People who smoke marijuana--even heavy, long-term marijuana users--do not appear to be at increased risk of developing lung cancer, according to a study to be presented at the American Thoracic Society International Conference on May 23rd. . . . 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 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 22,000 joints. Even these smokers did not have an increased risk of developing cancer. People who smoked more marijuana were not at any increased risk compared with those who smoked less marijuana or none at all. . . . The study found a 20-fold increased risk of lung cancer in people who smoked two or more packs of cigarettes a day. The more tobacco a person smoked, the greater the risk of developing both lung cancer and head and neck cancers, findings that were consistent with many previous studies.
An abstract of the study can be found here, with similar results were found in a Kaiser Permanente study whose abstract is here, and a third study in Washington State.
The investigators do not know why marijuana smoking does not cause cancer as they had expected based on existing models of harm caused by tobacco. A meta-analysis of a number of studies examining the health impacts of marijuana can be found here.
The results of the study were previously reported in 2005 at the June 26, 2005 annual conference of the International Cannabinoid Research Society (ICRS). The same researcher who conducted the Los Angeles, while not finding a link to cancer, has found a link between marijuana use and other respiratory ailments in another study.
Hat Tip to Coyote Gulch