When you drink things do change. Not always for the better. For her, the last 5 years have been one long party. When she was first asked she wasn't sure if she wanted to go. And now she can't say no.
Some drugs are addictive.
That means that after a time your body needs them. Without them you get shakes, sweats, cramps, all sorts of symptoms. Then it feels like you've got to have them. Like you haven't any choice. You've been taken over.