ARIJUANA HAS BEEN THE most common illicit substance used in the United States for several decades. 1,2 Understanding changes in the use of marijuana over time is important for a number of reasons. Marijuana use is associated with impaired educational attainment, 3 reduced workplace productivity, 4 and increased risk of use of other substances. 5 Marijuana use plays a major role in motor vehicle crashes 6 and has adverse effects on the respiratory and cardiovascular systems. 7-10 Marijuana use also is a necessary, although not a sufficient, condition for developing marijuana abuse and dependence as defined in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), which are clear indicators of problems in and of themselves. 11 Marijuana abuse is defined in the DSM-IV as repeated instances of use under hazardous conditions; repeated, clinically meaningful impairment in social/occupational/ educational functioning, or legal problems related to marijuana use. Marijuana dependence is defined in the DSM-IV as increased tolerance, compulsive use, impaired control, and continued use despite physical and psychological problems caused or exacerbated by use. Beyond the seriousness of these disorders in their own right, marijuana abuse and dependence increase the risk of other serious consequences, most significantly, major mood, anxiety, and personality psychopathology. 12-14

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