Unlike cigarette smoking, 20 years of typical marijuana smoking doesn’t appear to impair lung function, according to a report in the Jan. 11 issue of JAMA.
"With up to 7 joint-years of lifetime exposure (e.g., one joint per day for 7 years or one joint per week for 49 years), we found no evidence that increasing exposure to marijuana adversely affects pulmonary function," said Dr. Mark J. Pletcher of the department of epidemiology and biostatistics and the department of medicine, University of California, San Francisco, and his associates.
While heavier use may impair lung function, the number of such users was too small in this study cohort to allow reliable estimates. The data suggested a detrimental effect with heavier marijuana smoking, the investigators noted.
Previous studies of the pulmonary effects of long-term marijuana use have yielded inconsistent results. On the one hand, they have demonstrated "consistent evidence of airway mucosal injury and inflammation, as well as increased respiratory symptoms such as cough, phlegm production, and wheeze, similar to that seen in tobacco smokers." On the other hand, these appear to be short-term effects, and there has been no clear evidence of long-term damage to lung function.
Dr. Pletcher and his colleagues studied the issue using data from a large longitudinal study of coronary risk that closely followed the cigarette and marijuana smoking habits of 5,016 young adults in four U.S. communities from 1985 through 2006. As part of that study, the subjects (aged 18-30 years at baseline) underwent pulmonary function testing at baseline, 2 years, 5 years, 10 years, and 20 years.
Using that data, "we estimated both current intensity and lifetime cumulative exposure to tobacco and marijuana smoking and analyzed their associations with spirometric measures of pulmonary function over the 20 years of follow-up," the researchers said.
One joint-year of exposure was defined as the equivalent of 365 joints or filled pipe bowls smoked. Secondhand smoke exposure also was taken into account.
As expected, both current and lifetime tobacco smoking were associated with lower forced expiratory volume in 1 second (FEV1) and lower forced vital capacity (FVC). But unexpectedly, both current and lifetime marijuana smoking were associated with higher FEV1 and higher FVC, the authors wrote (JAMA 2012;307:173-81).
"For example, compared with zero exposure, FVC increased with greater lifetime exposure in joint-years, and FEV1 increased with greater lifetime exposure of up to 10 joint-years and then declined to ... greater than zero-exposure level," the investigators said.
Why marijuana smoking would increase lung capacity is unknown, but other studies have also found this effect. "Some investigators have proposed that the deep inspiratory maneuvers practiced by marijuana smokers could stretch the lungs, resulting in larger lung volumes. Another speculative possibility is strengthening of chest wall musculature or another ‘training’ effect that allows marijuana users to inspire more fully (closer to lung capacity) on spirometry testing," Dr. Pletcher and his associates said.
In a separate analysis of the data using different statistical methods, "we again found strong, dose-related associations between increasing exposure to tobacco and lower FEV1 and FVC," but no such associations for marijuana smoking. Only at very high levels of marijuana smoking was a detrimental effect on pulmonary function suggested.
"Hypothetically speaking, a positive effect from marijuana in the short term (the stretch/training effect) and a negative effect in the long term (damage from smoke exposure) should result in a nonlinear association such as the one we observed. According to this explanation, the predominant effect for FEV1 at very high exposure (more than 40 joint-years) reflects cumulative damage; the predominant effect for FVC at all levels of exposure is from the stretch/training mechanism," they noted.
"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," the investigators said.
This study was supported by the National Institute on Drug Abuse and the National Heart, Lung, and Blood Institute. Dr. Pletcher reported no conflicts of interest.