I don't know if it works for substances containing THC but it definitely works for amphetamines. My daily consumption of 60-120 mg of amphetamine (Adderall) plus regular (and long term) inhalation of methamphetamine (meth crystals, ice) were stopped 2 days before a urine test. One and a half hours before the test, a 16 oz bottle of Rely Detox - Tropical was consumed followed by about 24 oz of water. The test passed.
Due to the nature of my work, I am subject to urine tests several times a year. This was the first one I ever passed immediately. The results of all others - deemed inconclusive by the initial lab work - had to be reviewed by an MRO to establish a legal reason (i.e.valid prescription) for the substance to be in me.
I would clearly recommend this product to anyone using substances proscribed for certain types of employment by certain types of employers.