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The study included 611 people in the Los Angeles area who developed lung cancer, 601 people with cancer of the head or neck regions, and 1,040 cancer-free people matched on age, gender and neighborhood.
All the study participants were under age 60. They were asked about their lifetime use of marijuana, tobacco, alcohol, other drug use, diet, occupation, family history of cancer, and socioeconomic status. The heaviest marijuana users in the study had smoked more than 22,000 marijuana cigarettes, while moderately heavy users had smoked between 11,000 to 22,000 joints.
People who smoked more marijuana did not have any increased risk of cancer compared to those who smoked less marijuana or none at all, the study found.
The study was expected to be presented Tuesday at the American Thoracic Society International Conference, in San Diego.
The researchers said they were surprised by the findings.
"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," senior researcher Dr. Donald Tashkin, a professor of medicine at the David Geffen School of Medicine, University of California, Los Angeles, said in a prepared statement.
Tashkin said one possible explanation is that THC, a chemical in marijuana smoke, may encourage aging cells to die earlier than normal, reducing the likelihood that they'll transform into cancer cells.
-- Robert Preidt
SOURCE: American Thoracic Society, news release, May 23, 2006
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