People talk about how addictive methamphetamine is. I know people who tried and never wanted to try it again. It was the most unpleasant experience. Like you drank way too much coffee and had all the anxiety and jitters to go along with it. For other people, it's like heaven.
This goes back to the idea of "self-medication" as a root cause of drug addiction. People get psychologically addicted to a drug because it solves a psychological issue for them. Depression? It goes away when I take meth. Anxiety? Gone when I take oxy.
Thinking about it that way, we shouldn't really be that surprised when people have trouble beating an addiction. Even with all the negative side effects, would you want to stop taking a drug that cured (imperfectly) a severe problem you've had your whole life?