Dec 212012

Smoking marijuana moderately over years is strongly associated with small improvements in lung function, even compared to people who have never smoked cigarettes or marijuana, according to a study in JAMA. But the popular news media and the study authors downplayed that finding of the study, apparently to avoid sending a pro-marijuana message.

Mark Pletcher, Eric Vittiinghoff, Stefan Kertesz et al crunched numbers from the CARDIA study, which followed 5,115 young U.S. men and women for 20 years (1985-2006) collecting data on tobacco and marijuana use, and included spirometry from 20,777 clinic visits: an enormous trove of longitudinal data.

About half of participants reported smoking tobacco, marijuana, or both at least once during the 20 year study. Interestingly, the pot smokers tended to be taller and more active. Most reported smoking marijuana 2-3 times in the past month.

Among people who smoked tobacco or marijuana, compared to people who did not smoke either:

  • Smokers' lung function was worse, unsurprisingly (pack-a-day tobacco smokers had FEV1s 63 mL lower; 20-pack-year smokers' FEV1 were 101 mL lower);
  • But in marijuana smokers who had smoked up to 3,650 marijuana cigarettes (10 "joint-years"), FEV1 and FVC were higher than matched nonsmokers. At these common levels of marijuana use, there was a steady dose-response relationship: the more marijuana smoked, the better the lung function (FEV1 increase of 13 mL/joint-year).
  • Even in the heavier marijuana users, FVC remained significantly elevated by 76 mL over nonsmokers. Only those smoking large amounts of marijuana every day began to display decreases in lung function.
  • All these trends were highly statistically significant (p < 0.001), and supported by the large sample size and body of spirometric data.

It's interesting that the mass media largely declined to report this key finding of the paper, which seems straightforward: smoking marijuana is associated with slight improvements in lung function. Fox News sidestepped the issue completely. The New York Times acknowledged the improvement, but called it "minuscule." A difference of 130 mL (associated with 10 "joint-years") isn't huge, but it's not minuscule either, and the clear dose-response relationship makes it even more scientifically interesting, although any further discussion or investigation will most likely not be paid for with federal dollars.

The authors suggest the pot smokers trained their lungs to hold more air by regularly inhaling deeply, which sounds like a stretch, forgive the pun. Take a look at this study. Inspiratory muscle training can increase total lung capacity in healthy 21 year olds -- but only in those who regularly engage in pretty intense exercises (80% of sustained maximal inspiratory effort, 3 times a week). Would smoking marijuana a couple times a week produce that effect? (On the other hand, the JAMA study by Pletcher et al had a huge sample size, so might better detect small improvements due to "inhalational training" than this randomized trial of 40 people.)

Marijuana has long been used as a folk remedy by asthmatics because of the well-described bronchodilator effects of THC. No one has previously established a longer-term response of the airways or lung parenchyma to marijuana, but previous reviews controlling for tobacco use have concluded there is no good evidence that marijuana smoking worsens airway dynamics, or causes chronic obstructive pulmonary disease or lung cancer.

Pletcher MJ et al. Association Between Marijuana Exposure and Pulmonary Function Over 20 Years. JAMA 2012;307:173-181.

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Moderate pot smokers’ lung function better than nonsmokers