People who smoke both tobacco and marijuana may have a particularly high risk of developing chronic obstructive pulmonary disease (COPD), a new study suggests.

Cigarette smoking is a well-known risk factor for COPD, a group of lung diseases that includes emphysema and chronic bronchitis. In this study, people who smoked tobacco alone were at elevated risk.

On the other hand, the study failed to show a strong link between pot smoking, by itself, and a higher COPD risk.

The findings suggest that marijuana and cigarette smoking may act “synergistically” to promote COPD, the researchers report in the Canadian Medical Association Journal.

It’s possible, they speculate, that marijuana smoking sensitizes the airways, making them more vulnerable to the adverse effects of tobacco smoking.

The bottom line is that “smoking marijuana and cigarettes - even small amounts - is very harmful for your lungs, increasing the risk of COPD by several-fold,” lead researcher Dr. Wan C. Tan, of the University of British Columbia in Vancouver, told Reuters Health.

While the lung damage exacted by cigarette smoking is well-established, studies on the respiratory consequences of pot smoking have yielded conflicting results. One recent study, for example, linked daily marijuana use to a higher lung cancer risk, but other studies have failed to show such a connection.

For the current study, Tan’s team surveyed a random sample of 878 Canadian adults age 40 and older. Just over half (53.1 percent) had ever smoked cigarettes, while 45.5 percent had ever used marijuana.

The researchers found that compared with non-smokers, people who smoked cigarettes only were 50 percent more likely to have frequent respiratory symptoms and 2.7 times more likely to have COPD.

Meanwhile, men and women who smoked both tobacco and marijuana were more than twice as likely as non-smokers to have frequent respiratory symptoms, if they’d had more than 50 joints over a lifetime. Their risk of COPD was nearly three times higher than non-smokers.’

According to Tan, the findings do not mean that pot smoking, by itself, has no impact on COPD risk. She explained that there was an elevated risk linked to marijuana use, but in statistical terms, the increase was not significant. A study that included a larger number of marijuana-only smokers might detect a statistically significant risk.

“We would need a much larger study and a marijuana-smoking cessation study to have sufficient power to demonstrate the impact of marijuana on lung function and risk of COPD,” Tan said.

SOURCE: Canadian Medical Association Journal, April 14, 2009.