Their study, published in the December 1999 issue of the journal Cancer Epidemiology, Biomarkers & Prevention, is not the first to link marijuana to such cancers. Earlier research has shown marijuana cigarettes contain more tar and higher levels of certain cancer-causing chemicals than tobacco cigarettes. DNA mutations have been found in respiratory system cells of marijuana users and several case reports have found an unexpectedly high number of marijuana users among patients with cancers of the head and neck region, including the mouth, tongue, throat, and larynx.

This study, however, is the first to systematically compare head and neck cancer risk among marijuana users and non-users. The study looked at the relationship between marijuana use and head and neck cancers in 173 patients diagnosed with those diseases who were compared with 176 cancer-free patients.

Carcinogens in Marijuana

"This is the first [epidemiological] study to examine whether smoking marijuana increases risk of head and neck cancers," said Zuo-Feng Zhang, MD, lead author of the study and member of the Jonsson Cancer Center at the University of California Los Angeles. "Most people don’t think about marijuana in relationship to cancer. The carcinogens in marijuana are stronger than those in tobacco. The big message here is marijuana, like tobacco, can cause cancer."

According to the study, the more times per day a person smokes marijuana, the greater the risk of head and neck cancers. People who use marijuana habitually for many years increase their risk of head and neck cancers substantially. "If you smoke a little, your risk increases a little," Dr. Zhang said. "If you smoke a lot, your risk increases a lot." The average user in this study had two to three times the risk of non-users.

The results of the study are particularly important now, as people who became habitual marijuana smokers in the 1960s are reaching 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 developing head and neck cancers," Dr. Zhang said.

ACS Expert Agrees

Michael Thun, MD, vice president of epidemiology and surveillance research for the American Cancer Society (ACS), agreed the study strongly suggests habitual marijuana smoking increases the risk of head and neck cancers. "This is not surprising since many of the same cancer causing substances in tobacco smoke are present in marijuana smoke. Marijuana cigarettes generally deposit more tar in the respiratory tract than tobacco cigarettes," he said.

Marijuana is the most commonly used illegal drug in the US, according to the study. It is estimated about 31 percent of the US population 12 years or older has used the drug.

The study builds on previous studies of marijuana and cancer risk. The authors conclude large studies are needed to further examine the relationship between marijuana and increased risk of cancer.

The ACS estimates about 36,000 cancers of the mouth, tongue, throat, and larynx will be diagnosed this year in the US and about 10,000 people will die of these cancers. In addition to marijuana use, the main risk factors for these cancers are tobacco and alcohol use.