San Francisco ? It might take Californians a puff or two to get their heads around an apparent contradiction recently enshrined in state law.

The same marijuana smoke that doctors can recommend to ease cancer patients’ suffering must soon come with a warning saying it causes the disease.

State environmental regulators last month voted to place marijuana smoke on its list of hundreds of substances known to cause cancer. The decision could lead to warning signs in medical marijuana dispensaries and labels on packaged pot within a year.

A voter-approved measure made medical marijuana legal in California in 1996. Key backers included patients with serious illnesses such as cancer and AIDS who said pot helped them manage pain and nausea.

Medical marijuana advocates sought to downplay the significance of the state’s decision, arguing researchers have long known that the smoke contains cancer-causing compounds.

“This does not mean in any way that those carcinogens that appear in smoked marijuana, smoked cannabis, have any kind of causal relationship to cancer,” said Kris Hermes, spokesman for Americans for Safe Access, a pro-medical marijuana group.

Regulators disagree. Scientists with the state’s Office of Environmental Health Hazard Assessment reviewed 27 studies of the links between marijuana and cancer in humans. Though not all the studies showed a link, regulators found that “marijuana smoke was clearly shown, through scientifically valid testing according to generally accepted principles, to cause cancer,” according to an agency statement.

Dr. Thomas Mack, a University of Southern California epidemiologist and chairman of the committee, said the decision to list marijuana smoke as a cancer-causing substance should not surprise anyone.

Marijuana smoke and tobacco smoke share 33 of the same cancer-causing compounds, according to agency scientists.

Even so, the existing evidence is merely “suggestive” of a link between marijuana and cancer in humans, Mack said.

To counter the conclusion that smoking marijuana carries major health risks, advocates were quick to jump on the flaws in studies reviewed by the committee.

For instance, regulators reviewed three studies that found connections between marijuana and lung cancer. Of those, two were conducted in North Africa, where smokers regularly mix marijuana with tobacco, a problem the committee acknowledged.

The committee also considered a large 2006 study that found not only did marijuana smokers show no higher risk for cancer than nonsmokers but possibly even less.

“If they want to classify marijuana smoke as carcinogenic, then that is true. It contains carcinogens,” said Donald Tashkin, a longtime University of California, Los Angeles marijuana researcher who led the study. “That doesn’t mean it causes cancer.”

One possible explanation is that marijuana contains chemicals that have an anti-cancer effect that cancels out the carcinogens, though that has not been proven, Tashkin said.

Marijuana supporters have hailed Tashkin’s findings as evidence that pot can actually protect against cancer. He said he doesn’t know whether marijuana has that power or not.

But Tashkin himself believes the carcinogens present in pot mean it will never be approved by federal regulators as medicine.