Drugs have never actually been much of a problem within relationships for me - I myself am a fairly low level user, various different drugs ever few weeks in low amounts (It feels somewhat strange to actually classify myself as much, even knowing that "users" is a social construct of sorts.) but my current girlfriend is much keener on drugs of just about all and sundry varieties, it's definitely a "most-days" thing for her, and I dont see why this should bother me at all.
The emotions we share for each other and the time we enjoy together has absolutely no baring on what we choose to do with our bodies and minds - the notion of this sort of control and attachments completely alien to our relationship, as well as to each of us as people.
My previous girlfriend was somewhat like being in the perpetual company of a female teenage Oscar Wilde - If it could be inhaled, burnt or formed up into a tiny little pill, she wanted it in her body - that said, any and all stereotypes here would be pointless - she was and still is healthy, stable and possessed of a remarkable genius - but I digress, the point is, in this situation yet again, there was absolutely no feeling that it was really "anyone's business".
I'm always concerned for the welfare of the people I care about, but I don't feel at all that I have the right, nor do I have the desire to change their habits and how they treat themselves.
I honestly don't understand why people think it's any of their business... the whole "I'm your boyfriend/girlfriend and I want you to stop smoking weed/taking pills/snorting coke or what have you" thing is just strange to me.