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Occasional Marijuana Smoking Does Not Cause Long-Term Lung Damage

Infrequent smoking of cannabis over prolonged periods did not lead to a decrease in pulmonary function, according to a 20-year study reported in the January 11, 2012, Journal of the American Medical Association. These findings suggest that occasional recreational use of marijuana does not cause lung damage similar to that seen with regular tobacco use, but more study is needed of people who use medical marijuana more frequently.

Many people with HIV and hepatitis C use medicinal marijuana to stimulate appetite and relieve nausea and chronic pain. But some patients and providers are concerned that regular use might lead to lung damage including chronic obstructive pulmonary disease (COPD) and lung cancer, as happens with cigarette smoking.

Marijuana smoke contains many of the same components as tobacco smoke. People typically consume marijuana less frequently -- a few times per month on average for recreational users, up to several times per day for regular medical users -- compared with a pack or more of cigarettes per day for many tobacco users; however, marijuana users may inhale more deeply and hold the smoke longer in their lungs.

Mark Pletcher from the University of California San Francisco and fellow investigators with the long-term CARDIA (Coronary Artery Risk Development in Young Adults) study longitudinally evaluated pulmonary function and its association with marijuana smoking over 20 years (March 1985 through August  2006).

The analysis included 5115 men and women in 4 U.S. cities; the average age at study entry was 25 years. More than half of participants (54%) reported smoking marijuana, tobacco, or both at 1 or more examinations.

Lifetime marijuana exposure was expressed in "joint-years," with 1 joint-year of exposure being the equivalent of smoking 365 joints or full pipe bowls. The main outcome measures were 2 indicators of lung capacity -- forced expiratory volume during the first second of exhalation(FEV₁) and forced vital capacity (FVC) -- and their association with marijuana and tobacco smoking.

Results

• Marijuana use was nearly as common as tobacco smoking, but was generally lighter (median 2-3 uses of marijuana per month vs 8-9 cigarettes per day).
• Both current and lifetime tobacco exposure were linearly associated with lower FEV₁ and FVC.
• In contrast, the association between marijuana exposure and lung function was non-linear.
• At low exposure levels, FEV₁ and FVC actually increased or improved significantly (by 13 mL and 20 mL per joint-year, respectively)
• At higher exposure levels, however, these associations leveled off or even reversed, indicating impaired lung function.
• With very heavy marijuana use, the net association with FEV₁ was not significantly different from baseline, while the net association with FVC remained significantly greater than baseline.
• There was a non-significant decline in FEV₁ with more than 10 joint-years of lifetime exposure, but the association reached statistical significance with more than 20 episodes of marijuana use per month.

Based on these findings, the researchers concluded, "Occasional and low cumulative marijuana use was not associated with adverse effects on pulmonary function."

“Marijuana may have beneficial effects on pain control, appetite, mood, and management of other chronic symptoms," they elaborated in their discussion. "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," they continued, "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."

Further study is needed to determine whether these findings hold for daily medical marijuana users. Concerns about lung damage encourage some patients to use oral cannabis preparations, while others prefer vaporizers that release the active compounds in marijuana without the by-products of combustion.

Investigator affiliations: Department of Epidemiology and Biostatistics and Division of General Internal Medicine, University of California San Francisco, San Francisco, CA; Asthma-COPD Program, Division of Pulmonary and Critical Care Medicine, Northwestern University Feinberg School of Medicine, Chicago, IL; Department of Surgery and Division of Preventive Medicine, University of Alabama at Birmingham, Birmingham, AL; Center for Surgical, Medical and Acute Care Research and Transitions, Veterans Affairs Medical Center, Birmingham, AL; Division of Research, Kaiser Permanente of Northern California, Oakland, CA.

1/10/12

Reference

MJ Pletcher, E Vittinghoff, R Kalhan, S Kertesz, et al. Association Between Marijuana Exposure and Pulmonary Function Over 20 Years. Journal of the American Medical Association 307(2):115-213. January 11, 2012.

Other Source

JAMA and Archives Journals. Marijuana Use Not Associated With Adverse Effects on Lung Function. Press release. January 10, 2012.