People who smoke both tobacco and marijuana increase their risk of respiratory symptoms and chronic obstructive pulmonary disease (COPD), say University of British Columbia researchers.

COPD is the medical term used to describe diseased lungs and narrowed airways, a condition that can be fatal.

"This effect suggests 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," Dr. Wan Tan and co-authors wrote in a study published Monday in the Canadian Medical Association Journal.

Tan and his colleagues did not find any link between smoking only marijuana and increased risk of respiratory illness, but hedged their bets on marijuana's potential effects.

"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," Tan said in a release.

While tobacco smoking alone was associated with increased risk, smokers who reported using both tobacco and marijuana were nearly 2.5 times more likely than nonsmokers to have respiratory symptoms and almost three times more likely to have COPD.

Those who reported smoking only tobacco were 1.5 times more likely than non-smokers to have respiratory symptoms and more than 2.5 times more likely to have COPD.

The study surveyed 878 people in Vancouver who were at least 40 years old.

The researchers defined smokers as people who reported smoking at least 365 cigarettes in their lifetime. Participants were considered to be marijuana smokers if they smoked more than 50 marijuana cigarettes in their lifetime.

The researchers cautioned their study was limited by lack of data on 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 the University of California Los Angeles wrote 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 found no additional effect from the combination of marijuana and tobacco use on either chronic respiratory symptoms or abnormal lung function in younger smokers.