Are you saying that the type of drug one is addicted to, indicates the root psychological problem? Really?
NO sir/madam, it indicates the EXISTANCE of a psychological problem, and that also encompasses an inability to cope with peer pressure. When people have a problem or are in an epic stance in their lives, they turn to
illicit drugs in pursuit of seeking a safe harbor from their troubles - or - they could just want to have a fun time.
Irrespectively, there are correlations (not certain concrete indicators) between the type of drug and the mental state desired to relinquish some sort of mental distress or external pressure.
You probably wouldn't find the largest proportion of people who are enduring a divorce smoking a blunt with their friends, but rather, seeking an escape via consumption of alcohol.
Marijuana, on the other hand, is primarily used recreationally and occasional use of marijuana may not necessarily indicate that the person is overwhelmed with mental despair.
Schedule II controlled substances primarily constitute the lesser of the "recreational" drugs.
The more illicit Schedule I controlled substances and abuse thereof, may indicate that the person is enduring an exorbitant degree of mental crisis or giving in to external pressures without discretion or control.