Urinalysis doesn't test for "THC", they detect THC metabolites, the end result of your body breaking down the active ingredient in marijuana. There is no way to "flush" your system clean of these metabolites, the best you can hope for is to dilute your urine enough to not trigger a positive on the test, water is best for this. Since I'm sure you're not going to show up drunk to a drug test, alcohol isn't suitable for test day dilution.

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