It's the second case that has gone against manufacturer Monsanto, which was acquired for $63 billion last year by the German company. Bayer's shares dropped 10 percent to 62.70 euros on the Frankfurt exchange.
A federal court jury in San Francisco ruled unanimously Tuesday in a lawsuit against Monsanto. Attorneys say the trial could help determine the fate of hundreds of similar lawsuits. A second phase to the trial will determine whether the company is liable and if so, for how much.
Monsanto has argued that studies have established that Roundup's active ingredient, glyphosate, is safe. It has appealed a separate U.S. court decision last year in favor of a man who used Roundup.
Some countries have banned the use of the weed killer because of concerns about its potential impact on human and animal health. Others, including Germany, have resisted campaigners' calls to take Roundup off the market immediately, but say it will be phased out gradually.
German authorities are currently suing a website for publishing an official study into the dangers of glyphosate. The German Federal Institute for Risk Assessment cited copyright reasons Wednesday for wanting the website FragdenStaat.de, which facilitates citizens' freedom of information requests, to remove the document from its site.
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