A US ruling has prompted fresh questions about the health effects of Roundup - but scientists say the case shouldn't warrant a knee-jerk ban of the popular weedkiller here in New Zealand.

Over the weekend, a San Francisco jury ordered Monsanto to pay NZ$439m to a former school groundskeeper, on the basis its product had contributed to his cancer.

The lawsuit from Dewayne Johnson claimed Roundup caused his non-Hodgkin's lymphoma, which Monsanto denied.

The company insisted hundreds of studies have established that the active ingredient in Roundup - glyphosate - was safe and reportedly planned to appeal the decision.

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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."

Another toxicologist, Dr Belinda Cridge, said the terms of the US case were interesting as the plaintiff didn't need to conclusively demonstrate that glyphosate caused the cancer - only that it was a plausible contributing factor.

Also, she pointed out, Monsanto was unable to prove that glyphosate definitely did not cause the cancer.

Cridge said the IARC's re-classification itself was still being widely debated, but argued that its underlying finding - that glyphosate might cause cancer under the right conditions and exposures - still stood.

"Finally, it is important to consider the whole picture. Roundup isn't, and has never been, a safe panacea for all weed control.

"Scientists continue to learn more and more about this chemical and its effects."

Yet the alternative options weren't very appealing and many were still much worse for both people and the environment, she said.

"Roundup has been used extensively worldwide for a long time, it has a reasonably good safety record and has limited environmental effects - compared to the alternatives.

"Yes, improvement is needed, but for farmers, Roundup is one of the safer options currently available."

That point was echoed by Massey University weed scientist Dr Kerry Harrington.

"Hopefully the calls for it to be banned will take into consideration the low risk of problems and the crucial importance of this herbicide for sustainable weed control worldwide," he said.

"It is a concern if decisions on the use of glyphosate in New Zealand hinge on the outcome of a court case in US where a jury of ordinary members of the public had to decide about complex issues of toxicology."

University of Otago cancer epidemiologist Associate Professor Brian Cox further cautioned against reacting with a ban.

"A sudden reaction to one case in one US law court, that has not yet gone to the appeal court, is not an appropriate method of developing health policy in New Zealand."

Cridge's advice to people was not to use chemicals they didn't need to and to be rigorous about safety equipment.

"This applies to all the chemicals we use from home cleaners to industrial chemicals in the workplace to agrochemicals such as Roundup."