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Tashkin DP. 
“Does smoking marijuana increase the risk of chronic obstructive pulmonary disease?”. 
CMAJ. 2009 Apr 14;180(8):797-8.
Marijuana is the second most commonly smoked substance worldwide after tobacco.1 The constituents of marijuana smoke are qualitatively and, to a large extent, quantitatively similar to those of tobacco smoke, with the exceptions of Δ9-tetrahydrocannabinol (THC), found only in marijuana, and nicotine, found only in tobacco. Given these similarities, there is concern that the health risks of regular marijuana smoking may be similar to those of habitual tobacco smoking. Chronic obstructive pulmonary disease (COPD), which is associated with high morbidity and mortality, is among those risks.

Over the past 2 decades, studies have addressed the possible relation between smoking marijuana and COPD by systematically assessing respiratory symptoms and measuring lung function in smokers and nonsmokers of marijuana or tobacco.28 These cross-sectional2,3,6 and longitudinal4,5,7 studies have used convenience sampling, random or stratified random sampling or birth cohorts from the general community. One study examined lung structure using thoracic high-resolution computed tomography to identify macroscopic emphysema.8

In this issue of CMAJ, Tan and colleagues report the findings of a cross-sectional population-based study of the possible association between smoking marijuana and risk of COPD.9 Whereas previous studies have consistently reported an association between use of marijuana and chronic respiratory symptoms even in the absence of concomitant use of tobacco,2,3,6,8 Tan and colleagues did not report a similar finding for marijuana only. However, they found that the concurrent use of marijuana and tobacco appeared to synergistically increase respiratory symptoms and risk of COPD.
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