Researchers at UCLA's Jonsson Cancer Center are reporting, for the first time, that smoking marijuana may increase the risk of head and neck cancers.

Results of an epidemiological study of more than 340 people are outlined in an article published in today's (Dec. 17) edition of the peer-reviewed journal Cancer Epidemiology Biomarker and Prevention.

Previous laboratory and clinical studies have indicated that marijuana use may be related to molecular alterations in the respiratory tract, changes that may lead to cancer. This is the first study to examine whether smoking marijuana increases risk of head and neck cancers, said Dr. Zuo-Feng Zhang of UCLA's Jonsson Cancer Center, a professor in the Department of Epidemiology in the UCLA School of Public Health and director of the cancer epidemiology training program at UCLA.

"Most people don't think about marijuana in relationship to cancer," said Zhang, lead author of the journal article. "The carcinogens in marijuana are much stronger than those in tobacco. The big message here is that marijuana, like tobacco, can cause cancer."

Zhang studied the relationship between marijuana use and head and neck cancers in 173 patients diagnosed with those diseases. He compared those findings to 176 cancer-free control patients, and found that those who habitually smoked marijuana were at higher risk for head and neck cancers.

The epidemiological data was collected using a structured questionnaire, which queried patients about their histories of tobacco smoking, marijuana smoking and alcohol use. Zhang said researchers were able to evaluate the data on marijuana smoking independently from data on tobacco smoking and alcohol use, which also increase the risk of certain cancers.

The results of the study are particularly important now, Zhang said, as habitual marijuana smokers from the 1960s reach older ages. Because head and neck cancers -- cancers of the mouth, tongue, larynx and pharynx -- take many years to develop, people who smoked large amounts of marijuana in the 1960s may just now be contracting head and neck cancers, Zhang said.

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"In the '60s, we had very high numbers of people in their 20s smoking marijuana," Zhang said. "These people are just now getting to the ages at which they will get head and neck cancers. This is the time to study a risk like this."

The more times per day a person smokes marijuana, the greater his or her risk of head and neck cancers, according to the study. Additionally, people who use marijuana habitually for many years also increase their risk of head and neck cancers, Zhang said.

"If you smoke a little, your risk increases a little," Zhang said. "If you smoke a lot, your risk increases a lot."

Marijuana is the most commonly used illegal drug in the United States, Zhang said. It is estimated that about 31 percent of the U.S. population 12 years or older has used marijuana, according to the journal article.

Zhang's research builds on previous studies of marijuana and cancer risk. An article by UCLA cancer researchers published in the Aug. 19, 1998, issue of the Journal of the National Cancer Institute stated that habitual smoking of marijuana and crack cocaine causes the same kinds of molecular changes that precede the development of lung cancer in cigarette smokers.

"Now we have evidence that may link marijuana smoking to head and neck cancers," Zhang said. "Many people may think marijuana is harmless, but it's not."

In addition, the epidemiological study and the subsequent journal article also touch on the interplay between marijuana smoking and the genetic defect that prevents DNA from repairing itself. Some marijuana smokers with this genetic defect might not have the ability to repair DNA damage prompted by the habit. Zhang said these people are about 16 times more likely to develop head and neck cancers than non-marijuana smokers whose DNA repair function is operating normally.

Zhang said larger epidemiological studies are needed to replicate the results obtained by UCLA cancer researchers. One such study, funded by the National Institutes of Health, is being conducted now at UCLA.