CHICAGO — Smoking a joint once a week or a bit more apparently doesn’t harm the lungs, suggests a 20-year study that bolsters evidence that marijuana doesn’t do the kind of damage tobacco does.

The results, from one of the largest and longest studies on the health effects of marijuana, are hazier for heavy users – those who smoke two or more joints daily for several years. The data suggest that using marijuana that often might cause a decline in lung function, but there weren’t enough heavy users among the 5,000 young adults in the study to draw firm conclusions.

Still, the authors recommended “caution and moderation when marijuana use is considered.”

Marijuana is an illegal drug under federal law although some states allow its use for medical purposes.

The study by researchers at the University of California, San Francisco, and the University of Alabama at Birmingham was released Tuesday by the Journal of the American Medical Association.

The findings echo results in some smaller studies that showed while marijuana contains some of the same toxic chemicals as tobacco, it does not carry the same risks for lung disease.

It’s not clear why that is so, but it’s possible that the main active ingredient in marijuana, a chemical known as THC, makes the difference. THC causes the “high” that users feel. It also helps fight inflammation and may counteract the effects of more irritating chemicals in the drug, said Dr. Donald Tashkin, a marijuana researcher and an emeritus professor of medicine at the University of California, Los Angeles. Tashkin was not involved in the new study.

Study co-author Dr. Stefan Kertesz said there are other aspects of marijuana that may help explain the results.

Unlike cigarette smokers, marijuana users tend to breathe in deeply when they inhale a joint, which some researchers think might strengthen lung tissue. But the common lung function tests used in the study require the same kind of deep breathing that marijuana smokers are used to, so their good test results might partly reflect lots of practice, said Kertesz, a drug abuse researcher and preventive medicine specialist at the Alabama university.

The study authors analyzed data from participants in a 20-year federally funded health study in young adults that began in 1985. Their analysis was funded by the National Institute on Drug Abuse.

The study randomly enrolled 5,115 men and women aged 18 through 30 in four cities: Birmingham, Chicago, Oakland, Calif., and Minneapolis. Roughly equal numbers of blacks and whites took part, but no other minorities. Participants were periodically asked about recent marijuana or cigarette use and had several lung function tests during the study.

Overall, about 37 percent reported at least occasional marijuana use, and most users also reported having smoked cigarettes; 17 percent of participants said they’d smoked cigarettes but not marijuana. Those results are similar to national estimates.

On average, cigarette users smoked about 9 cigarettes daily, while average marijuana use was only a joint or two a few times a month – typical for U.S. marijuana users, Kertesz said.

The authors calculated the effects of tobacco and marijuana separately, both in people who used only one or the other, and in people who used both. They also considered other factors that could influence lung function, including air pollution in cities studied.

The analyses showed pot didn’t appear to harm lung function, but cigarettes did. Cigarette smokers’ test scores worsened steadily during the study. Smoking marijuana as often as one joint daily for seven years, or one joint weekly for 20 years was not linked with worse scores. Very few study participants smoked more often than that.

Like cigarette smokers, marijuana users can develop throat irritation and coughs, but the study didn’t focus on those. It also didn’t examine lung cancer, but other studies haven’t found any definitive link between marijuana use and cancer.

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