Regular heavy marijuana use may increase the risk of developing some head and neck cancers, according to a study published Thursday in the journal JAMA Otolaryngology – Head and Neck Surgery.
Studies have found that frequent cannabis users are 3.5 to 5 times more likely to develop head and neck cancer than those who don’t use it regularly.
Head and neck cancer is cancer of the mouth, throat, and nasal passages and is twice as common in men as in women, according to the Centers for Disease Control and Prevention.
This study adds to a somewhat ambiguous body of evidence that is just beginning to explore the effects of cannabis on cancer risk.
“It’s possible that the cannabinoids themselves are carcinogenic, and it’s possible that the smoke itself is carcinogenic,” said Dr. Niels Kokott, a head and neck surgeon at Keck Medicine of USC and lead author of the study.
Kokot and his team used 20 years of data from a global database called TriNetX, which included patient information from 64 medical centers across the United States. Of the more than 4 million people included, about 116,000 had been diagnosed with cannabis use disorder between April 2004 and April 2024. The diagnosis was based on self-reports of drug use; in other words, patients had to tell their health care providers how often they used cannabis and how it affected them.
The researchers compared people with cannabis use disorder to people of the same age and sex who participated in the study without cannabis use disorder. Most of the people with cannabis use disorder who participated in the study were white. None had a history of head and neck cancer before the study began.
Cannabis users in the study were more likely than non-users to develop all types of head and neck cancer, except for hypopharyngeal cancer (a type of cancer of the lower part of the larynx). The most common types of head and neck cancer in both groups were oral cavity and laryngeal cancer.
The study authors noted several limitations that could skew their findings: First, relying on self-reported data makes it very difficult to measure how many people use cannabis regularly.
To be in that group, people had to tell their health care provider at a consultation that they used marijuana very frequently, the equivalent of smoking one joint a day, said Dr. Joseph Califano, director of the Gleiberman Head and Neck Cancer Center at the University of California, San Diego Moores Cancer Center, who was not involved in the study.
The same goes for alcohol and tobacco use.
“The difficulty with this study, and almost all studies looking at cannabis use, is that there are simply no data,” said Califano, who wrote an editorial published with the new study.
The study also looked at head and neck cancer diagnoses at two time points: within one year of being diagnosed with cannabis use disorder and at least five years after diagnosis, but excluded cancers detected one to four years after a cannabis use disorder diagnosis. This secondary analysis was used to determine the strength of the association. If the association had persisted five years later, it would likely have been strong.
They found that after five or more years, the overall incidence of head and neck cancer remained elevated in those with cannabis use disorder, but the risk of certain types of cancer, such as oral cavity, laryngeal, and oropharyngeal cancer, was no longer significantly increased.
“It makes sense that there would be a cumulative effect over time,” said Dr. Glenn Hanna, a medical oncologist at the Dana-Farber Cancer Institute’s Head and Neck Tumor Center.
It’s still unclear how the amount, frequency or method of cannabis consumption affects cancer risk, but Califano noted that in the new study, the increased risk was associated with self-reported heavy cannabis users, not those who use it occasionally.
Previous studies looking at whether cannabis use increases the risk of head and neck cancer have had mixed results, and the new study highlights the need for continued research on the topic, especially as daily marijuana use becomes more widespread, Hanna said.
Although the study has limitations, “it still suggests there may be some association,” he said.
Cannabis and Cancer Risk
The cannabis plant contains over 100 cannabinoids, including THC and CBD, as well as hundreds of non-cannabinoid chemicals, and scientists are only just beginning to understand the effects, both positive and negative, that these compounds have on the human body.
“Cannabinoids are powerful drugs that have effects at recreational concentrations — you know it because it gets you high,” Califano said, noting that finding a truly pure form of a single cannabinoid like CBD is almost unheard of.
Hanna said smoking anything, including marijuana, activates inflammatory pathways that may play a role in the formation of cancer, and cannabinoids may also be carcinogenic in the body.
“There are biological reasons for the hypothesis that cannabis smoke is harmful, but the extent of the increased risk for head and neck cancer is unclear,” he said, noting that other risk factors, such as alcohol or tobacco use, are hard to pin down. About 20% of study participants with cannabis use disorder reported frequent tobacco or alcohol use, compared with about 2% of those without cannabis use disorder.
With that in mind, edibles may be safer than smoking marijuana, Hanna said, acknowledging that some of his patients use medical marijuana for health issues.
But Califano said that if future studies prove a clear link, there is reason to suspect that some of the cannabinoids themselves, rather than the smoke, may be causing the increased cancer risk.
“Cannabinoids have a variety of effects related to immune response and the development of cancer and the body’s ability to fight cancer,” he says, “so it’s not unreasonable to think that the cancer-related effects of cannabis use would be independent of whether it’s ingested, vaped, or mixed into brownies.”
Hanna and Califano acknowledge that cannabis can have both positive and negative health effects, and as research uncovers more about its effects on the human body, it’s likely that different cannabinoids will prove to have different effects on cancer risk.
“Some of them may be involved in cancer development, and some of them may actually inhibit cancer development,” Califano says.
This article originally appeared on NBCNews.com.
