substitution is too risky, messy, and just plain stupid. Simply follow the dilution guidelines of drinking lots of water, cranberry juice, gatorade, etc.. the day of your test. Make sure a day or two before, you test yourself with a simple at home drug test kit you can pick up for $14.99 at your local CVS pharmacy, then you will know for sure. I've had 8 drug tests in my life, only ever failed 1 and that was because I only gave myself 35-40hrs of being clean, didnt drink enough water the day of my test, and didnt buy an at home test kit. Every other one I've had, I've only needed 5-7 days of being clean, and I passed everytime. Keep in mind, you dont have to be COMPLETELY clean of the THC metabolites, you only need to be below the threshold of 50 ng/ml, anyone who does drug tests for a living (my mom used to) will tell you that. Good luck =)