Occasional marijuana use does not affect lung function: Study

By Dr. Ananya Mandal, MDJan 11 2012

A new study published in the Journal of the American Medical Association has shown that an occasional drag on marijuana does not appear to have long-term adverse effects on lung function.

Researchers from the University of Alabama at Birmingham and University of California at San Francisco analyzed marijuana and tobacco use among 5,000 black and white men from the national database, CARDIA (Coronary Artery Risk Development in Young Adults study), which was intended to determine heart disease risk factors over a 20-year period. They measured participants' lung function for air flow and lung volume five times throughout the study period.

Results showed that cigarette smokers saw lung function worsen throughout the 20-year period, but marijuana smokers did not. Only the heaviest pot smokers (more than 20 joints per month) showed decreased lung function throughout the study.

“The more typical amounts of marijuana use among Americans are occasional or low levels,” said Dr. Stefan Kertesz, assistant professor of medicine at the University of Alabama at Birmingham and principle investigator of the study. “From the standpoint of being a scientist, these data suggest that low and moderate range use of marijuana do not do long-term harm.”

But, he cautioned, this result should be taken with a pinch of salt. “As a primary care doctor, I see patients who have problems with drugs and alcohol,” continued Kertesz. “This [marijuana] is a complicated substance that has a lot of potential effects on human life and well-being.”

Among the study participants, the average pot smoker lit up two to three times per month. The average tobacco user smoked eight cigarettes per day. Those who smoked less than the heaviest actually saw a slight increase in air flow and lung function. But otherwise, researchers actually saw a slight increase in lung function among marijuana users. Kertesz said that the enhanced lung capacity could be due to the extended and heavy inhalations done while smoking marijuana rather than any beneficial effect.

Marijuana is the most commonly used illegal drug in the U.S. About 16.7 million Americans 12 and older reported using marijuana at least once in the month prior to a survey conducted in 2009 by the National Survey on Drug Use and Health. Still, the debate goes on as to whether pot should be legalized. So far, 16 states have legalized the substance for medical use to curb symptoms in patients with pain, AIDS, cancer and several other conditions.

“This research will no doubt contribute to the public dialogue on marijuana, but it is far from conclusive when weighing the risks versus benefits of smoked marijuana in different populations,” a spokesperson for the National Institute of Drug Abuse said in an email. “For example, it is not clear how these findings relate to patients whose levels of exposure to marijuana are not known, who may be vulnerable as a result of their illness, and/or who are using marijuana to attempt to treat symptoms related to chronic conditions.” NIDA noted that the results should not overshadow other established harmful effects of marijuana, such as adverse effects on cognition, potential for psychosis or panic during intoxication and the risk of addiction, which occurs in 9 percent of users.

Dr. Donald Tashkin, who has studied the relationship between marijuana smoking and lung function for more than 30 years as a professor of medicine at UCLA, says the study confirms what other research has also concluded. “This is a well-done study involving more subjects than in the past,” says Tashkin, who is not affiliated with the new study. “The public should take away it’s a confirmatory study, but larger and longer than previous studies demonstrating, once again, that smoking marijuana does not impair lung function, unlike tobacco.”

Tashkin says scientists have a theory that lung capacity is not affected in marijuana smokers because the chemical THC in marijuana has immunosuppressant properties that interfere with the development of respiratory issues such as COPD. He says this indicates there will be lower rates of COPD, but marijuana smokers are still at risk for chronic bronchitis, which means they tend to have increased cough and mucus. The study didn't look at the risk of lung cancer.

And Tashkin cautions about drawing overall conclusions from the new work: “We’re only talking about one end point. We’re not looking at lung cancer, chronic bronchitis symptoms. We are not looking at other effects, behavioral effects. We are looking at lung function.”

“I think a lot more work will need to be done to make any blanket statements about safety,” said Dr. Jeanette Tetrault, a substance abuse researcher at the Yale School of Medicine in New Haven, who wasn't tied to the new research. “These are only two measures of pulmonary function and don't really paint the entire picture” of the potential effects of marijuana on the lungs, she told Reuters Health.

Robert MacCoun, professor of public policy and law at University of California at Berkeley, said that while the study was carefully conducted, the results are purely correlational. “The results must be interpreted cautiously,” said MacCoun. “For example, it is difficult to be certain that any comparison between the non-smokers and marijuana smokers actually reflects effects of marijuana, rather than some other differences between the groups.” Experts agreed that the study does not provide evidence that marijuana smoking is healthy for the lungs, but that marijuana is indeed a complex substance. “I think what is most striking about the results is that we are so accustomed to studies emphasizing, and sometimes exaggerating, how dangerous marijuana is for users' health,” said MacCoun. “So this study is a cautionary note that we still have a lot to learn about this complex psychoactive plant.”