Drug Addiction Riding Behind Mental Illness?

Drug addiction, despite being seen by many as some sort of failing of the mind, is not listed as a mental health disorder. Substance abuse has escaped that category despite the fact that comparatively less troublesome ailments or sicknesses like social anxiety disorder and (in some circles) insomnia are listed as possible or acknowledged mental health conditions. The facts known about drug addiction show that it is a biological and physiological condition, with the body craving the effects that these narcotics have on the brain. The divide between mental disorder and drug addiction is a very thin, blurry line, but there is a line. However, recent research is starting to reveal information that is making this line seem even thinner and more blurred than it already is. It would appear that drug addiction and mental conditions, such as social anxiety disorder and depression, are not as distinct form one another as initially thought.
In layman's terms, when one person shows signs of being a drug addict, there's usually some sort of mental health condition riding the coattails, though not everyone who's crazy is a junkie, and not every drug addict is insane. The psychological problems tend to vary from patient to patient, though things like social anxiety disorder are common in teenage addicts, along with depression, performance anxiety, and a few behavioral disorders. Schizophrenia, bipolar and unipolar depression, and other personality disorders are also commonly observed to tag along with addictions, though not always with narcotics and other illegal drugs. Nicotine and alcohol addicts also tend to have a host of mental health problems riding in their wake as well.
Some recent studies are showing that damage to certain regions of the brain may be responsible for making people more likely to develop addictions, with the amygdala taking center stage in the study's findings. This does not take away anything from the natural addictive abilities of substances such as alcohol, opioids, and nicotine, but it does serve to explain why some people appear more likely to become addicts than others on a psychological level. The studies also discovered that addictions for people with damaged amygdala are not only more prone to addiction, they are also less likely to discern from one substance to another in their abuse. Findings showed that it didn't seem to matter what the substance was or what the effects it had on the mind and body were, so long as they had the potential to be habit-forming and the subjects were exposed to it regularly.
Obviously, since mental health problems such as social anxiety disorder and dissociative identity disorder can make someone more likely to become an addict, there are things that need consideration. A number of drug addicts can and do claim that external factors forced them into their substance abuse, with several of these reasons being highly similar to things that trigger mental illness. With psychological conditions now leading to substance abuse, is there now reason to believe that those who are genetically predisposed towards mental illness are, logically, also more likely to become addicts?