One of the most frequent questions posted here is whether drinking large amounts of fluid in the days to weeks before a urine test will help "flush" or clean out the THC metabolites from one's "system." Most answers correctly state that this practice is useless, though the questions keep coming. Here's an explanation that might clear things up: FACT: THC is stored in body fat and is in equilibrium with itself and its metabolites in the bloodstream. The natural course of things is for THC to go from fat to bloodstream to kidneys, where it (or its metabolites) is filtered into the urine. A sizable portion also is exreted in stool but to date I do not know of any stool-THC tests out there in use. FACT: The amount of THC delivered to the kidneys for filtration into the urine is determined by the THC concentration in blood and the glomerular filtration rate (GFR), which is simply a measure of how much blood is filtered by the kidneys in one minute. FACT: A person's GFR remains relatively constant through a process called autoregulation, by which reflex changes in vascular tone inside the kidney help keep GFR constant despite daily fluctuations in blood pressure or cardiac output, such as with exercise. FACT: Drinking excess fluids will make you urinate more, yes, but not by increasing GFR. The increase in urine with say, drinking a gallon of water, is a hormonally mediated change. Drinking a lot of water lowers the blood osmolality (the concentration of solutes in blood). The body needs to keep blood osmolality very tightly regulated, so the response to a fluid load is to excrete the excess water due to a shutdown of antidiuretic hormone, which diminshes the kidney's ability to reabsorb water in the kidney. However, GFR is unaffected. The same amount of blood is filtered by the kidneys per minute, and the same amount of THC is filtered into the urine per minute, it's just that less of that filtered load (which includes a lot of water) is reabsorbed later in the kidney due to the temporary shutdown of antidiuretic hormone. This hormone normally helps the kidney reabsorb filtered water, so we don't all pee out all of our water and die, but when osmolality is at stake the body must get rid of the excess water. FACT: The only times when drinking a lot of fluid will raise the GFR is if the GFR has been lowered to start with, most commonly due to dehydration. Even if someone has chronic kidney disease, drinking more water will not increase the GFR unless they too happen to be dehydrated. FACT: Pregnancy actually increases GFR due to expansion of blood flow to the kidney. The only ways to help "flush out" the "system" of THC are: 1)Get pregnant 2)Have a surgeon place a third kidney inside of you 3)Drink fluids to reverse dehydration, but once you're normally hydrated drinking any more fluids will be uselss. Don't be confused with dilution. Dilution methods do not help you rid your body of THC any faster. The same amount of THC per unit time is in your urine, it's just that it exists in a larger volume of urine when you drink a lot and therefore the concentration is decreased. Drinking a lot in the days before a test will keep you diluted most of the time, but the same amount of THC is in your body. Thus, don't bother with drinking a lot until you're ready to dilute and take your test. As for aerobic fat burning exercise, that will burn more fat, which places more THC into the bloodstream. Though GFR remains constant, the higher THC concentration in the blood that is being delivered to the kidneys will result in faster bodily elimination of THC. Thus, aerobic exercise WILL help "flush you" out. Just be careful in those few days before a test. The common dogma is that during the few days before a test, you want to exercise very little and pig out so as to go into "fat storage mode". This makes sense, since burning less fat before the test minimizes how much thc is being released into your bloodstream, filtered by your kidneys, and ending up in your urine to be DETECTED. IN SUMMARY: Drinking a lot in the days to weeks before your test will just keep you relatively diluted. It will NOT help you rid yourself of total bodily THC any faster. Save the fluids for when you're ready to dilute, and go for it!