Occasional marijuana use doesn't harm lungs, study finds

Smoking marijuana on an occasional basis does not appear to significantly damage the lungs, according to a new study.

The paper, published Tuesday in the Journal of the American Medical Association, adds to some previous research that has also failed to find a link between low or moderate exposure to marijuana smoke and lung damage. The issue has gained some importance in recent years, however, as marijuana usage rates rise and as states considered legalization of marijuana for medical or even recreational purposes.

Even a low level of cigarette smoke has been linked to a higher risk of respiratory problems, pulmonary disease and lung cancer - as has exposure to secondhand tobacco smoke. Thus, the idea that marijuana smoke may also be harmful is reasonable. Marijuana contains many of the same chemicals as tobacco smoke.

Researchers led by Mark Pletcher at the University of California, San Francisco, studied 5,115 men and women in four U.S. cities regarding their current and lifetime exposure to tobacco smoke and marijuana smoke and their lung function. The exposure to marijuana smoke was expressed by joint years, with smoking 365 joints or filled pipe bowls being equal to one "joint year."

The study showed that lung function declined with increased exposure to tobacco smoke. However, that same pattern was not seen with marijuana smoke. There was no evidence of lung function damage with seven joint years (or smoking one joint a day for seven years.) After 10 years, there was some decline in lung function as measured by the speed at which a person can blow out air.

The study should reassure people that medical uses of marijuana, such as for pain control, will not be offset by lung damage, the authors pointed out. But, they said, the study did not evaluate the effects of heavy marijuana smoking on the lungs.

"Our findings suggest that occasional use of marijuana for (medical) purposes may not be associated with adverse consequences on pulmonary function," Pletcher said in a news release. "On the other hand, our findings do suggest an accelerated decline in pulmonary function with heavier use - either very frequent use or frequent use over many years - and a resulting need for caution and moderation when marijuana use is considered."

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