Shares in Bayer have plunged more than 14% to their lowest in almost five years - wiping around $14 billion off the value of the company - after a California jury ordered its subsidiary Monsanto to pay $289m in damages last week.
A jury found Monsanto liable in a lawsuit alleging that the German company's glyphosate-based weedkillers, including its Roundup brand, caused cancer.
Jurors unanimously found Monsanto acted with "malice" and that Roundup and the professional grade version RangerPro contributed "substantially" to Dewayne Johnson's terminal illness.
Mr Johnson, a groundskeeper diagnosed in 2014 with non-Hodgkin's lymphoma, says he repeatedly used a generic form of RangerPro while working at a school in Benicia, California.
The case against Monsanto, which Bayer acquired this year for $63 billion, is the first of more than 5,000 similar lawsuits across the United States.
Monsanto said it would appeal against the verdict.
"Today's decision does not change the fact that more than 800 scientific studies and reviews ... support the fact that glyphosate does not cause cancer," it said in a statement.
Bayer shares were down 10.6% at 83.48 euro shortly before 8.30am Irish time, against a 0.6% decline for the German stock exchange, the DAX.
The owner of Bayer has said it believes it will be vindicated following the court ruling.
In a statement to RTÉ News, the head of Corporate Media Relations at Bayer said it believes the jury's verdict is "at odds with the weight of scientific evidence".
"The jury's verdict is just the first step in this case, and it remains subject to post-trial motions in the trial court and to an appeal, as announced by Monsanto. As this case proceeds, Bayer believes courts ultimately will find that Monsanto and glyphosate were not responsible for Mr Johnson's illness," the statement said.
In Europe, the EU Commission in December renewed the licence for glyphosate despite intense debate over its safety, though Germany and France have taken steps to phase out use of the weedkiller.
Originally an exclusive Monsanto brand, patent-free glyphosate herbicides are now sold by the global crop-protection industry.
