California is taking a stand against Roundup, Monsanto Company's (MON) weed killer, which various health organizations claim has an ingredient that causes cancer. A California judge issued a ruling Friday requiring Monsanto to label Roundup, which contains the chemical glyphosate, as a possible cancer threat. This comes despite the St. Louis-based agriculture giant insisting that the product poses no risk to people. The news sent Monsanto shares down 1.2% Friday. Monsanto said it will appeal the ruling. (See also: Monsanto Fights Back Against Roundup Health Claims.)

Farmers and home gardeners spray Roundup to combat weeds and other unwanted plants that threaten their crops. California, the nation's leading agricultural state, will now be the first state to require a label informing consumers that the product can be dangerous. However, Monsanto said that California officials illegally based their decision for carrying the warnings on an international health organization located in France, noted the Associated Press.

Monsanto attorney Trenton Norris said on Friday that the label would have immediate financial consequences for the company, reports the Associated Press. "It will absolutely be used in ways that will harm Monsanto," he said. Norris added that consumers would see the labels and stop buying the product. (See also: Top 3 Companies Owned by Monsanto.)

Introduced it in 1974 as an effective way to fight weeds without hurting crops, glyphosate is sold in more than 160 countries. California uses it on 250 types of crops, noted the Associated Press. However, glyphosate is also colorless and odorless, making it hard to detect. Those qualities are perceived by health officials as dangerous to consumers. The International Agency for Research on Cancer classifies the chemical as a "probable human carcinogen."

Monsanto shares closed Friday at $109.40, down 0.36%. The stock has risen 3.98% this year, compared with a 2.5% rise in the S&P 500 index (SPX). (See also: Dissecting Monsanto's Post-Earnings Stock Valuation.)

Want to learn how to invest?

Get a free 10 week email series that will teach you how to start investing.

Delivered twice a week, straight to your inbox.