Awhile back, the Washington Post ran an excellent article by Maia Szalavitz on what exactly addiction is, and how we should go about treating the Britneys and Lindsays out there.
To get to the article, click here. Below are excerpts:

Today’s most widely accepted definition of addiction — used in psychiatry’s latest edition of its diagnostic manual, the DSM-IV-TR — recognizes that compulsive use of a substance despite negative consequences is key. And that’s exactly what I experienced: At least six times, I made it through the physical sickness of heroin withdrawal — the shaking, diarrhea and vomiting — only to use again because I wanted the drug. This compulsive aspect helps explain why we can now consider video games and, yes, even potato chips more addictive than we did in the past.
But the DSM retains a focus on physical aspects of addiction: It calls addiction “substance dependence,” suggesting that physical need is critical. Tolerance and withdrawal are part of the criteria used to diagnose the condition, even though pain patients taking opioids as directed may experience both and not actually be addicted. Studies find that less than 1 percent of people who take pain medications and don’t have a past history of drug problems become addicted. Many pain patients who stop opioids after the source of their pain has been removed even undergo withdrawal without realizing it: It’s called “hospital flu.” But the vast majority have no difficulty refusing further medication.
As a result, experts — including NIDA director Nora Volkow — have called for the official name of the disorder to be changed from “substance dependence” to “addiction” in the next edition of the DSM. They say the confusion between physical dependence and addiction leads to under-treatment of pain: Surveys find many patients, even those who are dying, don’t receive enough medication for effective relief. Physicians are even criminally prosecuted for “over-prescribing” when patients with painful conditions become physically dependent on opioid drugs.