Addiction can be a combination of physical and psychological dependency. Many people are addicted to things psychologically while having no physical dependency. And in many cases the combination of physical and psychological dependency is what it all about.
Almost all addiction must be treated medically, because whether physical and / or psychological, it is still extremely difficult to rid yourself of it, and in many case cases can be the one thing that keeps you going.
I often think of religion as a form of addiction, because like most other addictions, it acts as a crutch for the person to lean on when they cannot cope on their own, providing either help for, or a way of blocking out, the other problems in that person’s life and allows them to continue their life without addressing the real issues.
I wouldn’t say that addiction is a choice as such, as few addicts would rationally choose to be addicts. They may choose to take whatever they are addicted to in order to help them with other problems they have in life, but not so much to be addicts as such (in fact most addicts refuse to admit that they are actually addicts and claim that they can control whatever they are addicted to, rather than being controlled by the addiction).
The term “choice” can be open to interpretation here though. If someone “chooses” to become a heroin addict because they can find no other way to deal with their problems in life, is that really a choice? Or is it more of a survival mechanism? Can clinging on to whatever you think may “save” you from the problem you are in really be considered to be “choosing”?