I've passed 2 drug tests with certo and both wer lab-tested. What I did was - I took 3-4 water pills in the morning and peed about 10 times. Then drank the certo mixed in a glass of water (tastes terrible so be prepared) then immediately drank another full glass of water. DONT PEE AFTER YOU TAKE IT UNTIL ITS TEST TIME. Also, if you take vitamin B12 it'll turn your pee yellow.
I've heard that certo coats your stomach and doesn't allow any of the THC to be released into your urine. It makes sense to me, and it works for me. Smoke up!