Science: Cannabis smoking does not impair lung function according
large long-term study
Cannabis does not impair lung function - at least not in the doses
inhaled by the majority of users, according to the largest and
longest study ever to consider the issue, which was published in the
Journal of the American Medical Association. US researchers conducted
a longitudinal study collecting repeated measurements of pulmonary
function and smoking over 20 years from 1985 to 2006 in 5115 men and
women. "Occasional and low cumulative marijuana use was not
associated with adverse effects on pulmonary function," authors
summarized the results. Lifetime exposure to cannabis cigarettes was
expressed in joint-years, with 1 joint-year of exposure equivalent to
smoking 365 joints (cannabis cigarettes) or filled pipe bowls.
Researchers found that measures of lung function - forced expiratory
volume in the first second of expiration and forced vital capacity -
actually improved slightly as young people reported using more
cannabis - at least up to 7 joint-years or 2,555 joints. "There's no
doubt marijuana triggers a cough," said Dr. Stefan Kertesz, from the
University of Alabama at Birmingham, who worked on the new study. But
questions have remained about the drug's longer-term effect on lung
functioning. Not surprisingly tobacco use was associated with
decreased lung function. But at least at moderate levels of cannabis
smoking, that didn't seem to be the case - in fact, the trend was
reversed. Lung volume and air flow rates both increased with each
joint-year in moderate users. "It's a very real increase (...) but
it's so small that I don't think that a person would feel a benefit
in terms of their breathing," Kertesz said. At the highest levels of
cannabis smoking lung function seemed to decline again, but the
researchers noted that there weren't enough heavy cannabis users in
their study population to be sure of that.
It is unlikely that cannabis puts users at risk of chronic
obstructive pulmonary disease, or COPD, as smoking tobacco does, says
Dr. Donald Tashkin, professor at the University of California in Los
Angeles, who studies the effects of cannabis on the lungs for decades
but wasn't involved in the new study. When it comes to diminished
lung function, "This particular potential complication of marijuana
smoking doesn't appear to be an important risk," he told the press
agency Reuters. "Therefore, people who are using marijuana for
medicinal purposes or recreationally at least could be reassured that
they're not harming their lungs in this way."
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(Source: Pletcher MJ, Vittinghoff E, Kalhan R, Richman J, Safford M,
Sidney S, Lin F, Kertesz S. Association between marijuana exposure
and pulmonary function over 20 years. JAMA 2012;307(2):173-81.)
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