You've heard it a hundred times before, but a big new study released Tuesday by the Journal of the American Medical Association offers more good news for light pot smokers who happen to be concerned about their health. For heavy pot smokers, the news is not so good. "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 Associated Press sums up the findings. "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." Of course, the study's authors issue the obligatory recommendation that "caution and moderation" should be taken when using marijuana. It's also important to note that the study did not look at the correlation between pot and cancer, though the AP's careful to point out that "other studies haven't found any definitive link between marijuana use and cancer." So what's the downside? Ask the Drug Enforcement Agency.