Head, Neck and Lung CancersWhile a handful of anecdotal reports and one small case-control study associate heavy marijuana use among younger adults with increased incidents of head, neck and lung cancers, no large scale population studies have replicated these results. Investigators at John Hopkins University in Baltimore reported that neither "lifetime use" nor "ever use" of cannabis were associated with head, neck or lung cancer in younger adults in a large, hospital-based case-control study of 164 oral cancer patients and 526 controls. Researchers concluded, "The balance of evidence from this, the largest case-control study addressing marijuana use and cancer to date, does not favor the idea that marijuana as commonly used in the community is a major causal factor for head, neck or lung cancer in young adults."
More recently, the results of a 2004 population-based case-control study of 407 individuals diagnosed with oral squamous cell carcinoma and 615 healthy controls found "no association" between cannabis use and incidents of oral cancer, regardless of how long, how much or how often individuals had used it. A second 2004 case-control study of 116 oral cancer patients and 207 matched controls also failed to identify any association between self-reported cannabis use and oral cancers in adults age 45 years old or younger, although only 10 percent of patients in the study identified themselves as heavy users of cannabis.
A 1997 retrospective cohort study examining the relationship of marijuana use to cancer incidence in 65,171 men and women 15 to 49 years of age in California found that cannabis use was not associated with increased risks of developing tobacco-use related cancers of the lung and upper aerodigestive tract, and in fact, no cases of lung cancer were identified among men and women who used marijuana but did not smoke tobacco. Critics charge that volunteers in the study were relatively young and that the follow up period was fairly short, arguing that "such a study could not have been expected to detect any relationship between marijuana and lung cancer if the lag period were comparable to that seen with tobacco," which typically occurs after at least 20 years of smoking cigarettes and/or among adults over age 60. The study's author responds: "n contrast to users of tobacco and alcohol, most cannabis users generally quit using cannabis relatively early in their adult lives. ... Therefore, even diseases that might be related to long-term use of cannabis (e.g. lung cancer) are unlikely to have a sizeable public health impact because most people who try cannabis do not become long-term users."
Government reviews investigating a possible link between cannabis use and lung cancer have also failed to find a definitive causal connection between the two. A 1998 report by the British House of Lords Science and Technology Committee concluded, "There is as yet no epidemiological evidence for an increase risk of lung cancer" in cannabis smokers, though authors did concede that studies have revealed cellular changes in the airways of cannabis smokers that could potentially be pre-cancerous. An 18-month study by the US National Academy of Science Institute of Medicine also concluded, "There is no conclusive evidence that marijuana causes cancer in humans, including cancers generally related to tobacco use," but added that cellular studies and a handful of poorly controlled case studies suggest that cannabis smoke may be "an important risk factor" for the development of upper aerodigestive or lung cancers. A 2002 Canadian Senate review further commented that among the small number of case studies present in the literature: "[N]one compare the prevalence of cancer with a control group or evaluates the use of cannabis in a standardized way. Interpretation is also limited by the fact the patients smoked tobacco and drank alcohol."
More recent reviews of the subject published in the journals Alcohol and Lancet Oncology reach similar conclusions. A review of two cohort studies and 14 case-control studies assessing the association of marijuana and cancer risk by Hashibe and colleagues concluded, "[R]esults of cohort studies have not revealed an increased risk of tobacco related cancers among marijuana smokers." Authors did highlight a pair of African case control studies citing marijuana use as a possible elevated risk factor for lung cancer, though they added that investigators failed to assess either the durations of cannabis use or quantify the amount of tobacco used by subjects in conjunction with marijuana. A second 2005 review by Hall and colleagues conclude, "There is a conspicuous lack of evidence on the association between cannabis smoking and lung cancers," and recommends the subject receive additional study.
A large US case-control study funded by the US National Institutes of Health assessing the effects of marijuana smoking on the risks of lung cancer and upper aerodigestive tract cancers among 2,400 Los Angeles County residents less than 60 years of age is ongoing. Preliminary data from the study, presented by investigators at the 2005 annual conference of the International Cannabinoid Research Society (ICRS), report that those who self-reported using moderate levels of cannabis had no greater odds of suffering from lung or UAT cancers than controls.
Source: Cannabis Smoke and Cancer: Assessing the Risk - NORML