“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.

The improvement wasn’t seen in the heaviest users, however. At high levels of marijuana use—for example, in those who smoked more than 20 times a month—FEV1 slipped back to levels seen in nonusers and a reduction was seen in, um, the most chronic smokers.  But FVC remained high in even the longest term, heaviest users.

So why might marijuana users have greater lung capacity than nonsmokers?  Consider Bill Clinton’s famous non-denial denial of his marijuana use, “I didn’t inhale” and Barack Obama’s retort that he “inhaled frequently; that was the point.”  Unlike cigarette smokers, cannabis users usually draw deeply on the joint or pipe— and hold each puff in, typically for as long as physically possible.

“In some ways, marijuana smoking is really a lot like doing a pulmonary function test,” Pletcher says. This “practice” or “exercise” might expand lung capacity and account for the unusual results.

He cautions, however, that long term exposure to marijuana smoke at the most extreme doses probably does damage the lungs, although he concedes that the evidence from the study on this point is “weak.”

The authors conclude:

Marijuana may have beneficial effects on pain control, appetite, mood, and management of other chronic symptoms. Our findings suggest that occasional use of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function. It is more difficult to estimate the potential effects of regular heavy use, because this pattern of use is relatively rare in our study sample; however, our findings do suggest an accelerated decline in pulmonary function with heavy use and a resulting need for caution and moderation when marijuana use is considered.

Study: Smoking Marijuana Not Linked with Lung Damage

Marijuana smokers performed better on tests of lung function compared to nonsmokers and cigarette smokers