Although it is tempting to speculate on the temporal order of the
relationship between marijuana use, cigarette use, and depression,
doing so would be misrepresenting the true developmental course of
these behaviors. Our sample was 14–15 years old at baseline, and
many had already smoked at least one cigarette; 38% used marijuana
at least once in the past three months; and 25% had clinically significant
depression score based on the CES-D at that time. Therefore, many of
the behaviors and symptoms of interest had already been expressed
prior to the start of the longitudinal study, making causal interpretations
difficult. Participants were oversampled for prior cigarette use to
maximize chances to observe escalation (and not for marijuana use or
symptoms of depression); however, the high number of individuals
who use marijuana and who have high depression scores suggests
that adolescents who have ever tried a cigarette by 15 are at high-risk
for not only cigarette smoking, but also marijuana use, and higher
symptoms of depression.Our findings suggest that this may be especially
true for males.