When discussing this topic I really think you need to look at the DRUG, SET and SETTING and how this applies to an individual's behaviour. Being addicted to a substance ie. going into some sort of withdrawal etc, is one thing when you don't have that drug. Being addicted to an experience, ie. clubbing while on a substance is something else.
Particularly a substance taken in a context or a setting that provides a very powerful experience. If an individual has mental health issues or is dealing with just the day to day lifes little setbacks or anxieties, going out and clubbing and taking pills may be the most meaningful experience for someone in in dull and dreary life. Lets face for some, the weekends by sheer definition are far more exciting than our weekday existences. To remove this experience, the drug, the people, music etc, when taken away from someone could be quite traumatic. Whereby the individual may manifest signs of withdrawal. I just think it's extremely difficult to say a substance is addictive without examining these other factors. I think the whole concept of addiction is such a subjective experience.