Health & Science

Health series: Does marijuana cause lung cancer? Is secondhand smoke safe?

An elderly man has chronic pain that keeps him up at night, so he smokes marijuana most nights before he and his wife go to bed. But is he putting one or both of them at risk?

There’s a growing body of research that suggests marijuana can help with conditions such as nausea and pain while posing only modest health risks for adults. But there are broader public health questions to consider, including the health risks associated with secondhand smoke.

“We often hear there are no negative effects,” said Kevin Alexander, who works with addicts at Hoag Hospital’s ASPIRE program in Newport Beach. “But we need more research and information on how it would affect us as a community and the societal impacts.”

The Cannifornian is continuing an occasional series that surveys current research and interviews experts on common questions about the health impacts of marijuana use. Have a question you’d like us to consider? Email tips@thecannifornian.com.

Q: Does smoking marijuana cause lung cancer?

A: The cancer link appears increasingly weak, though more research is needed.

Marijuana contains many compounds also found in tobacco, including some known to cause cancer. That’s triggered reports suggesting that smoking marijuana must be more dangerous, since it’s typically inhaled more deeply and held in the lungs longer than tobacco.

(Nhat V. Meyer/Bay Area News Group Photo Illustration)

However, the research so far suggests otherwise.

A comprehensive 2014 study in the International Journal of Cancer found “little evidence for an increased risk of lung cancer,” even among heavy or long-term cannabis smokers. Those results are buoyed by a number of other large studies, including an examination out of UCLA in 2006 that was funded by the National Institute on Drug Abuse.

One possible explanation is that marijuana users typically don’t smoke as often as tobacco users.

Dr. Donald Tashkin, a pulmonologist who led the UCLA study, suggested marijuana doesn’t pose the same cancer risk as tobacco because THC has been found to slow the growth of some cancers.

THC is known to reduce inflammation, too, Tashkin points out, which may explain why there also doesn’t appear to be a link between marijuana and chronic obstructive pulmonary disease, which plagues cigarette smokers.


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Several studies have shown a correlation between heavy marijuana smoking and other respiratory conditions, such as chronic bronchitis.

There’s an alternative, advocates say: Don’t smoke it. While edibles and concentrates come with their own set of risks, lung issues aren’t among them.

Q: Is secondhand pot smoke dangerous?

A: The jury is still out, but experts say best to avoid it if possible.

First, there’s the potential – albeit a small one – for a “contact high.”

Nonsmokers who were in a car or other small, unventilated space with heavy marijuana smokers showed some of the same temporary, minor memory and coordination problems as the smokers themselves, according to a study out of Johns Hopkins University School of Medicine. Some exposed nonsmokers even tested positive for the drug.

Those are considered extreme conditions. The National Institute on Drug Abuse notes that a contact high is highly unlikely, since very little THC, or the psychoactive compound in cannabis, is exhaled back into the air.

The more serious concern is whether secondhand pot smoke poses the same deadly risks as exposure to secondhand tobacco smoke. That’s where the science isn’t settled, though research largely points to no.

As Tashkin’s studies above show, even firsthand marijuana smoke doesn’t seem to pose an increased risk of lung cancer or chronic pulmonary obstruction disease as cigarette smoke.

(Jane Tyska/Archive)

However, a 2014 study on rats by cardiovascular researcher Matthew Springer at UC San Francisco found secondhand marijuana smoke restricts blood vessels much like tobacco smoke. That can increase chances of a heart attack, particularly for people who have other risk conditions.

California law says people can’t smoke medical or recreational marijuana in areas where tobacco is banned or within 1,000 feet of school or youth centers. And Prop. 64 bans recreational consumption in public.

To be safe, experts recommend making sure there’s good ventilation if you’re around marijuana smokers. Or suggest other methods of ingestion, such as vaporizing, edibles or tinctures.