Water, if you drink enough, will in fact reduce the amount of nanograms/ml of amphetamines that can be detected from a urinalysis (drug test). Urinating will release small amounts of the drug's metabolites from your system because this type of drug IS water soluble (unlike marijuana's active chemical THC which is strictly fat soluble, meaning you canNOT urinate it out), and if you drink juices full of antioxidants (cranberry juice, blueberry juice, decaffinated green tea, etc) it will push an even higher concentration out of your body than just H2O. Drinking acidic beverages will speed up the process even more, for instance orange juice or even vinegar if you really trying to rush the process. (Acid breaks down the detection rate of amphetamines 3 times faster than normal.) Also, if you do decide to just dilute your urine prior to the drug test, be sure to take multiple B12 vitamins to ensure your pee is a yellow color rather than clear. A clear sample could result in an automatic POSITIVE result from the lab technician and you could be required to produce another sample that day.
