Occasional marijuana use does not appear to have long-term adverse effects on lung function, according to new research published in the Journal of the American Medical Association.
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.
Measuring participants' lung function for air flow and lung volume five times throughout the study period, the researchers found 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, the research should not be viewed as a green light to toke up.
"As a primary care doctor, I see patients who have problems with drugs and alcohol," Kertesz said. "This 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.
While an adult male blows out about four liters of air in one second, those who occasionally smoked weed could blow out those four liters, plus another 50 milliliters -- about one-seventh of a soda can. 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 United States. 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."
As an institute that studies drug abuse, 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.
Some health experts have questioned whether the study's findings are conclusive. 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," MacCoun said. "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," MacCoun said. "So this study is a cautionary note that we still have a lot to learn about this complex psychoactive plant."