Pot Increases Risk of Lung Disease for Cigarette Smokers
Those who smoke tobacco products and marijuana increase the risk of respiratory problems and lung disease, according to a study published this week by the Canadian Medical Association Journal. However, those surveyed who smoked only pot showed no increased risks of chronic obstructive pulmonary disease (COPD).
The purpose of the study was to determine the combined and independent effects of tobacco and marijuana smoking on respiratory symptoms and COPD in the general population.
The study surveyed 878 people aged 40 years and older in Vancouver, Canada, and was part of the Burden of Obstructive Lung Disease (BOLD) Initiative that sought to determine the prevalence of COPD in adults in the older population. COPD is characterized by diseased lungs and narrowed airways and is associated with high mortality.
The difference between this study and others is that the subjects were older and the prevalence of tobacco smoking was lower in participants.
The Canadian Medical Association Journal said marijuana is the most widely used illegal drug in the world. After tobacco, it is the second most widely smoked substance in the general population, with a conservatively estimated 11 million users in the United States and 160 million users worldwide.
For this survey, 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 three times more likely to have COPD as defined by testing the measure of breath.
"We were able to detect a significant synergistic effect between marijuana smoking and tobacco smoking," wrote Dr. Wan Tan of the University of British Columbia and St. Paul's Hospital, and the study’s 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."
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.
In a related commentary, Dr. Donald Tashkin of UCLA writes that "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."
Previous studies have failed to find an additive effect of marijuana and tobacco on either chronic respiratory symptoms or abnormal lung function in younger smokers. Dr. Tashkin said, "We can be close to concluding that marijuana smoking by itself does not lead to COPD."
However, Dr. Wan Tan and coauthors conclude that, "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."
Concerns about the impact of marijuana on lung health are based on the fact that both the gaseous and the particulate phases of tobacco and cannabis smoke contain a similar range of harmful chemicals. The adverse effects of tobacco smoke on the lungs are well established. By contrast, the potential impact on lung health of marijuana smoke, with its wide range of toxins, is poorly understood. The study claims that short-term, heavy marijuana smoking among young adults can worsen lung function, and long-term pot smoking has been linked to an increase in respiratory symptoms.
