Bayer shares plunged as much as 14 percent on Monday, losing about $14 billion in value, after newly acquired Monsanto was ordered to pay $289 million in damages in the first of possibly thousands of USA lawsuits over alleged links between a weedkiller and cancer.
Glyphosate, which is most commonly sold under the name Roundup, is a wildly popular herbicide manufactured by Monsanto.
Monsanto denies that glyphosate, the world's most widely used herbicide, causes cancer and says decades of scientific studies have shown the chemical to be safe for human use.
Jurors unanimously found that Monsanto - which vowed to appeal - acted with "malice" and that its weed killers Roundup and the professional grade version RangerPro contributed "substantially" to Dewayne Johnson's terminal illness.
Friday was also a losing day for Monsanto, which Bayer AG controversiallyacquiredfor $62.5 billion in June.
Just after the IARC issued its glyphosate monograph, Portier signed lucrative contracts with a couple of big civil litigation law firms to work as an expert witness asserting that glyphosate likely caused specific cases of non-Hodgkin lymphoma.
He called on Monsanto to "put consumer safety first over profits". The jury found that Monsanto had failed to warn Johnson of the cancer risks posed by its weedkillers.
Working for a school district in Benicia, California, about 40 miles east of San Francisco, Mr Johnson mixed and sprayed hundreds of gallons of Roundup. He was diagnosed with non-Hodgkin's lymphoma in 2014.
Farmers fear that cutting the use of glyphosate herbicides will hurt their crop yield. The German company is due to begin integrating Monsanto's operations into its own later this month.
After the trial, Scott Partridge, Monsanto's vice president for global strategy, said the documents had been "taken completely out of context", according to The Guardian.
The case against Monsanto, bought by Bayer this year for $63 billion, is the first of more than 5,000 similar lawsuits over the company's glyphosate-based weedkillers, including its Roundup brand, across the United States. "It would be nice if we could tell people why they get it, but we can't".