Researchers found that multiple measures of lung function actually improved slightly as young people reported using more marijuana -- at least up to a couple thousand lifetime joints.
The new data come from a long-term study of more than 5,000 young adults in Oakland, Chicago, Minneapolis and Birmingham. From 1985 until 2006, researchers regularly asked participants about their past and current use of cigarettes and marijuana. They also tested how much air their lungs could hold and the maximum rate of air flow out of their lungs.