The study has followed more than 5,000 young adults in four cities for more than two decades. More than half of the people in the study reported smoking tobacco, marijuana, or both.
Over time, researchers repeatedly checked two measures of lung function: One was a test that measured the amount of air forcefully exhaled in a single second. The second test measured the total amount of air exhaled after taking the deepest possible breath.
Those tests help doctors diagnose chronic, irreversible breathing problems like chronic obstructive pulmonary disease (COPD).
Cigarette smoking is a leading cause of COPD. And marijuana smoke contains many of the same chemicals as tobacco smoke.
As more states legalize marijuana — 16 states and the District of Columbia now allow its medical use — experts have worried that the kinds of lung damage caused by cigarettes could also be brought on by pot smoking.
Indeed, cigarette smokers in the study saw their lung function drop significantly over 20 years.
But that didn’t happen to people who only smoked marijuana.
In fact, the study found that the lung function of most marijuana smokers actually improved slightly over time.
‘These Are Not the Cheech and Chongs of the World’
A healthy adult man can blow out about a gallon of air in one second, says researcher Stefan Kertesz, MD, an assistant professor of medicine at the University of Alabama at Birmingham.
Pot smokers, on average, were able to blow out that gallon of air plus about 50 milliliters.
“That’s roughly one-sixth of a size of a can of soda,” Kertesz says. “It’s not anything anybody would notice.”
The results do have to be put into the proper context, though.
Most marijuana users in the study were light smokers. “These are not the Cheech and Chongs of the world,” Kertesz says.
The average number of times a person using marijuana in the study said they lit up was two to three times per month.
But even in regular users, researchers say they still saw no evidence of breathing problems.
In fact, researchers estimated that lung capacity would stay slightly elevated even if a person had smoked as much as a joint a day for seven years, or two to three joints a day for three years.
The study is published in the Journal of the American Medical Association.
Marijuana Smoking May Stretch the Lungs
But researchers are quick to say that the small improvement seen in this study may have more to do with the way people smoke marijuana — by taking and holding deep breaths — than it does with any actual benefit of the drug.
And although they didn’t find any long-term breathing problems associated with occasional pot smoking, it has been linked to some short-term irritation.
“Marijuana does irritate airways, and certainly anyone who’s heard someone cough after smoking marijuana knows that,” says Kertesz. “Is this actually a real benefit to lung health? Probably not.”
What’s more, there was some evidence that very heavy users — those who smoked the equivalent of a joint a day for 40 years or lit up more than 25 times a month — might lose lung function.
But because the number of heavy users in the study was small, researchers say they aren’t sure whether those trends are valid or not.
Kertesz says the study should reassure people who smoke the drug for medical reasons.
But he says the study doesn’t mean marijuana is safe. It was narrowly focused on lung function. It didn’t look at other possible dangers like cancer.
“One study about one aspect of [lung] function is simply a small part of the puzzle of figuring out what the impacts of this substance are,” Kertesz says.
Experts agree that the study shouldn’t be a reason for people to light up.
“This is a well-designed, well-described study,” says Jeanette M. Tetrault, MD, an assistant professor of medicine at Yale School of Medicine in New Haven, Conn.
“The jury is still out about smoking marijuana, especially with heavy smokers and long-term chronic use,” says Tetrault, who has reviewed the health effects of smoking marijuana on the lungs but was not involved in the current study. “There are a lot of studies that are conflicting.”
Advice to Patients
Donald P. Tashkin, MD, medical director of the pulmonary function laboratory at the David Geffen School of Medicine at UCLA, has spent his career studying the health effects of marijuana.
He says this study is helpful because it was relatively large and followed people for a long time, which gives him confidence in the results.
“The main thrust of the paper has confirmed previous results indicating that marijuana in the amounts in which it is customarily smoked does not impair lung function,” he says.
His own study of heavy, habitual marijuana smokers — people who smoked the equivalent of a joint a day for 50 years — found no harmful effect on lung function.
But he says none of these studies should be taken as the last word.
Other experts agree.
Barry J. Make, MD, co-director of the COPD program at National Jewish Health in Denver, says it can take years and even decades for the lungs to become so damaged by smoking that it would affect airflow, the measure of lung function used in the study.
“It’s doesn’t mean that there isn’t more damage that you can’t see with these tests,” says Make.
Until more is known, experts say if you are using the drug, it might be safer not to smoke it.
“The smoke in marijuana contains thousands of ingredients, many of which are toxic and noxious and have the potential, at least, to cause airway injury,” Tashkin says. “In an ideal world, it would be preferable to take it in another form.”
By Brenda Goodman, MA
WebMD Health News Reviewed by Laura J. Martin, MD