Drink lots of fluids before the test. Also called flushing, this method remains one of the easiest, cheapest, and most successful ways to get a false negative on a pee test. Pee tests do not measure the amount of drugs in urine; they measure the amount of metabolites in the urine. Drugs are ingested and then come out of the system as metabolites. Drinking more water will help your body produce metabolites that aren’t tainted by drugs.
Urinalysis checks for 31 different metabolites caused by marijuana, 4 caused by cocaine, 3 caused by opiates, 1 caused by Phencyclidine, and 5 caused by amphetamines.
Be careful not to over-hydrate. Although rare, it is possible to drink too much water and damage your internal organs, enough so to become fatal. This is called water intoxication.