Study links heavy marijuana use with risk of head, neck cancers

People who use marijuana at high levels are putting themselves at more than three times the risk for head and neck cancers, new research warns.

The study is perhaps the most rigorous ever conducted on the issue, tracking the medical records of over 4 million U.S. adults for 20 years.

Given the surging popularity of marijuana, cases of head and neck cancers "attributable to cannabis will continue to increase, and perhaps dramatically," wrote a team of oncology experts led by Dr. Joseph Califano of the University of California, San Diego.

Their commentary accompanied the new findings, which were published Thursday in the journal JAMA Otolaryngology-Head & Neck Surgery .

The study was co-led by Dr. Niels Kokot , a professor of clinical otolaryngology-head & neck surgery at the University of Southern California (USC), in Los Angeles.

Head and neck cancers include tumors of the mouth, throat, windpipe and salivary glands. According to the American Cancer Society , these cancers affect more than 58,000 Americans each year, with a death toll of more than 12,000.

Kokot's team noted that cannabis, especially when smoked, is far from harmless to human cells.

Tobacco smoke has long been a big risk factor for head and neck cancers, and Kokot's team noted that "the smoke content of cannabis contains carcinogens similar to those found in tobacco."

There's also evidence that the cannabinoid compounds found in weed could have "tumor-promoting activities," although the jury is out on any definite links to cancer.

The new study focused on patient information from a major database of U.S. medical records.

Kokot's team first looked at over 116,000 records for people diagnosed with problematic "cannabis-related disorder," meaning they used the drug daily.

They compared rates of head and neck cancers in that group to those of another 4 million patients without cannabis use disorder.

"We found that the relative risk of developing head and neck cancer for those with cannabis-related disorders ranged from 3.5 to 5 times that of those without cannabis-related disorders," the researchers reported.

The study couldn't confirm cause-and effect, but the authors noted that risks tended to rise as years spent with a cannabis use disorder increased.

Risks for specific cancers could rise dramatically.

For example, chronic marijuana users had nearly five times the risk or oropharyngeal (tongue, palate, upper throat and tonsils) tumors, and more than eight times the odds for cancers of the larynx (the area of the throat around the voice box).

Smoked marijuana is probably the most carcinogenic means of ingesting the drug, the USC team said, and it might be even more harmful than cigarette smoking .

"Cannabis smoking is typically unfiltered and consumed through deeper breaths than tobacco," they noted.

The findings were enough to convince one head and neck cancer specialist of the dangers.

"Now we know that there is a link between cannabis use and head and neck cancer -- I would say that we've proven that link," said Dr. Michael Blasco . He directs head and neck oncology and reconstruction at Northwell Health in Staten Island, N.Y.

"We don't necessarily know what the threshold is that seems to greatly increase the risk, but we know that heavy users have higher rates, so I would 100 percent tell patients that there's a link between head and neck cancer and cannabis use," he said.

The new study supports the notion that marijuana is far from harmless, Blasco added.

"This is a part of a growing body of literature that's showing that there are measurable negative health effects from cannabis use," he said. "Whether that's effects on the lungs, whether that's effects on the mood and addiction. And then this is part of a growing body of literature that suggests association with cancers. So, we're learning more and more about the long-term health effects of cannabis."

More information

Find out more about marijuana's links to cancer at the American Cancer Society .

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