A US court has issued a tentative decision to allow a new trial that could overturn an August ruling awarding US$289M in damages against German chemicals and pharmaceutical giant Bayer AG for failing to warn users of the alleged cancer risks of its weedkillers.

Shares in Bayer – which completed its US$63bn acquisition of Monsanto in June – rose after the 10 October decision by San Francisco Superior Court of California Judge Suzanne Bolanos, Reuters reported.

Bayer originally lost 10% of its value after a jury in California found on 10 August that ex-groundskeeper Dewayn Johnson had contracted cancer from years of exposure to Monsanto’s glyphosate-containing Roundup and Ranger Pro herbicides and that the firm had neglected to warn him and others of the cancer risk.

Monsanto asked Bolanos in court filings on 18 September to set aside the entire verdict, reduce the award or grant a new trial.

In her ruling, Judge Bolanos said that Johnson had failed to meet his burden of producing clear and convincing evidence of malice or oppression by Monsanto, a requirement for allowing a jury to award punitive damages.

However, investors and analysts still cautioned that the tentative ruling was just one step in an uncertain legal battle, the Wall Street Journal (WSJ) reported.

“It isn't yet clear when Judge Bolanos may complete the ruling and Johnson is likely to appeal if it is completed,” the WSJ wrote.

Obtaining a new trial or achieving a significant reduction in damages is crucial for Bayer, as it works to integrate Monsanto, the US seed and pesticide giant that also produces GM seeds such as soyabeans and corn.

Johnson’s case was the first to go on trial over allegations of glyphosate causing cancer but Bayer is facing more than 8,700 similar cases from plaintiffs in the USA, with cases set to start in February.

The US Environmental Protection Agency found in 2017 that glyphosate was not likely to cause cancer in humans, contradicting a 2015 World Health Organization report that classified glyphosate as “probably carcinogenic to humans.”

Johnson’s case, filed in 2016, was fast-tracked for trial due to the severity of his non-Hodgkin’s lymphoma, a cancer of the lymph system, Reuters said.