FILE - In an April 20, 2005 file photo, a University of Colorado freshman, who did not want to be identified, joins a crowd smoking marijuana during a "420" gathering at Farrand Field at the University of Colorado in Boulder, Colo. People who started using marijuana persistently before age 18 risk losing some of their IQ by the time they're 38, a long-running study says. In contrast, even long-term chronic users who started after age 18 showed no such effect, suggesting the drug holds some particular toxicity for the developing brain. (AP Photo/Longmont Daily Times-Call, Richard M. Hackett, File) MANDATORY CREDIT, MAGS OUT, NO SALES

Associated Press
FILE - In an April 20, 2005 file photo, a University of Colorado freshman, who did not want to be identified, joins a crowd smoking marijuana during a "420" gathering at Farrand Field at the University of Colorado in Boulder, Colo. People who started using marijuana persistently before age 18 risk losing some of their IQ by the time they're 38, a long-running study says. In contrast, even long-term chronic users who started after age 18 showed no such effect, suggesting the drug holds some particular toxicity for the developing brain. (AP Photo/Longmont Daily Times-Call, Richard M. Hackett, File)  MANDATORY CREDIT, MAGS OUT, NO SALES
FILE - In an April 20, 2005 file photo, a University of Colorado freshman, who did not want to be identified, joins a crowd smoking marijuana during a "420" gathering at Farrand Field at the University of Colorado in Boulder, Colo. People who started using marijuana persistently before age 18 risk losing some of their IQ by the time they're 38, a long-running study says. In contrast, even long-term chronic users who started after age 18 showed no such effect, suggesting the drug holds some particular toxicity for the developing brain. (AP Photo/Longmont Daily Times-Call, Richard M. Hackett, File) MANDATORY CREDIT, MAGS OUT, NO SALES
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