Giving to an addict - of any kind - is tantamount to being party to a slow murder of another. In handing over money you've entered into that agreement. We're as responsible and accountable to our selves/god as much as we are to others. So if I wouldn't give money to a friend or family member who's an addict due to 'tough love', why would I think any less of an identified street addict? Life's tough enough for those who've made the addict choice, without me adding to their woes.
Now giving of food is a different matter...although in my part of the world it needs to be opened first or the person begging will often try to return it to the store for cash (esp if they're an addict)or sell to other homeless/addicts.