Shares in Bayer plunged more than 10 percent to their lowest in almost two years after a California jury ordered the German company's subsidiary Monsanto to pay $289 million in damages last week.
A jury found Monsanto liable in a lawsuit alleging that the company's glyphosate-based weedkillers, including its Roundup brand, caused cancer. 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 on Friday that 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 percent at 83.48 euros at 0728 GMT, against a 0.6 decline for the DAX index.
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