"This effect suggests that smoking marijuana may act as a primer, or sensitizer, in the airways to amplify the adverse effects of tobacco on respiratory health," says study researcher Wan C. Tan, MD, of the University of British Columbia and St. Paul's Hospital in Vancouver, Canada.

The study is among the first to suggest a synergistic relationship between marijuana and tobacco use among older people who are most at risk for COPD .

Smokers in the study who reported using both tobacco and marijuana were three times as likely as nonsmokers to have clinically confirmed COPD ( chronic obstructive pulmonary disease ); people who smoked only cigarettes had a slightly lower risk.

April 13, 2009 -- People who smoke both cigarettes and marijuana have a greater risk for developing the progressive lung disease COPD than cigarette smokers who don't smoke pot, a new study shows.

COPD Risk

About 12 million Americans are currently living with a diagnosis of COPD; an equal number are believed to have the disease and not know it, according to the National Heart, Lung and Blood Institute.

In the U.S, the term COPD includes both emphysema and chronic bronchitis. With COPD, breathing becomes more difficult over time.

Largely caused by cigarette smoking, COPD is the fourth leading cause of death in the U.S.

While the link between tobacco and COPD is well established, far less is known about the impact of marijuana use on the lungs.

Some studies have found that even short-term heavy marijuana smoking can worsen lung function, while others have not shown this association.

Even less is known about the combined effects of smoking cigarettes and pot, Tan tells WebMD.

The study by Tan and colleagues included 878 residents of Vancouver, Canada participating in a larger investigation examining COPD prevalence.

Participants were considered tobacco smokers if they had smoked at least 365 cigarettes in their lifetime, and were considered marijuana smokers if they reported having ever smoked pot. The researchers defined "substantial" marijuana use as having smoked at least 50 marijuana cigarettes.

The average cigarette smoker in the study had smoked for 16 years, while the self-described pot smokers had smoked an average of 84 marijuana cigarettes.