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Ashley May | USA TODAY

Time

Reed Saxon, AP

Hundreds of lawsuits claiming Monsanto's weed killer Roundup caused cancer were given the green light to proceed to trial, a San Francisco judge ruled Tuesday.

The chemical glyphosate, an active ingredient in Roundup, is at the center of the debate. Cancer victims and families presenting cases say Monsanto knew about the ingredient's risk for years, but failed to warn buyers. U.S. District Judge Vince Chhabria said there's “rather weak" evidence the ingredient causes cancer, but the opinions of three experts linking glyphosate and non-Hodgkin’s lymphoma were not “junk science."

Monsanto Vice President Scott Partridge denies any connection between glyphosate and cancer. In the past, Monsanto sued California’s Office of Environmental Health Hazard Assessment for adding to a list of cancer causing chemicals, and lost.

There is conflicted evidence linking glyphosate and cancer.

The U.S. Environmental Protection Agency has gone back and forth on considering glyphosate a possible carcinogen. In a review of the chemical last year, it concluded glyphosate is likely not a carcinogen. But, the World Health Organization has classified glyphosate as "probably carcinogenic."

A separate San Francisco trial, the first case where a jury has heard cancer allegations against Roundup, is under way to determine whether glyphosate caused a school groundskeeper's non-Hodgkin’s lymphoma.