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 recommend "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 showing that 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.

The chemical also helps fight inflammation and might 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 might help explain the results.

Unlike cigarette smokers, marijuana users tend to breathe in deeply when they inhale a joint, which some researchers believe 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.