NEW YORK -- People who smoke marijuana may be at less risk of developing lung cancer than tobacco smokers, according to a study presented yesterday.
The study of 2,200 people in Los Angeles found that even heavy marijuana smokers were no more likely to develop lung, head, or neck cancer than nonusers, in contrast with tobacco users, whose risk increases the more they smoke.
The findings seemed to be a surprise; marijuana smoke has some of the same cancer-causing substances as tobacco smoke, often in higher concentrations, said the senior researcher, Donald Tashkin, a professor at the David Geffen School of Medicine at the University of California, Los Angeles. One possible explanation is that THC, or tetrahydrocannabinol, a key ingredient in marijuana not present in tobacco, may inhibit tumor growth, he said.
``You can't give marijuana a completely clean bill of health," said Tashkin, who planned a presentation of the study yesterday before the American Thoracic Society. ``I wouldn't give any smoke substance a clean bill of health. All you can say is we haven't been able to confirm our suspicions that marijuana might be a risk factor for lung and head and neck cancer."
About 1,200 adults under 60 with cancer of the lung, tongue, mouth, throat, or esophagus, took part in the study, as well as about 1,000 without cancer. The study ran from 1999 to 2003.
Marijuana use was found to have been no greater or less in any of the groups, 44 percent of those with lung cancer, 41 percent with head or neck cancers, and 42 percent of those without cancer, Tashkin said.
Other studies had suggested that marijuana smoking was a risk factor for cancer, Tashkin said. Marijuana smokers inhale more deeply than tobacco smokers, and often hold the smoke in their lungs more than four times longer, depositing more tar, he said.
The results of Tashkin's study corroborated some earlier research, said Paul Armentano of the National Organization for the Reform of Marijuana Laws, which advocates legalizing marijuana use. The study was funded by the National Institutes of Health's National Institute on Drug Abuse, Tashkin said.