Joints and baggies sold at California’s medical marijuana dispensaries will soon carry a new warning label. Next to tags like “Purple Haze” and “White Widow” will be the advisory: Contents may cause cancer when smoked.

On Friday, California added marijuana smoke to its official list of known carcinogens, joining the ranks of arsenic, asbestos and DDT. Pot brownies, lollipops and other non-inhalables are not affected by the new ruling.

Scientists found the pungent smoke shares many of the same harmful properties as tobacco smoke, warranting its inclusion on the Proposition 65 warning list. The law requires the state to publish a list of chemicals known to cause cancer or reproductive toxicity, and businesses and government agencies must post warnings when they use such chemicals or sell products containing them.

“Marijuana smoke is a mixture of different chemicals, and a number of those were already on the Prop. 65 list,” said Allan Hirsch, chief deputy director of the state Office of Environmental Health Hazard Assessment, which made the designation.

From a health perspective, pot advocates say the ruling was not surprising, given the state’s track record on documenting the harmful effects of all kinds of smoke inhalation. However, some are worried about its political implications as advocates attempt to legalize recreational use of marijuana.

“I definitely have concerns about law enforcement and politicians who aren’t in favor of medical marijuana or law reform, who would use this designation to further restrict access,” said Allen St. Pierre, executive director of NORML, the National Organization for the Reform of Marijuana Law, based in Washington, D.C. “This will be a vexing series of conversations and negotiations.”

Though marijuana smoke was added to the list Friday, the labeling requirements won’t go into effect until June 19, 2010. Only medical marijuana dispensaries with 10 employees or more are required to post a warning either in their shop or on the products themselves. Violations of the law carry a fine up to $2,500 a violation per day.

The panel of scientists at the state Office of Environmental Health Hazards Assessment reviewed an extensive body of research finding links between marijuana smoke and cancer, particularly head and neck cancers.

“There’s not one single piece of evidence that was a slam dunk,” said Dr. George Alexeeff,﻿ deputy director for scientific affairs at the OEHHA. “But the pieces together form a very compelling argument.”

The panel did not consider studies showing medical benefits of marijuana, like reducing nausea and restoring appetite after chemotherapy or slowing the progression of glaucoma.

“Singling out marijuana is gratuitous,” said Assemblyman Tom Ammiano, D-San Francisco, who introduced a bill to legalize recreational marijuana. “Many, many symptoms of disease can be alleviated through smoking marijuana.”

St. Pierre says the ruling may factor into the larger philosophical debate about the role public health officials may play in adult choices. He hopes the national conversation will turn to minimizing the harmful consequences of smoking pot, rather than aiming for a utopian drug-free society.

Because the ruling highlighted the harmful effects of smoking pot, St. Pierre believes it may accelerate what he calls the “pharmaceuticalization” of the drug: marijuana pills, arm patches, under-tongue sprays and suppositories. His organization has even paid for studies showing the benefits of the vaporizer, a device that heats marijuana to a very high temperature to release THC, the active substance in marijuana, in a vapor that is cleaner and more pure than smoke.

“In years to come, kids won’t be smoking blunts or bongs, they’re going to be vaporizing,” he said, remembering a young man who recently asked to take his photo with him. “He wore a T-shirt that said ‘Got Vape?’ “

Contact April Dembosky at adembosky@mercurynews.com or 408-920-5064.