There are certain activities I will not participate in if my best friend is not there. I believe in a buddy system...for the most part. Say if I were one to take part in drinking on occasion. If my best friend was not there I would not be drinking. If I were a smoker the same stipulations apply. That doesn't mean he needs to be there to partake with me; it means he's there to ensure my well being. His well-being matters to me just as much.
So when I tell you I'm not putting whatever it is in my body I meant that for an entirely different reason. I probably do trust you, but not with my life and it's well being.
