California can require Monsanto to label its popular weed-killer Roundup as a possible cancer threat despite an insistence from the chemical giant that it poses no risk to people, a judge tentatively ruled Friday.

California would be the first U.S. 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 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’s 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.”