JOHN SANDERS WORKED in the orange and grapefruit groves in Redlands, California, for more than 30 years. First as a ranch hand, then as a farm worker, he was responsible for keeping the weeds around the citrus trees in check. Roundup, the Monsanto weed killer, was his weapon of choice, and he sprayed it on the plants from a hand-held atomizer year-round.

Frank Tanner, who owned a landscaping business, is also a Californian and former Roundup user. Tanner relied on the herbicide starting in 1974, and between 2000 and 2006 sprayed between 50 and 70 gallons of it a year, sometimes from a backpack, other times from a 200-gallon drum that he rolled on a cart next to him.

The two men have other things in common, too: After being regularly exposed to Roundup, both developed non-Hodgkin lymphoma, a blood cancer that starts in the lymph cells. And, as of April, both are plaintiffs in a suit filed against Monsanto that marks a turning point in the pitched battle over the most widely used agricultural chemical in history.

Until recently, the fight over Roundup has mostly focused on its active ingredient, glyphosate. But mounting evidence, including one study published in February, shows it’s not only glyphosate that’s dangerous, but also chemicals listed as “inert ingredients” in some formulations of Roundup and other glyphosate-based weed killers. Though they have been in herbicides — and our environment — for decades, these chemicals have evaded scientific scrutiny and regulation in large part because the companies that make and use them have concealed their identity as trade secrets.

Now, as environmental scientists have begun to puzzle out the mysterious chemicals sold along with glyphosate, evidence that these so-called inert ingredients are harmful has begun to hit U.S. courts. In addition to Sanders and Tanner, at least four people who developed non-Hodgkin lymphoma after using Roundup have sued Monsanto in recent months, citing the dangers of both glyphosate and the co-formulants sold with it. As Tanner and Sanders’s complaint puts it: Monsanto “knew or should have known that Roundup is more toxic than glyphosate alone and that safety studies of Roundup, Roundup’s adjuvants and ‘inert’ ingredients” were necessary.

Research on these chemicals seems to have played a role in the stark disagreement over glyphosate’s safety that has played out on the international stage over the last year. In March 2015, using research on both glyphosate alone and the complete formulations of Roundup and other herbicides, the World Health Organization’s International Agency for Research on Cancer (IARC) declared glyphosate a probable human carcinogen. The IARC report noted an association between non-Hodgkin lymphoma and glyphosate, significant evidence that the chemical caused cancer in lab animals, and strong evidence that it damaged human DNA.

Meanwhile, in November the European Food Safety Authority issued a report concluding that the active ingredient in Roundup was “unlikely to pose a carcinogenic hazard to humans.” The discrepancy might be explained by the fact that the EFSA report included only studies looking at the effects of glyphosate alone. Another reason the agencies may have differed, according to 94 environmental health experts from around the world, is that IARC considered only independent studies, while the EFSA report included data from unpublished industry-submitted studies, which were cited with redacted footnotes.

On Friday, April 29, the Environmental Protection Agency weighed in — briefly — when it posted a long-awaited report on the reregistration of glyphosate concluding that the herbicide is “not likely to be carcinogenic to humans.” But the agency removed the report and 13 related documents from its website the following Monday, saying the publication had been an error. The U.S. House of Representatives Committee on Science, Space and Technology is looking into the EPA’s “apparent mishandling” of the glyphosate report, and the EPA said it will release the reregistration materials by the end of this year.