Correspondence to: Dr. Donald P. Tashkin, Department of Medicine, David Geffen School of Medicine, University of California Los Angeles (UCLA), 10833 Le Conte Ave., Los Angeles CA 90095-1690, USA; fax 310 206-5088; dtashkin@mednet.ucla.edu

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 9-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.

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