Drug use is another idol that we use to fill the void that God should be occupying. I think it is very telling that much drug use goes out of control - if it controls you, God is not leading your life and it is viewed as a separation from Him (i.e. sin).
This is pretty abstract and could apply to a large number of things. Drug use is not the "you're going straight to hell" behavior that supposed godly hard-liners make it out to be. I do, however, believe it to be immoral, along with a whole host of other common behaviors.
The idea here is not that I am holier that thou. Indeed, I do things which I believe to be immoral. However, this will not lead me to rationalize, compartmentalize and justify what I do.