Your mention of susceptibility to addiction reminded me of a sad irony. The population that's among the most susceptible to addiction are those with untreated ADHD. ADHD impairs your ability to consciously choose your actions -- many doctors think ADHD is a misnomer and Executive Functioning Disorder would be a better one. It drastically increases one's stimulation-seeking behavior, since adrenaline acts as a form of natural medication. (This is why you find higher-than-expected numbers of first responders and military personnel with ADHD, and why they tend to be so calm in a crisis.)
Unfortunately, recreational drugs are a one-two punch. First, the activity of taking recreational drugs is highly stimulating in and of itself, due to its risk, which means that the simple act of taking something, anything, can become damn near irresistible. Second, the effects of many of these drugs actually do end up acting as a quasi-medication. Cocaine, for instance, is quite chemically similar to Ritalin; meth is an amphetamine like Adderall; opiates seem to act almost as stimulants for many with ADD (including me), and also help to deaden the hyperactivity a bit; alcohol mercifully slows down your thinking; and so on.
The irony then is that many drug addicts would significantly benefit from a prescription of Adderall or Ritalin, but unfortunately these medications are very difficult to have prescribed if you have a history of addiction. I understand why this is the case, but I can't help but feel sad for those who unfortunately may never get the treatment (or even the diagnosis) they need to rein in their lives.