Using marijuana daily for years may raise the overall risk of head and neck cancers three- to five-fold, according to a new study that analyzed millions of medical records.“Our research shows that people who use cannabis, particularly those with a cannabis use disorder, are significantly more likely to develop head and neck cancers compared to those who do not use cannabis,” said senior study author Dr. Niels Kokot, a professor of clinical otolaryngology-head and neck surgery at the Keck School of Medicine at the University of Southern California in Los Angeles.Cannabis use disorder is diagnosed when a person has two or more of such symptoms as craving weed, becoming tolerant to its effects, using more than intended, using marijuana even though it causes problems in life, using it in high-risk situations, experiencing withdrawal and being unable to quit, according to the US Centers for Disease Control and Prevention.“While our study did not differentiate between methods of cannabis consumption, cannabis is most commonly consumed by smoking,” Kokot said in an email. “The association we found likely pertains mainly to smoked cannabis.”Some 69% of people with a diagnosis of oral or throat cancer will survive five years or longer after their diagnosis, according to the National Cancer Institute. If the cancer metastasizes, however, that rate drops to 14%. About 61% of people diagnosed with cancer of the larynx will be alive five years later — a rate that drops to 16% if the cancer spreads.The study used insurance data to look at the association of cannabis use disorder with head and neck cancers, said Dr. Joseph Califano, the Iris and Matthew Strauss Chancellor’s Endowed Chair in Head and Neck Surgery at the University of California, San Diego. He was not involved in the study.“The researchers used a huge, huge dataset, which is really extraordinary, and there is enormous power in looking at numbers this large when we typically only see small studies,” said Califano, who is also the director of UC San Diego’s Hanna and Mark Gleiberman Head and Neck Cancer Center.“On average, people with cannabis use disorder smoke about a joint today and do so for at least a couple years, if not longer,” said Califano, who coauthored an editorial published Thursday in JAMA Otolaryngology–Head & Neck Surgery in conjunction with the new study.However, he added, the study does not find an association between “the occasional recreational use of marijuana and head and neck cancer.”Causes of head and neck cancersIn the United States, head and neck cancers make up 4% of all cancers, with more than 71,000 new cases and more than 16,000 deaths expected in 2024, according to the National Foundation for Cancer Research.Tobacco use, which includes smoking cigarettes, cigars, pipes and smokeless tobacco, and the use of alcohol are the two most common causes of head and neck cancers, experts say. Other risk factors include poor oral hygiene; gastroesophageal reflux disease, or GERD; a weakened immune system; and a diet low in fruits and vegetables. Occupational risk factors include exposure to asbestos and wood dust.A growing number of head and neck cancers are due to infection with the human papillomavirus or HPV, or the Epstein-Barr virus, or EBV, which are DNA viruses that affect genes, making them susceptible to malignancies.Epstein-Barr virus is linked to infectious mononucleosis, also called the “kissing disease,” as well as various cancers. Researchers estimate that 90% of the world’s population is infected with EBV. A vaccine is available for HPV, which is linked to a high risk of developing cervical cancer and some non-Hodgkin lymphomas.It’s possible to be infected with both viruses at once, and that combination is responsible for 38% of all virus-associated cancers, according to research.How might cannabis cause cancers?The study, published Thursday in JAMA Otolaryngology–Head & Neck Surgery, analyzed a database of 4 million electronic health records and found more than 116,000 diagnoses of cannabis use disorder among people with head and neck cancers. Those men and women, whose average age was 46, were then matched with people who also had head and neck cancers but were not diagnosed with cannabis use disorder.The analysis showed that people with cannabis use disorder were about 2.5 times more likely to develop an oral cancer; nearly five times more likely to develop oropharyngeal cancer, which is cancer of the soft palate, tonsils and back of the throat; and over eight times more likely to develop cancer of the larynx. The findings held true for all age groups, according to the study.Smoke is one key reason for the association between smoking weed and head and neck cancers, experts say. Smoking tobacco doubles the risk of developing head and neck squamous cell carcinoma, which makes up 90% of all head and neck cancers. Heavier tobacco use leads to even higher risk, and when alcohol is added, the risk is even greater.Due to the way marijuana is smoked — unfiltered and breathed in deeply and held in the lungs and throat for a few seconds — the risk from cannabis smoke could be even greater, experts say.Another key to the puzzle of how cannabis causes cancer: Research has found a link between various cannabinoids and tumor growth. There are more than 100 cannabinoids — biological compounds in the cannabis plant that bind to cannabinoid receptors in the human body, according to the National Center for Complementary and Integrative Health. All told, there are about 540 chemicals in each marijuana plant.Tetrahydrocannabinol, or THC, is the substance that makes one euphoric, while cannabidiol, or CBD, has been shown to have medicinal uses for childhood seizures and epilepsy.“Part of the research we have already published shows that THC or THC-like compounds can certainly accelerate tumor growth,” Califano said. “We also have some data to show that cannabinoids enhance the growth of HPV-related throat cancers.”With the use of marijuana rising, there is concern that diagnoses of head and neck cancers could increase in coming years, Kokot said.“Especially as (marijuana) becomes more widely legalized and socially accepted, we may see a corresponding rise in head and neck cancer cases if the association is confirmed,” he said.“This underscores the importance to inform people about the potential risks and conduct further research to understand the long-term impacts of cannabis use on cancer development.”