an herbicide containing both glyphosate and another contentious substance known as 2,4-D. The US Environmental Protection Agency (EPA) approved Enlist Duo last year.

IARC will likely evaluate glyphosate again in a few years once more research has been conducted, Blair said. Until then, the panel's decision stands in opposition to what many other groups have said for years.

"Individual scientists have different opinions, different groups have different opinions," Blair told VICE News. "The IARC process has stood the test of time."

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