Dr Brent Moore and colleagues from Yale University in New Haven, Connecticut, examined the respiratory effects of marijuana and tobacco use in a nationally representative sample of 6728 individuals aged between 20 and 59 years.

Participants were interviewed in person and asked to undergo a medical examination, including measurement of lung function. Marijuana smokers were defined as those who had used the drug at least 100 times in their lifetime and at least once in the past month.

The findings, published in the Journal of General Internal Medicine, showed that marijuana users reported smoking on an average of 10 days out of the previous 30, with 16% reporting daily or near daily use. Notably, 77% of marijuana smokers also smoked tobacco.

Further analysis revealed that, after taking into account age, gender, tobacco use and asthma, smoking marijuana was significantly associated with an increased risk of respiratory symptoms.

Specifically, compared with nonusers, both tobacco and marijuana smokers had increased risks of chronic bronchitis, cough on most days, phlegm production, shortness of breath, wheezing and abnormal chest sounds.

"Because more than 2 million adult Americans are heavy marijuana smokers, these risks represent a potentially large health burden," said Dr Moore. "Marijuana smokers use more medical services for respiratory problems, and these demands are likely to increase as the population of heavy marijuana smokers ages."

The researchers therefore point out that doctors' efforts to prevent and reduce marijuana use, such as advising patients to quit and providing referrals for support and assistance, may result in "substantial public health benefits".