Does Marijuana Worsen COPD Symptoms in Cigarette Smokers?

Key Point

Smoking marijuana in addition to cigarettes may worsen respiratory symptoms in patients with COPD.

SAN FRANCISCO—Investigations into the potentially harmful effects of smoking marijuana have had conflicting results, although little research has examined the interaction between marijuana and cigarette smoking as it relates to pulmonary function. A study presented at the 103rd American Thoracic Society International Conference has shown that smoking marijuana may in fact exacerbate respiratory symptoms in current tobacco smokers with COPD.

“In our study we actually show that marijuana use among current cigarette smokers has an additive effect not only on the symptoms but the development of chronic obstructive lung disease,” explained Wan-Cheng Tan, MD, of St. Paul’s Hospital in Vancouver, British Columbia. Dr. Tan is also Honorary Professor in the Department of Medicine, Respiratory Division, at the University of British Columbia.

Dr. Tan and colleagues gathered data from a group of 648 persons ages 18 to 39 and a group of 878 persons ages 40 and older; both groups were randomly selected from a population-based study on lung health. The participants answered questions on cigarette and marijuana smoking as well as on respiratory symptoms. The two cohorts included persons who smoked only cigarettes, only marijuana, or both cigarettes and marijuana, as well as persons who smoked neither.

In the younger age-group, 49% had smoked marijuana at least once and 17% were current marijuana smokers, while in the group of persons ages 40 and older, 46% of the participants had smoked marijuana at least once and 13% were current marijuana smokers. Prevalence rates in the younger group were 31% and 16% for having ever smoked cigarettes and current cigarette smoking, respectively, compared with corresponding rates of 52% and 13% in the older group. A total of 76% of younger cigarette smokers also smoked marijuana, while 58% of older cigarette smokers also smoked marijuana. In both age groups, the prevalence rate of smoking both marijuana and cigarettes was 30%.

“In Vancouver, as in many cities in North America, marijuana smoking is as prevalent, if not more so, than cigarette smoking,” Dr. Tan said. “It’s a habit that is very popular, right across all age-groups.”

The study found that the odds of cigarette smokers developing respiratory symptoms was 2.36 times that of nonsmokers, while the odds of patients who smoked both cigarettes and marijuana developing respiratory symptoms was 18.28 times that of people who smoked neither, after adjustment for cigarette smoking. In other words, smoking marijuana in addition to cigarettes corresponded to a nearly eightfold increase in risk.

The study also found that among people 40 and older, smokers were 2.53 times more likely than nonsmokers to develop COPD, while smokers of both cigarettes and marijuana were 3.43 times more likely than people who smoked neither to develop COPD, after adjustment for cigarette smoking.

The adjusted odds ratio for respiratory symptoms was 1.17 in current cigarette smokers and 1.83 in both cigarette and marijuana smokers. However, adverse effects on respiratory function or COPD could not be linked to marijuana smoking alone. Dr. Tan explained that studying marijuana smoking alone opens the possibility for several confounding factors. “Those who smoke marijuana alone are not very heavy smokers,” she said. “They are healthier, they are younger, they are richer, they are more educated. So all those factors have hindered our efforts at looking at this effect” on respiratory symptoms and COPD.

“Most people think that marijuana smoking is pretty innocuous and that it does not do any harm, despite evidence to the contrary,” Dr. Tan said. “It’s a kind of recreational smoking that people do treat quite lightly.” She added, “We can’t say for sure that smoking marijuana is a risk factor—it hasn’t been proven by longitudinal studies—but I think this study is the first to show that it is actually related to COPD in cigarette smokers.”            

—John Merriman

Reference
Moore BA, Augustson EM, Moser RP, Budney AJ. Respiratory effects of marijuana and tobacco use in a U.S. sample. J Gen Intern Med. 2005;20(1):33-37.
Tetrault JM, Crothers K, Moore BA, et al. Effects of marijuana smoking on pulmonary function and respiratory complications: a systemic review. Arch Intern Med. 2007; 167(3):221-228.

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