Marijuana does not impair lung function—at least not in the doses inhaled by the majority of users, according to the largest and longest study ever to consider the issue, which was published today in the Journal of the American Medical Association.
Researchers working on a long-term study of risk factors for cardiovascular disease (the Coronary Artery Risk Development in Young Adults or CARDIA study) tested the lung function of 5115 young adults over the course of 20 years, starting in 1985 when they were aged 18 to 30.
They found that marijuana use was almost as common as cigarette smoking in the sample, which was designed to reflect the U.S. population. Among participants, the average marijuana user toked 2-3 times a month, while the average tobacco user smoked eight cigarettes a day. Those who smoked both tended to do so slightly more frequently than those who smoked only cigarettes or only marijuana.
The study was “well conducted” and is “essentially confirmatory of the findings from several previous studies that have examined the association between marijuana smoking and lung function,” says Dr. Donald Tashkin, professor of medicine at UCLA and a leading scientist in the area. He was not associated with the new research.
“The major strengths of this study are that it included a far larger number of subjects followed for longer than any of these previous studies,” he adds.
MORE: Teen Drug Use: Marijuana Up, Cigarettes and Alcohol Down
While tobacco smokers showed the expected drop in lung function over time, the new research found that marijuana smoke had unexpected and apparently positive effects. Low to moderate users actually showed increased lung capacity compared to nonsmokers on two tests, known as FEV1 and FVC. FEV1 is the amount of air someone breathes out in the first second after taking the deepest possible breath; FVC is the total volume of air exhaled after the deepest inhalation. “FEV1 and FVC both actually increased with moderate and occasional use of marijuana,” says Dr. Mark Pletcher, associate professor of epidemiology and biostatistics at the University of California, San Francisco and the lead author of the study.
That was a bit of a surprise, says Pletcher, since “There are clearly adverse effects from tobacco use and marijuana smoke has a lot of the same constituents as tobacco smoke does so we thought it might have some of the same harmful effects. It’s a weird effect to see and we couldn’t make it go away,” he adds, explaining that the researchers used statistical models to look for errors or other factors that could explain the apparent benefit and did not find them.
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