Shares in Bayer AG (BAYRY) plunged more than 10% in early morning trade on Monday after Monsanto lost a case and looked set for more legal trouble in the near future.

A San Francisco court ordered Monsanto, the German pharmaceutical and chemical company's subsidiary, to pay $289 million in damages for failing to warn groundskeeper Dewayne Johnson and other consumers about the cancer risks posed by its Roundup brand, the most popular weed killer in the world.

Opening the Floodgates

The verdict against Monsanto, which Bayer acquired in June for $63 billion, has placed a dark cloud over its parent company’s shares. There are currently more than 5,000 similar lawsuits in the U.S. against Monsanto’s weed killers, according to Reuters, with Johnson’s trial coming first because doctors claimed that he was close to dying. (See also: Monsanto Shareholders Approve Bayer Buyout.)

Monsanto said it will appeal the verdict, adding that more than 800 scientific studies and reviews support its claim that glyphosate, the ingredient used in Roundup, does not cause cancer. "We will appeal this decision and continue to vigorously defend this product, which has a 40-year history of safe use and continues to be a vital, effective and safe tool for farmers and others," Monsanto Vice President Scott Partridge said, according to CNN.

Johnson's attorney Timothy Litzenburg told CNN that Monsanto’s appeal will still prove costly for Bayer as the company will have to pay interest on the damages, believed to be about $25 million a year, while the court reviews its findings.

In 2015, the World Health Organization's International Agency for Research on Cancer said the key ingredient in Roundup, glyphosate, is "probably carcinogenic to humans." However, separate studies suggest otherwise.

"More than 800 scientific studies, the US EPA, the National Institutes of Health and regulators around the world have concluded that glyphosate is safe for use and does not cause cancer," said Monsanto's vice president of strategy, according to CNN.

Amid ongoing debates over its safety, the European Union’s commission opted to renew its license for glyphosate in December, even though both Germany and France have taken steps to phase out use of the weed killer.

Monsanto, a major producer of genetically modified seeds and foods, has a history of being targeted by environmental and health activists. (See also: 13F: Buffett Increased Apple, Monsanto & Teva.)