The lack of a relationship between marijuana use frequency and
symptoms of depression in females, in comparison to males, is intriguing.
As depression is more common in females, and substance use is
more common in males, these findings require further pursuit. It may
be that depression in females is more heterogeneous, and there
may be a subgroup of females for whom marijuana use frequency
and depression symptoms are more closely linked, like we found
among males. Longitudinal studies of individuals at an early age
can more closely disentangle the semiology of substance use and depression,
and whether another underlying phenotype might predict
risk for both conditions in some significant portion of substance
users. It will be important for future studies to better understand factors
that influence the link between marijuana and cigarette use in
adolescence.