Lighting up a joint or hitting the bong a few times a month doesn't seem to cause a great deal of lung damage, a new study suggests.
That's not to say, of course, that marijuana is some kind of wonder drug: there do appear to be health risks to heavy pot smoking – that is, smoking a joint or two a day for several years.
But the study's main findings back what many pot lovers have said for years: occasional pot use doesn't do the kind of lung damage that smoking does.
The researchers behind the 20-year-long study can't say for sure why pot smoking doesn't hurt lung function as much as cigarette smoking does. But it's likely because pot users tend to smoke much less than cigarette smokers do.
Indeed, median tobacco use among the smokers was 8 to 9 cigarettes a day; median pot use, meanwhile, was just 2 to 3 times a month.
The study appears in JAMA, the Journal of the American Medical Association. It looked at more than 5,000 young adults in Oakland, Chicago, Minneapolis and Birmingham, Alabama. Around 2,300 didn't smoke at all, 851 were exclusive cigarette smokers, 795 were exclusive pot smokers, and the rest smoked both.
From 1985 until 2006, researchers regularly asked the participants about their current and past use of cigarettes and marijuana. The researchers did not distinguish between smoking method (joint, pipe, "bong", etc.)
They then assessed the volunteers' lung function using a test that measures how much air their lungs could hold, as well the maximum rate of air flow the volunteers could push out of their lungs.
Not surprisingly, cigarette smokers fared poorly in the tests. The more they smoked over time or had smoked in their life, the worse their lungs performed on both tests.
But among the moderate pot smokers, "we found no evidence that increasing exposure to marijuana adversely affects pulmonary function," the researchers write.
In fact, occasional pot smokers actually showed improvements in the two measurements of lung function over the study course, though the improvements were quite small.
The study authors said it's possible that because pot users tend to breathe in deeply when they inhale, they regularly stretch and strengthen their lung tissue. The lung function tests in the study are designed to assess exactly the kind of deep breathing that marijuana smokers are used to.
But improvements in lung function decline with heavy pot use. Those who smoked pot more than 20 times a month saw a decline in lung capacity over the study period. But it took until seven "joint-years" -- a joint or a bong a day for seven years or 1 joint/week for 49 years – before their lung function test started to decline.
The researchers note their findings are important given that marijuana has been found to have beneficial effects on pain control, appetite and mood.
"Our findings suggest that occasional use of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function," they write.
The study authors note a few weaknesses with their study. First, all cigarette and marijuana use was self-reported, a method that can be subject to "recall error."
As well, the authors note that the two lung function tests they used are not the only measure of lung health.
