People who smoke both tobacco and marijuana increase their risk of respiratory symptoms and chronic obstructive pulmonary disease (COPD), a new study has found.
Smoking only marijuana, however, was not associated with increased risks, the CMAJ study claimed.
The study, which surveyed 878 people aged 40 years or more in Vancouver, Canada, was part of the Burden of Obstructive Lung Disease (BOLD) Initiative that sought to determine the prevalence of COPD in adults over 40 years in the general population.
OPD is characterized by diseased lungs and narrowed airways and is associated with high mortality.
The researchers defined smokers as people who reported smoking at least 365 cigarettes in their lifetime, and a history of marijuana smoking as self-reporting of any previous smoking.
While tobacco smoking was associated with increased risk, smokers who reported using both tobacco and marijuana were 2.5 times more likely than nonsmokers to have respiratory symptoms and almost 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 coauthors.
"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," they added.
The researchers were limited by lack of data on the potential variations in marijuana potency, on differences in inhalation and the number of smokers who combine both substances in the same cigarette.