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Effects of Marijuana on the Lung and its Defenses against Infection and Cancer

Department of Medicine, UCLA School of Medicine, Los Angeles, USA

Habitual marijuana use may lead to the following effects on the lung: (1) acute and chronic bronchitis; (2) extensive microscopic abnormalities in the cells lining the bronchial passages, some of which may be pre-malignant; (3) overexpression of molecular markers of progression to lung cancer in bronchial tissue; (4) abnormally increased accumulation of inflammatory cells (alveolar macrophages) in the lung; and (5) impairment in the function of these immune-effector cells (reduced ability to kill micro-organisms and tumour cells) and in their ability to produce protective inflammatory cytokines. Clinically, the major pulmonary consequences that may ensue from regular marijuana use are pulmonary infections and respiratory cancer. Infections of the lung are more likely in marijuana users due to a combination of smoking-related damage to the ciliated cells in the bronchial passages (the lung's first line of defense against inhaled micro-organisms) and marijuana-related impairment in the function of alveolar macrophages (the principal immune cells in the lung responsible for defending it against infection). Patients with pre-existing immune deficits due to AIDS or cancer chemotherapy might be expected to be particularly vulnerable to marijuana-related pulmonary infections. Furthermore, biochemical, cellular, genetic, animal and human studies all suggest that marijuana may be an important risk factor for the development of respiratory cancer. However, proof that habitual use of marijuana does in fact lead to respiratory cancer must await the results of well-designed case-control epidemiologic studies that should now be feasible after the passage of 30 years since the initiation of widespread marijuana use among young individuals in our society in the mid-1960s.

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