This has worked in 100% of the cases for people I know, including myself (like 9-for-9 or so).
I believe it is also available at the Vitamin Shoppe, if you have one of those near you.
Also, make sure it's a piss test - one of my buddies actually had to take a hair test. Luckily for him, he was well-prepared and had bought the masking shampoo as well.