I've never done drugs or smoked - not good for your health, hard to stop when started and (most importantly in my opinion) my parents would kill me if I did it even once.
As for alcohol, I think I'm intolerant to it. I can't stand the taste and aftertaste (not even a weak cider) and if I do force myself to swallow it - as I did when I was younger and my dad gave me half a pint of cider - it really mucks about with my stomach.
Why anyone thinks its a good idea to drink loads is beyond me; I hate it and get scared of my family if any of them has too much to drink as they are suddenly much harder to predict and their emotions are all out of balance - meaning they can become quite a pain in the neck or overly emotional in an instant and so I then have to spend my time watching over them until they eventually settle down.
By the end of it all, I'm often exhausted. The next morning, they often only have a few memories of the night before so are often shocked when i tell them some of the things they got up to during their drunken mood swing.
Due to my sensitivities, I can't eat or drink certain things. For example, I can't drink coke as the acid burns my tongue, screws up my insides and leaves a chemical aftertaste in my mouth.
As such, I would definitely say I'm a straight-edge kind of guy.