Light pot smoking easy on lungs

People who smoke marijuana for recreational or medical purposes might now breathe easier. Scientists report in the Jan. 11 Journal of the American Medical Association that occasional cannabis users don’t experience any loss of lung function.

In a 20-year study that included lung tests and a specific accounting of marijuana use, scientists also found that people who smoke more than 20 times a month and accumulate many years of use might have a slight drop in lung capacity over time. But the researchers are unsure of that finding since it was based on scant data.

The study is the longest ever conducted that measures cannabis smoking and lung function, uses standard lung measurements and includes thousands of volunteers, says Donald Tashkin, a pulmonologist at UCLA who wasn’t involved in the study. “That makes it important,” he says.

The data, he says, also suggest that marijuana is not a significant risk factor for chronic obstructive pulmonary disease, which includes emphysema. COPD is marked by loss of lung function and is typically caused by tobacco smoking.

The researchers tapped into a health study of 5,115 young adults recruited in 1985 and given lung tests periodically until 2006. The volunteers revealed whether and how often they smoked tobacco, marijuana or both. Most marijuana users in the study reported light use — a few times a month on average during the two decades.

After accounting for tobacco use, sex, race, body size and even pollution and secondhand smoke exposure, the researchers found that these light marijuana users had above-average scores for their age on lung function tests where they blew air into a gauge. People averaging somewhat higher use fared no better or worse than peers their age, while those who used cannabis at least 20 times a month for years showed hints of slightly reduced lung capacity, says study coauthor Stefan Kertesz, an internist at the University of Alabama at Birmingham and the Birmingham Veterans Affairs Medical Center.  

Jeanette Tetrault, an internist and addiction specialist at Yale University School of Medicine, says the study is methodologically sound and the results are believable.  But the potential for lung damage from frequent use makes a cautionary point, she says. “My concern is that giving the green light on light smoking might lead to heavy smoking.”

Besides, Tetrault says, a lung function test “doesn’t describe the entire picture.” Cannabis smoking causes increased coughing, phlegm production and wheezing, she says.

Kertesz agrees but notes that the large airways, the ones primarily irritated by marijuana smoke, seem to bounce back well. “Our bodies have evolved to recover from such short-term insults,” he says.

Marijuana has been approved for medical purposes in 16 states. Pharmacologist Karen Wright of Lancaster University in England says that while the new findings are reassuring for low-dose users, people who use medical marijuana regularly might be better off if they don’t have to inhale combustion fumes to do so. Researchers have made some progress on that front, having developed a spray containing two active cannabis components (SN: 6/19/2010, p. 16). “We should find better ways to administer cannabis compounds that are proven clinically to have a therapeutic benefit,” Wright says.