It seems logical that inhaling enough smoke will give you lung cancer. But a new study of Los Angeles residents suggests that smoking marijuana--even more than 22,000 joints in a lifetime--doesn't increase cancer risk. The results surprise many researchers, who point out marijuana has other ill health effects.
Decades of research have shown that cigarette smoking dramatically increases the risk of certain cancers. But controversy surrounds the risk of smoking weed. A 1999 study of blood donors suggested a link between marijuana and head and neck cancer, but a larger study in 2004 found no such connection. Still, work in the lab suggests marijuana can be dangerous. For example, pot smoke contains more of some cancer-causing chemicals than cigarettes do, thanks to the filterless nature of joints.
In hopes of settling the debate, pulmonologist Donald Tashkin of the University of California, Los Angeles, and colleagues assembled the largest study to date. They identified cancer patients through the University of Southern California Tumor Registry, which compiles cancer data in Los Angeles County. From 1999 to 2003, 611 Los Angelinos age 60 and under came down with lung cancer, and 601 developed head and neck cancers, each a kind of malignancy that smokers would most likely suffer from. The team then identified more than 1000 control individuals in L.A. who did not have cancer. The researchers matched these individuals to cancer patients by age, gender, and other factors such as the neighborhood in which they lived. In confidential interviews, Tashkin's group determined marijuana usage as well as other risk factors for cancer such as cigarette smoking.
Statistical analysis revealed that smoking joints did not increase the risk of coming down with these cancers. About half of cancer patients and controls smoked marijuana, but more than 80% of cancer patients were current or former cigarette smokers. Even heavy tokers--who reported smoking a total of about 22,000 joints over their lifetime--did not have increased risk compared to nonsmokers. The researchers will present their findings tomorrow at the American Thoracic Society International Conference in San Diego.
The results surprised Tashkin. "I wouldn't give [marijuana] a clean bill of health, but at least this study says if there is a risk, it's very small," he says. Still, he says, marijuana has been shown to suppress the immune system and may increase the risk of pneumonia.
As for why marijuana use doesn't seem to increase cancer risk, pulmonary critical care researcher John Hansen-Flaschen of the University of Pennsylvania points out that cigarette smokers puff a lot more cigarettes than do marijuana users--a smoker with a 2-pack-a-day habit lights up 292,000 cigarettes over 20 years, for example. That's probably because marijuana isn't nearly as addictive as tobacco, says epidemiologist Steve Schwartz of the Fred Hutchinson Cancer Research Center in Seattle, who conducted the 2004 study.