Marijuana and the lung: hysteria or cause for concern?

Increasing cannabis use and legalisation highlights the paucity of data we have on the safety of cannabis smoking for respiratory health. Unfortunately, concurrent use of tobacco among marijuana smokers makes it difficult to untangle individual effect of marijuana smoking. Chronic cannabis only smoking has been shown in large cohort studies to reduce forced expiratory volume in 1 s/forced vital capacity via increasing forced vital capacity in chronic use contrary to the picture seen in tobacco smoking. The cause of this is unclear and there are various proposed mechanisms including respiratory muscle training secondary to method of inhalation and acute anti-inflammatory effect and bronchodilation of cannabis on the airways. While cannabis smoke has been shown to increase symptoms of chronic bronchitis, it has not been definitively shown to be associated with shortness of breath or irreversible airway changes. The evidence surrounding the development of lung cancer is less clear; however, preliminary evidence does not suggest association. Bullous lung disease associated with marijuana use has long been observed in clinical practice but published evidence is limited to a total of 57 published cases and only one cross-sectional study looking at radiological changes among chronic users which did not report any increase in macroscopic emphysema. More studies are required to elucidate these missing points to further guide risk stratification, clinical diagnosis and management. KEY POINTS: Cannabis smoking has increased and is likely to increase further with relaxation of legalisation and medicinal use of cannabinoids. Chronic marijuana smoking often produces symptoms similar to those of chronic tobacco smoking such as cough, sputum production, shortness of breath and wheeze. Cessation of marijuana smoking is associated with a reduction in respiratory symptoms and no increased risk of chronic bronchitis. Spirometry changes seen in chronic marijuana smokers appear to differ from those in chronic tobacco smokers. In chronic marijuana smokers there is an increase in FVC as opposed to a definite decrease in FEV(1). Multiple case series have demonstrated peripheral bullae in marijuana smokers, but no observational studies have elucidated the risk. There is currently no clear association between cannabis smoking and lung cancer, although the research is currently limited. EDUCATIONAL AIMS: To update readers on legalisation of recreational and medicinal cannabis. To summarise the evidence base surrounding the respiratory effects of inhaled marijuana use. To provide clinicians with an understanding of the main differences between cannabis and tobacco to be able to apply this to patient education. To highlight common respiratory problems among cannabis users and the need for recreational drug history taking.