Marijuana smoking also did not appear to increase the risk of head and neck cancers, such as cancer of the tongue, mouth, throat, or esophagus, the study found.
The findings were a surprise to the researchers. "We expected that we would find that a history of heavy marijuana use--more than 500-1,000 uses--would increase the risk of cancer from several years to decades after exposure to marijuana," said the senior researcher, Donald Tashkin, M.D., Professor of Medicine at the David Geffen School of Medicine at UCLA in Los Angeles.
The study looked at 611 people in Los Angeles County who developed lung cancer, 601 who developed cancer of the head or neck regions, and 1,040 people without cancer who were matched on age, gender and neighborhood. The researchers used the University of Southern California Tumor Registry, which is notified as soon as a patient in Los Angeles County receives a diagnosis of cancer.
They limited the study to people under age 60. "If you were born prior to 1940, you were unlikely to be exposed to marijuana use during your teens and 20s--the time of peak marijuana use," Dr. Tashkin said. People who were exposed to marijuana use in their youth are just now getting to the age when cancer typically starts to develop, he added.
Subjects were asked about lifetime use of marijuana, tobacco and alcohol, as well as other drugs, their diet, occupation, family history of cancer and socioeconomic status. The subjects' reported use of marijuana was similar to that found in other surveys, Dr. Tashkin noted.
The heaviest smokers in the study had smoked more than 22,000 marijuana cigarettes, or joints, while moderately heavy smokers had smoked between 11,000 to 22
Contact: Jim Augustine or Bill Glitz
American Thoracic Society