Respiratory effects of cannabis smoking: A literature review

Background: With increasing cannabis use, physicians need to know more about its respiratory effects. However, there are few long term studies of cannabis smoking, mostly due to legality issues and the confounding effects of tobacco. Aims: We reviewed the effect of chronic cannabis use on lung function, particularly FEV 1 , FVC and FEV 1 /FVC ratio. Methods: 19 out of 256 English-language publications, prior to June 2015, from MEDLINE, Scopus, and Web of Science databases, which reported lung function in chronic cannabis users, were examined. Results: 8 studies reported no significant changes in FEV 1 /FVC ratio, while 6 found a significant decrease (0.5%-1.9%) in chronic marijuana only smokers compared to controls. While most reports omitted absolute FVC results, 3 large studies demonstrated increased FVC with marijuana smoking. 4 studies found a dose-related reduction in FEV 1 /FVC. All 8 studies examining respiratory symptoms reported an increase with cannabis smoking (odds ratio up to 3.0). The larger studies suggested that increased FVC may cause the reduced FEV 1 /FVC with chronic marijuana use (Hancox, et al. ERJ 2010;35:42-7). This contrasts starkly with airflow obstruction in tobacco smoking. Also, FEV 1 appears to have a non-linear relationship with cannabis smoking: increasing up to 7 joint-years, but declining afterwards (Pletcher, et al. JAMA 2012;307:173-81). Acute bronchodilator effects of cannabis, on top of chronic effects, may partly explain these results. Conclusions: Cannabis, like tobacco, smoking causes chronic bronchitis but more work is needed to explain the different effects on lung function and address issues such as effects on small airways, imaging and histology.