I feel more secure with a drink specifically made for clearing your system. Find a headshop/smokeshop, ask the guys if they will give you a double your money back guarantee, and get one.

I cant remember what I used, but I used it twice and both times I passed with flying colors. I felt really good afterwords too (but not right after, I felt horrible right after). Cost a whopping $30, but I found a dude that would give me $60 back if I failed. Sold me!