Marijuana smoke has joined tobacco smoke and hundreds of other chemicals on a list of substances that California regulators say cause cancer.
The ruling Friday by the Office of Environmental Health Hazard Assessment likely will force pot shops with 10 or more employees to post warnings.
Final guidelines are expected by the time warning requirements take effect in a year.
The listing only applies to marijuana smoke, not the plant itself.
Spokesman Sam Delson says the state agency found marijuana smoke contains 33 of the same harmful chemicals as tobacco smoke.
Delson says the findings came from a review of more than 30 scientific papers.
California’s Proposition 65 requires businesses to warn consumers of chemicals that cause cancer or reproductive harm.