Smoking a joint from time to time won't damage the lungs, even after years of drug use, according to a new study, which disproves one of the major concerns about marijuana — that smoking it must be just as risky as lighting up a cigarette.

SAN FRANCISCO — Smoking a joint from time to time won’t damage the lungs, even after years of drug use, according to a study led by University of California, San Francisco (UCSF) researchers that disproves one of the major concerns about marijuana — that smoking it must be just as risky as lighting up a cigarette.

The study, results of which were published Tuesday in the Journal of the American Medical Association, found the lung capacity of people who smoked pot was not diminished by regular toking, even in those who smoked once or twice a week.

Only heavy marijuana users — those who smoked 20 or more joints a month — saw a negative impact on the pulmonary system, but that level of marijuana use is unusual, researchers said.

In fact, they said, it may be that marijuana smoke doesn’t affect lung function the way tobacco does simply because people don’t smoke as many joints as they do cigarettes.

The results should reassure doctors and patients who are considering using marijuana for medical care, primarily to ease pain and nausea, said Dr. Mark Pletcher, a UCSF epidemiologist and lead author of the study.