I don't want this to be too off topic, I learned something kind of interesting last week while watching the trial in Dallas of Chante Mallard (girl who hit homeless man and parked him in the garage hanging out of her windshield) anyway.....she stated under oath that she used to pass her drug test at work by putting a drop of bleach in her urine.
The best advice, as stated in the post above, is to clean your system out but it depends on how long what particular drug stays in your system. Marijuana stays in your system for 30 days - I didn't know that either. I just found this to be of interest, I of course do not use illegal drugs.