Even among those heavy, long-term marijuana users, the risk of head and neck cancers including cancer of the tongue, mouth, throat and esophagus does not seem elevated compared with that in those who did not smoke, the study concludes.

    The findings shocked the researchers as they had expected the heavy marijuana users would have to face an increased risk of cancer.

    Donald Tashkin, M.D., Professor of Medicine at the David Geffen School of Medicine at UCLA in Los Angeles and colleagues conducted the study of 611 Los Angeles County lung cancer patients, 601 patients with head and neck cancers and 1,040 people without cancer.

    They found no increased cancer risk for even the heaviest pot smokers. People who smoked more marijuana did not seem to have a higher risk than those who smoked less or none.

    However, the study did find that 80 percent of lung cancer cases and 70 percent of patients with cancer of the head and neck had smoked tobacco, indicating smoking tobacco is linked with cancer risk.

    People who smoked two or more packs of cigarettes a day faced a 20-fold increased risk of developing lung cancer. There is also a dose-response correlation between tobacco smoking and risk of lung cancer and head and neck cancers.

    The current findings regarding the link between marijuana smoke and cancer is contradictory to previous studies in which other researchers found marijuana abuse has the potential to increase the risk of cancer of the lungs and other parts of the respiratory tract because it contains irritants and carcinogens.

    An earlier study concludes that marijuana smoking doubled or tripled the risk of developing cancer of the head or neck.

    The current findings are surprising also because marijuana users usually inhale more deeply and hold their breath longer than tobacco smokers do, which increases the lungs' exposure to carcinogenic smoke.

    Tashkin said one possible explanation is that tetrahydrocannabinol(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.

    Despite the new findings, experts warn, people should stay away from marijuana because smoking marijuana can cause other problems such as respiratory problems, loss of motor skills and increased heart rate. Enditem

    (Agencies)