A study published in the Canadian Medical Association Journal shows that for  those who already smoke regular cigarettes adding marijuana to the mix can increase the risk of respiratory symptoms and COPD – chronic obstructive pulmonary disease. CPOD is characterized by narrowed airways and diseased lungs and has a high rate of mortality in diagnosed patients.

Smoking marijuana alone though was not associated with the same increase in risk.

The study was a part of the Burden of Obstructive Lung Disease Initiative (or BOLD) The study surveyed 878 people over the age of forty in the Vancouver, Canada area. The Initiative was created to determine how prevalent CPOD was amongst the general population of over 40s.

For the purposes of their study researchers defined smokers as people who reported having smoked at least 365 cigarettes in their lifetime and marijuana smokers as self reporting of any previous use of the drug.

It was little surprise to the researchers that those who smoked tobacco were at an increased risk, but they also found that in those who smoked both tobacco and marijuana were 2.5 times more likely to have respiratory symptoms than non smokers and 3 times more likely to have COPD as defined by spirometric testing.

“We were able to detect a significant synergistic effect between marijuana smoking and tobacco smoking,” write Dr. Wan Tan of the University of British Columbia and St. Paul’s Hospital and a coauthor on the study. “This effect suggests that smoking marijuana (at least in relatively low doses) may act as a primer, or sensitizer, in the airways to amplify the adverse effects of tobacco on respiratory health.”

The researchers do admit that they were hampered in their research by a lack of available data about the potency of the marijuana used and the number of patients who might mix the two substances within the same “cigarette”.

Dr Donald Tashkin is from UCLA and was not a part of the research group that completed this study. However after reading it he says “the findings of Tan and colleagues add to the limited evidence of an association between marijuana use and COPD because their study focuses on an older population (aged 40 or older) that is at greater risk of COPD.” He adds “we can be close to concluding that marijuana smoking by itself does not lead to COPD.”

Dr Wan Tan and his colleagues are more cautious about the interpretation of their findings though. They concluded “Although our study had insufficient power to show an association between marijuana alone and increased risk for COPD, it remains uncertain whether marijuana by itself is harmful for the lungs. Larger studies are needed to address this critically important issue in the future.”