From NIDA website http://www.drugabuse.gov/news-events/news-releases
NIH-funded (National Institutes of Health - USA) research shows that long-term marijuana is associated with impaired intellectual functioning, especially if usage starts during the teen years. Over 1,000 study participants were given neuropsychological tests in early adolescence, prior to initiation of marijuana use, and then re-tested in mid adulthood. Study members with more persistent marijuana dependence showed greater IQ decline and greater impairment across five different cognitive domains, especially executive function (The cognitive process that regulates an individual's ability to organize thoughts and activities, prioritize tasks, manage time efficiently, and make decisions and processing speed). This is the first long-term prospective study to test young people before their first use of marijuana and again after 20+ years of use. The study was thus able to rule out pre-existing differences in IQ between heavy marijuana users and others; it is also significant for including degree of cannabis exposure and age of onset as factors. Those who started use during the teen years showed greater IQ decline than those who began use as adults. These latter results are especially troubling, given recent data showing increased marijuana use among teens over the last five years, along with declines in perceived risk of harm associated with use. The results of this study are consistent with the notion that cannabis may actually cause some of the neuropsychological deficits seen in regular cannabis users.