One Marijuana Joint a Week for 49 Years Doesn’t Harm Lungs, Research Finds

Occasional pot smoking doesn’t appear to harm users’ lungs the way regular tobacco use does, according to a 20-year study.

At the level of one marijuana cigarette a week for 49 years, or one joint a day for 7 years, there was no decline in lung function, according to a paper released today by the Journal of the American Medical Association.

Marijuana is the most-frequently used illegal substance in the United States, according to the National Institute on Drug Abuse’s Monitoring the Future Study. Pot smoke contains many of the same constituent chemicals as cigarette smoke, and until now it hasn’t been clear whether regular marijuana smoking led to the same injuries to the lungs, the authors said.

“The thing the dorm stoners in college have to worry about when they grow up is how to get rid of their cigarette-smoking habit if they still have it, and how their grades worked out,” said Stefan Keresz, a study author and associate professor of medicine at the University of Alabama at Birmingham, in a telephone interview.

The study, which followed more than 5,000 people for 20 years, is unusual in marijuana research because it involved the general population, Keresz said. Usually scientists study addicts because they are easier to find. However, most pot users don’t get addicted and it’s important to know how the drug use affects them, he said.

“This tells us long-term lung considerations aren’t the thing to focus on,” he said.

Tobacco Versus Pot

More than half of the people in the study reported current marijuana smoking, tobacco smoking, or both at one or more examinations. Tobacco smokers had eight to nine cigarettes a day at the peak of their use, versus marijuana smokers reporting about two or three episodes in 30 days.

At more than 10 years of smoking a daily joint, lung function seemed to decline although not to a degree that was statistically significant, the study found.

Marijuana is classified by the U.S. government as a Schedule I drug, which declares it has no medical use. Previous studies have shown that the drug can be used to treat glaucoma, multiple sclerosis, nausea and pain. Intoxication from the drug can cause distorted perception, difficulty in thinking, impaired coordination and memory problems, according to the National Institute on Drug Abuse.

Lung function was assessed using two tests, the volume of expiration in the first second of exhaling, and forced vital capacity, the volume of air exhaled after a full inhalation. Higher numbers, in both measures, indicate better lung function.

Lung Function

Patients who smoked only tobacco had 24 fewer milliliters of volume in the first second of an exhalation than an average nonsmoker; marijuana smokers who didn’t smoke tobacco had 0.7 more milliliters. The pot-only smokers also had 8.2 milliliters more air exhaled after a full inhalation, compared with those who smoked nothing, and the tobacco-only smokers had 19 milliliters less.

The way pot is smoked -- taking a long deep breath, holding it, and then forcefully expelling it -- may also prepare people to ace the lung function tests, Keresz said.

“Marijuana is clearly an irritative smoke for the lungs,” Keresz said, citing coughing after taking a “hit” as an illustration. “But this is may be a small degree of reassurance to people who may have used it in the past.”

To contact the reporter on this story: Elizabeth Lopatto in New York at elopatto@bloomberg.net

To contact the editor responsible for this story: Reg Gale at rgale5@bloomberg.net

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