California can require Monsanto to label its popular weed killer Roundup as a possible cancer threat even though the chemical giant insists it poses no risk to people, a judge tentatively ruled Friday.
California would be the first state to order such labeling if it carries out the proposal.
Monsanto had sued the nation's leading agricultural state, saying California officials illegally based their decision for carrying the warnings on findings by an international health organization based in France.
Monsanto attorney Trenton Norris argued in court Friday that the labels would have immediate financial consequences for the company. He said many consumers would see the labels and stop buying Roundup.
"It will absolutely be used in ways that will harm Monsanto," he said.
After the hearing, the firm said in a statement that it will challenge the tentative ruling.
Critics take issue with Roundup's main ingredient, glyphosate, which has no color or smell. Monsanto introduced it in 1974 as an effective way of killing weeds while leaving crops and plants intact.
It is sold in more than 160 countries, and farmers in California use it on 250 types of crops.
The chemical is not restricted by the U.S. Environmental Protection Agency, which says it has "low toxicity" and recommends people avoid entering a field for 12 hours after it has been applied.
But the International Agency for Research on Cancer, a Lyon, France-based branch of the U.N. World Health Organization, classified the chemical as a "probable human carcinogen."
Shortly afterward, the most populated U.S. state took its first step in 2015 to require the warning labels.
St. Louis-based Monsanto contends that California is delegating its authority to an unelected foreign body with no accountability to U.S. or state officials in violation of the California Constitution.
Attorneys for California consider the International Agency for Research on Cancer the "gold standard" for identifying carcinogens, and they rely on its findings along with several states, the federal government and other countries, court papers say.
Fresno County Superior Court Judge Kristi Kapetan has yet to issue a formal decision, which she said would come soon.
California regulators are waiting for the formal ruling before moving forward with the warnings, said Sam Delson, a spokesman for the state Office of Environmental Health Hazard Assessment.
Once a chemical is added to a list of probable carcinogens, the manufacturer has a year before it must attach the label, he said.
Teri McCall believes a warning would have saved her husband, Jack, who toted a backpack of Roundup for more than 30 years to spray weeds on their 20-acre avocado and apple farm. He died of cancer in late 2015.
"I just don't think my husband would have taken that risk if he had known," said McCall, one of dozens nationwide who are suing Monsanto, claiming the chemical gave them or a loved one cancer.
But farmer Paul Betancourt, who has been using Roundup for more than three decades on his almond and cotton crops, says he does not know anyone who has gotten sick from it.
"You've got to treat it with a level of respect, like anything else," he said. "Gasoline will cause cancer if you bathe in the stuff."