Moderate marijuana use does not affect lung function
MedWire News: Occasional and low cumulative use of marijuana does not appear to affect lung function adversely, US researchers report.
"With marijuana use increasing… knowing whether it causes lasting damage to lung function is important for public-health messaging and medical use of marijuana," said the study's senior author, Stefan Kertesz (University of Alabama at Birmingham) in a statement to the press.
First author Mark Pletcher (University of California, San Francisco) and colleagues add that marijuana smoke contains many of the same constituents as tobacco smoke, but it is unclear whether it has similar adverse effects on pulmonary function.
As reported in JAMA, the researchers therefore analyzed associations between marijuana use (both current and lifetime exposure) and pulmonary function among 5115 participants of the Coronary Artery Risk Development in Young Adults study.
They collected repeated measurements of pulmonary function and smoking over the course of 20 years (1985-2006). Lifetime exposure to marijuana joints was expressed in joint-years, with 1 joint-year of exposure equivalent to smoking 365 joints or filled pipe bowls.
Pulmonary function was assessed by forced expiratory volume in the first 1 second of expiration (FEV1; a measure of air flow) and forced vital capacity (FVC; a measure of lung capacity).
The researchers found that more than half of participants (54%) reported current marijuana smoking, tobacco smoking, or both at one or more of the examinations. Marijuana exposure was nearly as common as tobacco exposure but was mostly light (median, 2-3 episodes per month), they say.
Tobacco exposure, both current and lifetime, was linearly associated with lower FEV1 and FVC, compared with no exposure.
By contrast, current and lifetime exposure to marijuana was associated with higher FVC, while lifetime exposure was associated with higher FEV1, compared with no exposure.
These associations, however, were not linear: at low levels of exposure (<7 joint-years over a lifetime), FEV1 increased by a significant 13 mL per joint-year and FVC by a significant 20 mL per joint-year, but at higher levels of exposure, the associations leveled or even reversed.
Indeed, at more than 10 joint-years of lifetime exposure the researchers observed a nonsignificant decline in FEV1 of -2.2 mL per joint-year. They also observed a significant decline in FEV1 of -3.2 mL per marijuana smoking episode per month at levels above 20 episodes per month.
FVC, on the other hand, remained significantly elevated even in heavy users, with a net association of 76 mL observed at 20 joint-years.
"At levels of marijuana exposure commonly seen in Americans, occasional marijuana use was associated with increases in lung air flow rates and increases in lung capacity," said Kertesz. "Those increases were not large, but they were statistically significant. And the data showed that even up to moderately high-use levels - one joint a day for 7 years - there is no evidence of decreased air-flow rates or lung volumes."
Kertesz pointed out that the study did not examine other ways in which smoking marijuana could affect a person's health and insisted that the findings do not advocate the use of marijuana.
"Marijuana is still an illegal drug, and it has many complicated effects on the human body and its function," he concluded. "In our findings we see hints of harm in pulmonary function with heavy use, and other studies have shown that marijuana use increases a user's likelihood of a heart attack… and impairs the immune system's ability to fight disease."
By Laura Cowen