Two NIDA-funded studies identify health risks that
underscore the importance of curbing marijuana abuse.

BY PATRICK ZICKLER, NIDA Notes Contributing Writer

A large new epidemiological study suggests that marijuana smoke can cause the same types of respiratory damage as tobacco smoke. Significant associations between marijuana smoking and a variety of respiratory diseases also have been confirmed by an extensive review of clinical literature.

MONITORING THE EFFECTS OF TOBACCO AND MARIJUANA

Dr. Brent Moore and colleagues at Yale University, the National Cancer Institute, and the University of Vermont evaluated data from a nationally representative sample of 6,728 adults. Their analysis indicated that a history of more than 100 lifetime episodes of smoking marijuana, with at least one episode in the past month, increased an individual's risk of chronic bronchitis, coughing on most days, wheezing, chest sounds without a cold, and increased phlegm.

"The most significant difference between tobacco smoke and marijuana smoke is their principal active ingredients—nicotine in tobacco and delta-9-tetrahydrocannabinol (THC) in marijuana. Beyond that, marijuana contains at least as much tar and half again as many carcinogens as smoke from conventional tobacco," says Dr. Moore. "Quitting marijuana smoking may benefit respiratory health as much as quitting cigarettes, in addition to the clear and considerable health, psychological, and social benefits of no longer abusing an illicit drug."

The information Dr. Moore and his colleagues analyzed was gathered through the third National Health and Nutrition Examination Survey (NHANES III), conducted between 1988 and 1994. Participants included 4,789 nonsmokers of either tobacco or marijuana; 1,525 smokers of tobacco but not marijuana; 320 smokers of both marijuana and tobacco; and 94 who smoked marijuana only. On average, marijuana abusers had smoked the drug on 10 of the preceding 30 days, with 16 percent reporting daily or almost daily smoking. Tobacco smokers consumed roughly the same number of cigarettes—averaging 19.2 per day—whether or not they also smoked marijuana. Survey participants answered questions about their experiences of a range of respiratory symptoms and were examined for signs of respiratory abnormalities.

RESEARCHERS CITE PUBLIC HEALTH IMPACT More than 2 million adult Americans are heavy marijuana smokers, and increased efforts to prevent marijuana use may have significant public health benefits.

The researchers concluded that tobacco smokers who also smoked marijuana had a higher prevalence of most respiratory symptoms than tobacco-only smokers. Compared with tobacco-only smokers, however, those who also smoked marijuana were less likely to have had pneumonia during the previous year or to show spirometric evidence of obstructive pulmonary disorder. Commenting on this finding, Dr. Moore says that it is important to note that the marijuana smokers in the sample were significantly younger (average age 31.2 years) than the tobacco smokers (average age 41.5 years). "The marijuana-related respiratory effects correspond to a relatively young population, and NHANES III did not ask participants older than age 59 about drug use," he adds. "It is likely that respiratory effects will be higher in older marijuana smokers, and, because of the high prevalence of tobacco use among marijuana smokers, there appears to be an increased risk for illness due to cumulative effects of smoking both drugs."

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MARIJUANA'S LONG-TERM PULMONARY EFFECTS

Further evidence of marijuana's respiratory toxicity emerged from a study conducted by Dr. Donald Tashkin at the University of California, Los Angeles. Dr. Tashkin conducted an extensive review of clinical and epidemiological research to determine the extent to which chronic marijuana smoking might lead to long-term pulmonary effects and diseases similar to those caused by tobacco. Unlike the NHANES III data examined by Dr. Moore, the studies evaluated by Dr. Tashkin made it possible to assess a possible association between marijuana smoking and respiratory cancers.

The results of animal and cell culture studies are mixed with respect to the carcinogenic effects of THC, some studies showing that THC promotes lung cancer growth and others showing an anti-tumoral effect on a variety of malignancies. Although the results of epidemiological studies are also mixed, a large, recently completed case-control study has failed to find a direct link between marijuana use (including heavy use) and lung, throat, or other head and neck cancers. "Nevertheless, there is evidence that suggests precarcinogenic effects in respiratory tissue," Dr. Tashkin says. "Biopsies of bronchial tissue provide evidence that regular marijuana smoking injures airway epithelial cells, leading to dysregulation of bronchial epithelial cell growth and eventually to possible malignant changes." Moreover, he adds, because marijuana smokers typically hold their breath four times as long as tobacco smokers after inhaling, marijuana smoking deposits significantly more tar and known carcinogens within the tar, such as polycyclic aromatic hydrocarbons, in the airways. In addition to precancerous changes, Dr. Tashkin found that marijuana smoking is associated with a range of damaging pulmonary effects, including inhibition of the tumor-killing and bactericidal activity of alveolar macrophages, the primary immune cells within the lung.

Taken together, Dr. Tashkin's survey of clinical and epidemiological studies and Dr. Moore's assessment of self-reported and clinically observed effects provide an extensive catalog of respiratory and pulmonary damage associated with marijuana smoking. Smokers are subject to:

• Coughing and phlegm production on most days;
• Wheezing and other chest sounds;
• Acute and chronic bronchitis;
• Injury to airway tissue, including edema (swelling), increased vascularity, and increased mucus secretion; and
• Impaired function of immune system components (alveolar macrophages) in the lungs.

SOURCE

Moore, B.A., et al. Respiratory effects of marijuana and tobacco use in a U.S. sample. Journal of General Internal Medicine 20(1):33-37, 2005. [Full Text]

Tashkin, D.P. Smoked marijuana as a cause of lung injury. Monaldi Archives for Chest Disease 63(2):93-100, 2005. [Abstract]

Hashibe, M., et al. Marijuana use and aerodigestive tract cancers: a population-based case control study. Cancer Epidemiology, Biomarkers & Prevention (In Press).

Volume 21, Number 1 (October 2006)