Researchers at UCLA's Jonsson Cancer Center are reporting, for the firsttime, that smoking marijuana may increase the risk of head and neck cancers.
Results of an epidemiological study of more than 340 people are outlinedin an article published in the Dec. 17 edition of the peer-reviewed journalCancer Epidemiology Biomarker and Prevention.
Previous laboratory and clinical studies have indicated that marijuanause may be related to molecular alterations in the respiratory tract, changesthat may lead to cancer. This is the first study to examine whether smokingmarijuana increases risk of head and neck cancers, said Dr. Zuo-Feng Zhangof UCLA's Jonsson Cancer Center, a professor in the Department of Epidemiologyin the UCLA School of Public Health and director of the cancer epidemiologytraining program at UCLA.
"Most people don't think about marijuana in relationship to cancer,"said Zhang, lead author of the journal article. "The carcinogens inmarijuana are much stronger than those in tobacco. The big message hereis that marijuana, like tobacco, can cause cancer."
Zhang studied the relationship between marijuana use and head and neckcancers in 173 patients diagnosed with those diseases. He compared thosefindings to 176 cancer-free control patients, and found that those whohabitually 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, marijuanasmoking and alcohol use. Zhang said researchers were able to evaluate thedata on marijuana smoking independently from data on tobacco smoking andalcohol 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. Becausehead and neck cancers - cancers of the mouth, tongue, larynx and pharynx- take many years to develop, people who smoked large amounts of marijuanain the 1960s may just now be contracting head and neck cancers, Zhang said.
"In the '60s, we had very high numbers of people in their 20s smokingmarijuana," Zhang said. "These people are just now getting tothe ages at which they will get head and neck cancers. This is the timeto study a risk like this."
The more times per day a person smokes marijuana, the greater his orher risk of head and neck cancers, according to the study. Additionally,people who use marijuana habitually for many years also increase theirrisk of head and neck cancers, Zhang said.
"If you smoke a little, your risk increases a little," Zhangsaid. "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. population12 years or older has used marijuana, according to the journal article.
Zhang's research builds on previous studies of marijuana and cancerrisk. An article by UCLA cancer researchers published in the Aug. 19, 1998,issue of the Journal of the National Cancer Institute stated that habitualsmoking of marijuana and crack cocaine causes the same kinds of molecularchanges that precede the development of lung cancer in cigarette smokers.
"Now we have evidence that may link marijuana smoking to head andneck cancers," Zhang said. "Many people may think marijuana isharmless, but it's not."
In addition, the epidemiological study and the subsequent journal articlealso touch on the interplay between marijuana smoking and the genetic defectthat prevents DNA from repairing itself. Some marijuana smokers with thisgenetic defect might not have the ability to repair DNA damage promptedby the habit. Zhang said these people are about 16 times more likely todevelop head and neck cancers than non-marijuana smokers, whose DNA repairfunction is operating normally.
Zhang said larger epidemiological studies are needed to replicate theresults obtained by UCLA cancer researchers. One such study, funded bythe National Institutes of Health, is being conducted now at UCLA.