Investigators at St. Paul's Hospital in Vancouver, British Columbia, surveyed a random sample of 878 people aged 40 years or older about their respiratory history and their history of tobacco and marijuana smoking.

Researchers reported that the concurrent use of tobacco and marijuana was associated with an increased risk of COPD. They determined that cigarette-only smokers were approximately two-and-a-half times more likely than nonsmokers to develop COPD, while those subjects who reported smoking both substances were approximately three times more likely to develop the disease.

"The risks of respiratory symptoms and of COPD were related to a synergistic interaction between marijuana and tobacco," they concluded.

By contrast, investigators found that "smoking only marijuana was not associated with an increased risk of respiratory symptoms or COPD."

An accompanying commentary by UCLA pulmonary investigator Donald Tashkin added, "Given the consistently reported absence of an association between use of marijuana and abnormal diffusing capacity or signs of macroscopic emphysema, we can be close to concluding that smoking marijuana by itself does not lead to COPD."

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. Full text of the study, "Marijuana and chronic obstructive lung disease: a population-based study," is online at: http://www.cmaj.ca/cgi/content/full/180/8/814.