Beginning next year, California might require consumers buying the herbicide Roundup to be warned that the product contains a substance that could cause cancer.

The California Environmental Protection Agency recently said it plans to list glyphosate, the active ingredient in Roundup and many other herbicides, as a potential cancer-causing agent under the state’s law on safe drinking water and toxic substances, otherwise known as Proposition 65.

California took the action based on a classification this year by the International Agency for Research on Cancer, an arm of the World Health Organization, which labeled glyphosate as “probably carcinogenic to humans.”

Roundup is a brand owned by agriculture giant Monsanto, but it is distributed and marketed for retail sale by Marysville-based Scotts Miracle-Gro.

Scotts spent $300 million in May to secure its long-term rights to sell Roundup and expand distribution to China and other global markets, as well as offer Roundup as part of its Scotts Lawn Service business.

At the time of the agreement, Scotts CEO James Hagedorn said Roundup was critical to the company’s success and “a key driver of shareholder value.”

Scotts believes the product is safe and that approvals and studies from a wide swath of regulatory bodies, including other arms of the WHO, back up that belief.

“We continue to be confident in the science and the regulatory process,” said Jim King, Scotts spokesman, “and continue to believe the product is safe for consumers to use.”

Roundup contributed $111 million in sales for Scotts through the first nine months of its current fiscal year, about 10 percent more than at this point last year.

The warning won’t damage Scotts’ business, said Jim Barrett, an analyst with C.L. King and Associates. Consumers know there is some risk when using Roundup and competitors such as Spectracide, which also contains glyphosate, he said.

“There are plenty of warning statements on the label anyway,” Barrett said. “If all it is is a warning statement, I would view it as a fairly minor event.”

Monsanto disagreed with the International Agency for Research on Cancer’s classification of glyphosate as a “probable carcinogen.” That designation means there is not enough evidence to say with certainty that it causes cancer, but there is some evidence to support the finding.

Other substances and activities the IARC puts in the same category as glyphosate include nitrates, expmmosure to wood smoke, high-temperature frying of food, glass-making, working as a hairdresser and malaria.

“Regulatory agencies have already reviewed all the key studies examined by IARC and arrived at the overwhelming consensus that glyphosate poses no unreasonable risks to humans or the environment when used according to label instructions,” said Monsanto spokeswoman Charla Lord in an email. “No regulatory agency in the world considers glyphosate to be a carcinogen.”

Glyphosate is used around the globe in more than 160 countries. It is the most widely applied herbicide, partly because of Monsanto’s ability to create genetically modified corn and soybean plants that are resistant to glyphosate — giving farmers the ability to kill weeds without damaging crops.

California has a 30-day comment period before a substance is listed, according to Proposition 65, and companies have up to a year to comply with labeling requirements.

“During the upcoming comment period, we will provide detailed scientific information to (the state) about the safety of glyphosate,” Lord said, “and work to ensure that any listing will not affect glyphosate use or sales in California.”