Large Population-Based Case-Control Study
SAN DIEGO—Despite previous evidence suggesting that smoking marijuana predisposes users to lung cancer, a new large, population-based, case-control study shows no such association even among long-term, heavy users. The data were reported here at the American Thoracic Society Annual Meeting.
Donald Tashkin, MD, Professor of Medicine and Medical Director of the Pulmonary Function Laboratory at UCLA School of Medicine, who presented the results, explained that biochemical and preclinical studies suggest that marijuana smoke might be a risk factor for the development of lung cancer. Heavy habitual use has produced an accelerated, malignant change in lung tissue in culture and caused molecular changes in bronchial biopsy tissue grown in lab experiments.
In addition, marijuana smoke contains several known carcinogens. Smoking marijuana deposits four times as much tar in the lung compared with smoking a similar quantity of cigarettes. And marijuana joints are not filtered and are more loosely packed than cigarettes, so more particles are delivered to the lungs than with tobacco.
Problems with Earlier Research
Most epidemiologic studies have been limited by small numbers of heavy, long-term marijuana users and by sources of possible bias, Dr. Tashkin said. He and his colleagues assessed the possible associations between marijuana use, including heavy, long-term use, and the risk of lung cancer in middle-aged adults living in Los Angeles County.
The researchers conducted a population-based case-control study by identifying lung cancer cases, ages 18 to 59, through rapid ascertainment by the Los Angeles County Cancer Surveillance Program. Controls were matched to cases on age, gender, and neighborhood, which served as a surrogate for socioeconomic status and exposure to pollutants.
Personal, face-to-face interviews were completed for 6 lung cancer cases and 1,040 controls.
Data were collected on lifetime use of marijuana (measured in joint-years), use of tobacco, alcohol, and other drugs, diet, occupation, family history of lung cancer, and exposure to second-hand tobacco smoke.
A logistic regression analysis was used to estimate the effect of marijuana use on lung cancer risk, adjusting for age, gender, race/ethnicity, education, cumulative tobacco smoking, and alcohol use.
Marijuana use was separated out into less than one joint-year, less than 10, 30, and 60 joint-years, and more than 60 joint-years, which translated into more than 22,000 joints for heavy users.
The researchers found no increased risk of lung cancer from marijuana smokers compared with controls, although they did find a 20-fold increased risk of lung cancer among those who smoked two or more packs of cigarettes a day.
“The odds ratio was always less than one for marijuana use, except for lung cancer at the lowest level of use, where the odds ratio was exactly one,” Dr. Tashkin said. “We found no association, not even a suggestion of a positive association, of marijuana use—even heavy, long-term use—and lung cancer after controlling for tobacco smoking and other potential confounders.”
James Jett, MD, Professor of Medicine at Mayo College of Medicine, a lung cancer expert who was not involved with the research, called the study well done. “The results moved me from thinking that there was an association of marijuana smoking with lung cancer to a more neutral position that there is evidence on both sides.”
Epidemiologic Data Remain Conflicted
Yet, epidemiologic data remain conflicted. “There is a case-control study from Tunisia now in press that shows marijuana use does increase the risk of lung cancer,” Dr. Jett said. He also pointed out that a cancer-marijuana link could emerge as baby boomers age and that a large percentage (60%) of the marijuana smokers in Dr. Tashkin's study also smoked cigarettes.
Doesn't Mean Marijuana Is Innocuous
The results should not be taken as a blank check to smoke marijuana, Dr. Tashkin said. Tetrahydrocannibinol, the active ingredient in marijuana, is a known immunosuppressant that interferes with the production of prostimulatory cytokines, including gamma interferon. Marijuana smokers might be at increased risk for respiratory infections, but epidemiologic evidence shows mixed results in the development of pneumonia among marijuana smokers, he said.
Do the results suggest there is a protective effect from smoking marijuana? While the odds ratios were less than one, there is no statistical evidence, such as a dose response, to suggest a protective effect, Dr. Tashkin said.
As a case-control study, it does have limitations, he added. There is the potential of selection bias—60% of the lung cancer cases could not be interviewed because they were not alive, while 80% of controls were alive—and the data were self-reported.
In addition, the patients were under age 69, the average age for lung cancer patients, and therefore more likely to have some genetic abnormality that predisposed them to disease.
Dr. Tashkin pointed out that the toxic substances in marijuana smoke may affect an individual's susceptibility to develop lung cancer. People with a genetic predisposition to developing lung cancer should certainly avoid any assault on the lungs, he said.