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.

The letters allege that the businesses failed to warn buyers about specific fungicides and pesticides associated with cannabis cultivation, as well as the hazards of marijuana smoke.

Companies that do not comply can be penalized up to $2,500 per violation per day, a potentially significant sum for mass manufacturers or mom-and-pop shops that failed to post a required sign for months. To avoid litigation, many defendants agree to five-figure settlements that require them to reformulate or affix a warning label to their products.

The most prolific letter-sender appears to be a man named Michael Murphy, who filed copies of more than 600 violation notices with the Attorney General’s Office on May 5 alone. He is identified as manager of Clean Cannabis Initiative.

“The motivation is really just choice,” said Murphy’s attorney, Mark Morrison of the Morrison Law Firm in Newport Beach. “Consumers really just need to have the facts for each consumer to make the best choice.”

Morrison described Murphy as a “concerned consumer” who’s “very much into clean eating and living” and had the products targeted in the Prop. 65 letters lab tested. Murphy doesn’t think manufacturers of “clean” marijuana products should have to compete with companies that don’t label chemicals, he said. Murphy is not in the marijuana industry himself, Morrison added.

Other law firms representing Prop. 65 complainants who have noticed marijuana businesses include Bush & Henry of Santa Rosa and Brodsky & Smith of Bala Cynwyd, Pennsylvania, and Beverly Hills.

Lara L. DeCaro, a partner with Leland, Parachini, Steinberg, Matzger & Melnick in San Francisco, represents dispensaries that received Prop. 65 letters.

“Most of it’s bogus,” DeCaro said. Most of the dispensaries had the appropriate signs and won’t be participating in any settlements, she said.

“These lawyers … go after low-hanging fruit,” said DeCaro.

The state issued draft regulations for both medical and recreational marijuana in May that set maximum allowable levels for three of the chemicals most frequently cited in the marijuana-related Prop. 65 letters: carbaryl, malathion and myclobutanil. Edibles, flowers and other marijuana products that exceed those levels would be banned from retail sale.

Those regulations are still being reviewed; revisions that will affect thousands of new recreational-use licensees launching operations in January are expected to be released in the fall. Marijuana products that meet the proposed standards may still need to carry warning labels under Prop. 65.

State law gives the attorney general and certain city attorneys the ability to take action against alleged violators once they are notified by a private complainant. If those agencies don’t take legal action, private-citizen plaintiffs can pursue settlements or litigation.

Although the 60-day notice to public agencies has passed on many of Murphy’s letters, Morrison said he’s waiting to see what state Attorney General Xavier Becerra does before taking any action.

A Becerra spokeswoman said the state Department of Justice “is reviewing the allegations of Proposition 65 violations in the notices and the private enforcer’s basis for sending the notices.” She declined further comment.