Letters Section Editor: Stephen J. Lurie,
MD, PhD, Senior Editor.
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.
Lynskey MT, Statham DJ, Martin NG, Heath AC, Bucholz KK, Madden PAF, Nelson EC, Slutske WS. Early Exposure to Marijuana and Risk of Later Drug Use—Reply. JAMA. 2003;290(3):330-331. doi:10.1001/jama.290.3.330-a