More than £8 billion ($15 billion) was wiped off the value of the German company behind Roundup after a court ruled the weedkiller caused cancer.
Shares in pharma and chemicals group Bayer fell 11 per cent on the stock market in Frankfurt – slashing its value from £78b to £69.5b.
The slump came after Bayer's US subsidiary Monsanto was ordered to pay £226 million damages to school groundsman Dewayne Johnson who claimed its weedkillers gave him non-Hodgkin's lymphoma in 2014.
The firm is appealing the verdict, insisting glyphosate, a key ingredient in its weedkillers, is not carcinogenic.
The US case
The lawsuit brought by Johnson was the first to go to trial among hundreds filed in state and federal courts in the United States saying Roundup causes non-Hodgkin's lymphoma, which Monsanto denies.
Jurors in state Superior Court agreed the product contributed to Johnson's cancer and the company should have provided a label warning of the potential health hazard. Johnson's attorneys sought and won US$39m ($59m) in compensatory damages and US$250m of the US$373m they wanted in punitive damages.
"This jury found Monsanto acted with malice and oppression because they knew what they were doing was wrong and doing it with reckless disregard for human life," said Robert F. Kennedy Jr., a member of Johnson's legal team.
"This should send a strong message to the boardroom of Monsanto."
Monsanto has denied a link between the active ingredient in Roundup — glyphosate — and cancer, saying hundreds of studies have established that glyphosate is safe.
Johnson used Roundup and a similar product, Ranger Pro, as a pest control manager at a San Francisco Bay Area school district, his lawyers said. He sprayed large quantities from a 50-gallon tank attached to a truck, and during gusty winds, the product would cover his face, said Brent Wisner, one of his attorneys.
Once, when a hose broke, the weed killer soaked his entire body.
Johnson read the label and even contacted the company after developing a rash but was never warned it could cause cancer, Wisner said. He was diagnosed with non-Hodgkin's lymphoma in 2014 at age 42.
"The simple fact is he is going to die. It's just a matter of time," Wisner told the jury in his opening statement last month.
But George Lombardi, an attorney for Monsanto, said non-Hodgkin's lymphoma takes years to develop, so Johnson's cancer must have started well before he began working at the school district.
The US Environmental Protection Agency says Roundup's active ingredient is safe for people when used in accordance with label directions.
However, the France-based International Agency for Research on Cancer, which is part of the World Health Organisation, classified it as a "probable human carcinogen" in 2015. And California added glyphosate to its list of chemicals known to cause cancer.
New Zealand's position
In agricultural New Zealand, where debate about the use of glyphosate has increased over recent years, the Environmental Protection Authority considered Roundup safe to use and the US ruling hadn't changed that position.
However, Associate Environment Minister Eugenie Sage said she'd be asking the agency to consider adding Roundup to its hazardous substance reassessment list in the light of the decision.
Sage's move has been backed by University of Canterbury toxicologist Professor Ian Shaw, who noted the World Health Organisation's International Agency for Research on Cancer (IARC) had already classified glyphosate as possibly carcinogenic to humans.
New Zealand conducted its own review of the IARC re-classification and concluded glyphosate was unlikely to be a human carcinogen or genotoxic, which swayed authorities not to reconsider the regulatory status of glyphosate-containing herbicides.
Shaw was critical of the New Zealand review's findings, particularly as the work didn't appear to have covered the possibility of non-genotoxic carcinogenesis, or that some chemicals that caused cancer didn't directly alter genes.
"I do not think we should base our regulatory decisions on a US court case, but I do think that the evidence that glyphosate is possibly a carcinogen in humans is robust."
-With Daily Mail, AP