Alcohol. I avoid alcohol due to the fact that my father was an alcoholic. This leads to a couple of reasons. For a start, I have seen how debilitating alcoholism first-hand (he was a violent drunk; he'd abuse myself, my mother and my sister) and don't want to go down that route. Also, there is evidence to suggest that it's hereditary so I'm probably genetically inclined towards alcoholism. I also have a VERY addictive personality.
I read this, looked at your location, and proceeded to laugh my *** off.