Nobody likes weeds, but a potentially harmful ingredient in a popular herbicide is more cause for concern. And soon gardeners in California could see it on a warning label.

Glyphosate, the active ingredient in Monsanto's Roundup weed killer, will be added to the state's list of chemicals known to cause cancer effective July 7, California's Environmental Protection Agency's Office of Environmental Health Hazard Assessment (OEHHA) said in a statement.

Listing glyphosate as a carcinogen would require companies selling the chemical within the state to add warning labels to their products. Monsanto and other companies that use glyphosate would have about one year to relabel their products or take them off California store shelves, according to Reuters.

In September 2015, California's OEHHA declared its intention to list glyphosate as a known cancer-causing chemical. Monsanto filed suit, citing the unconstitutionality of the listing mechanism and the OEHHA filed a motion for dismissal. The two groups have been going back and forth since then. Monsanto has appealed a trial court decision and has vowed to continue its legal challenge.

The OEHHA's list of carcinogens, known as the Proposition 65 list, is a list of "chemicals that are known to cause cancer or birth defects or other reproductive harm." Businesses that use chemicals on the list are required to warn people before exposing them to the chemical, and that includes labeling a consumer product like Roundup.

Known, but disputed, dangers

One of the primary reasons for the OEHHA to look more closely at glyphosate was a 2014 study from the cancer research arm of the World Health Organization (WHO), which issued a report concluding that glyphosate is "probably carcinogenic" to humans.

The report, compiled by the International Agency for Research on Cancer, evaluated the existing research on the herbicide and its effect on humans and lab animals. Researchers found that there was "convincing evidence" that glyphosate can cause cancer in laboratory animals. They also noted "limited evidence" of risk of non-Hodgkin's lymphoma among people who were exposed to glyphosate through their work and had traces of the chemical in their blood and urine.

"There was sufficient evidence in animals that glyphosate is carcinogenic and evidence that it can cause some of the steps towards cancer in cells, but only a limited number of studies in humans, so it was classified as a probable carcinogen," said professor Lin Fritschi, a co-author of the study, which was published in the Lancet online.

These reasons led the panel to conclude that glyphosate was "probably carcinogenic to humans," but the panel stopped short of naming the chemical a conclusive carcinogenic.

The agency looked at several insecticides and pesticides, rating them according to four levels of risk: known carcinogen, probable or possible carcinogen, not classifiable and probably not carcinogenic, according to the Chicago Tribune.

Monsanto, the manufacturer of glyphosate, rejected the panel's findings. "We don’t know how IARC could reach a conclusion that is such a dramatic departure from the conclusion reached by all regulatory agencies around the globe,” said Philip Miller, Monsanto’s vice president of global regulatory affairs, in a prepared statement.

Several other environmental health regulators from around the world, including the Environmental Protection Agency (EPA), previously concluded that glyphosate has "low acute toxicity."

Glyphosate is used in more than 750 agricultural, household and forestry products worldwide, and its use is only predicted to increase now that Monsanto has released genetically modified "Roundup Ready" seeds that are resistant to it. Environmental advocates hope that this new report will force health regulators to take a closer look at Roundup and its possible effects on humans and the planet.

This story was originally published in March 2015 and has been updated with new information.