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Letters
July 16, 2003

Early Exposure to Marijuana and Risk of Later Drug Use—Reply

Author Affiliations
 

Letters Section Editor: Stephen J. Lurie, MD, PhD, Senior Editor.

JAMA. 2003;290(3):330-331. doi:10.1001/jama.290.3.330-a

In Reply: We disagree with Dr Schier and colleagues that the high rate of alcohol dependence in our sample invaliditates our conclusions. First, we reported lifetime prevalence, whereas previous work only presented 12-month estimates.1 Furthermore, the rates we reported were for twin pairs discordant for early cannabis use. Early substance use is associated with increased risks for substance dependence and thus elevated rates of alcohol (and other) drug dependence would be expected among early cannabis users. Because a large component of the association between early use and later risks of dependence likely arises from genetic and shared environmental factors preceding cannabis use, and because co-twins of early users would have been exposed to these same risk factors, they would also be expected to have elevated rates of alcohol dependence. Thus, the elevated rate of alcohol dependence reflects and reinforces our rationale for conducting the study. Finally, it demonstrates that a large component of previously observed associations between early cannabis use and later drug use can be attributed to heritable and shared environmental factors. In this respect, it is noteworthy that the strength of the associations that we report is substantially lower than those reported by previous studies that have not included such stringent control.

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