Background With increasing cannabis use, physicians need to know more about its respiratory effects. However, there are few long term studies of cannabis smoking, due to legality issues and confounding effects of tobacco.

Aims We reviewed effect of chronic cannabis use on respiratory symptoms and lung function, particularly FEV1, FVC and FEV1/FVC ratio.

Methods 19 out of 256 English-language publications, prior to June 2015, from MEDLINE, Scopus, and Web of Science databases, reporting lung function in chronic cannabis users, were examined.

Results 11 cross-sectional studies and 8 observational cohort studies were included. All 9 studies (n = 11,848) examining respiratory symptoms reported an increase with cannabis smoking (odds ratio up to 3.0). 2 studies (n = 1,336) reported that quitting cannabis with/without tobacco reduced chronic bronchitis symptoms to those of never cannabis smokers.

8 studies (n = 9,939) reported no significant changes in FEV1/FVC; 6 (n = 3,722) 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 (n = 13,858) demonstrated increased FVC with marijuana smoking. 4 studies (n = 13, 674) found dose-related reductions in FEV1/FVC. 7 studies associated chronic cannabis smoking with other evidence of airflow obstruction [increased airway resistance in 3; (0.03 to 0.38 cm H₂O/L/s), reduced specific airway conductance in 4; (0.007 to 0.07 mL/s/cm H₂O/L)].

The larger studies (n = 13,858) suggested increased FVC may cause reduced FEV1/FVC chronically.¹ This contrasts with airflow obstruction in tobacco smoking. Anti-inflammatory or acute bronchodilator effects of cannabis, on top of chronic effects, may partly explain these results.

Conclusions Cannabis, like tobacco, smoking causes chronic bronchitis but increased FVC is more consistently found than reduced FEV1. No studies in marijuana smokers have found a linear decline in FEV1 with time. More work is needed to explain the differing effects on lung function and to examine effects on small airways, imaging and histology.

Reference

1. Hancox RJ, et al. Effect of cannabis smoking on lung function: a population-based cohort study. Eur Respir J 2010;35:42–7.