Effects of Marijuana Smoking on the Lung

Regular smoking of marijuana by itself causes visible and
microscopic injury to the large airways that is consistently
associated with an increased likelihood of symptoms of chronic
bronchitis that subside after cessation of use. On the other hand,
habitual use of marijuana alone does not appear to lead to
significant abnormalities in lung function when assessed either
cross-sectionally or longitudinally, except for possible increases
in lung volumes and modest increases in airway resistance of
unclear clinical significance. Therefore, no clear link to chronic
obstructive pulmonary disease has been established. Although
marijuana smoke contains a number of carcinogens and
cocarcinogens, findings from a limited number of well-designed
epidemiological studies do not suggest an increased risk for the
development of either lung or upper airway cancer from light or
moderate use, although evidence is mixed concerning possible
carcinogenic risks of heavy, long-term use. Although regular
marijuana smoking leads to bronchial epithelial ciliary loss and
impairs the microbicidal function of alveolar macrophages,
evidence is inconclusive regarding possible associated risks for
lower respiratory tract infection. Several case reports have
implicated marijuana smoking as an etiologic factor in
pneumothorax/pneumomediastinum and bullous lung disease,
although evidence of a possible causal link from epidemiologic
studies is lacking. In summary, the accumulated weight of
evidence implies far lower risks for pulmonary complications of
even regular heavy use of marijuana compared with the grave
pulmonary consequences of tobacco.