Hundreds of medical marijuana-related businesses in California have been hit with product labeling violation letters since January, putting the businesses at risk of costly litigation and signaling potential trouble for the state's soon-to-launch recreational industry.

Approximately 800 letters, sent by law firms representing a handful of plaintiffs, allege violations of Proposition 65, California's Safe Drinking Water and Toxic Enforcement Act. The law, approved by voters in 1986, requires businesses to warn consumers about products that contain one or more of 900 chemicals declared by the state to cause cancer, birth defects or other health dangers.

The recent wave of Prop. 65 letters targeted medical marijuana dispensaries, producers of cannabis-infused edibles and vape cartridge manufacturers, according to notices filed with the Attorney General's Office.