For a while I dated a chick who was doing drugs WAY TOO much. I wouldn't really have minded, but she started getting real self-destructive. When we started going out, she was more like me, every weekend on something, but still moderation and the ability to stop, if she had too.
She started dealing more and more regularly, decided to drop out of school, nearly got kicked out of home (was for a little while)
I started getting real, real worried about her, and I would try and go whole weekends with out either of us doing anything. It was no problem for me, but it was for her. I finally suggested she books herself into rehab (I was tactful though), she exploded (I suspect she was high - she kept trying to hide it from me I think, not really sure either way though). It went like this for another couple of weeks, but then I thought "enough’s enough", and I gave her the ultimatum of "I'll stand by you, I won't leave you, but you have to fix your problem". She went a bit ballistic, and we broke up, I was really sad coz I really liked her (unfortunately).
We totally drifted apart, right down to social contacts. She got arrested a few months later and bla bla bla (you can pick what happened next)
From what I've heard more recently though, shes picked herself up, and is working and stuff, and most importantly is either right off drugs, or is taking in more moderation.
But to cut a long story short (too late) be very careful getting involved with someone who is into drugs more than you are, if you are a moderate user.
That was not really answering anything specific to this topic, but I think is kinda relevant.