Drinking Water to Pass a Drug Test: Does It Work?


Can you flush your system of THC and its metabolites by drinking lots of water or other liquids in order to pass a drug test for marijuana? Or is this myth? Nobody has directly studied it, but we can explore kidney physiology and cannabinoid pharmacology to see whether it is possible.

The controversy of drinking water to flush out THC

Today I am focusing on another common marijuana drug testing belief: that you can “flush out your system” of THC and its metabolites by drinking lots of water or other liquids (I will refer to this as hyper-hydration)

Don’t confuse this with drinking water immediately before the test to dilute the urine (which is a whole different topic). This is referring to drinking lots of water in the days and weeks leading up to the test with the hope that more THC or metabolites will be cleared out.

The hyper-hydration claim is highly controversial. About half of websites recommend it and the other half claim that drinking excess water will not help you pass a marijuana drug test.

For example, jezebel and wikihow fall into the pro-hydration camp (if you are getting your drug testing advice from wikihow, please stab yourself in the eye with a fork):

“Drink 20 to 40 ounces of sports drink per day to restore your electrolytes and keep you hydrated. Drink roughly 1 gallon (3.8 L) of water to flush your body of toxins. Drink the tea and other diuretics throughout the day as well.”

“Drink lots and lots and lots of water—gallons of it—from the time you learn about your test up until the big moment.”

On the other hand, others like Simple Cannabis and “THC Detox Master” (their words, not mine) fall into the anti-hydration camp:

“Drinking plenty of water several days before the test is of no use. Water does not help clean the THC accumulated in the fat cells.”

“…Drinking excess water does not help your body eliminate marijuana and its metabolites from your body, nor does it speed up this process.”

I love scientific controversy! But you probably just want to know whether it is worth the trouble of pissing every 30 minutes for the next two weeks so you don’t lose your job. Fair enough, let’s get this settled.

Basics of drug excretion by the kidneys


Nobody has ever studied whether hyper-hydration affects elimination of THC or its metabolites. However, we can still use our knowledge of pharmacology and physiology to arrive at an answer for whether drinking a lot of fluids has any chance of helping you pass a drug test for marijuana.

These are your kidneys. One of their jobs is to remove toxins, including drugs, from your blood. It does this by the process of renal excretion. There are several important processes involved in renal excretion that are relevant for our discussion today:

  1. Filtration: Your kidneys use a filtration system where your plasma (the aqueous part of your blood) is filtered into the renal tubule. About 20% of the plasma that goes to your kidneys is filtered.
  2. Reabsorption: After a molecule is filtered into the renal tubule, it can be reabsorbed back into the blood. Any molecule that can easily cross membranes (and most drugs can) will be highly reabsorbed.
  3. Secretion: Some molecules are are actively pumped  from the blood into the renal tubules by transporters.

How hyper-hydration affects renal excretion

The effect of hyper-hydration on the elimination of THC and its metabolites has not been studied, but we do understand how it affects the processes of the kidney.

Many people would think that drinking a lot of water would increase filtration rate. However, one study showed that in the short term, high hydration decreases filtration rate, and another study showed that drinking extra water has no effect at all on filtration rate over the long term.

Once you understand how the kidneys control the amount of urine production, it actually makes sense that filtration wouldn’t change by drinking more fluids. The water filtered by your kidneys into the renal tubule is almost entirely reabsorbed. Your kidneys do not need to create more urine by increasing filtration, they do it simply by reabsorbing less water.

What is the impact of hyper-hydration on the reabsorption of drugs and other molecules? In several cases, a modest decrease was observed in reabsorption. Why? As water is reabsorbed from the renal tubule to the blood, the drug becomes more concentrated in the urine. This concentration gradient drives reabsorption of the molecule into the blood.

When less water is reabsorbed, there is less of a concentration gradient and also the urine flow is faster, so there is less time for the molecule to cross the membrane into the blood. The net result is that drinking extra water can reduce reabsorption, which may modestly increase renal excretion of some drugs.

There is no evidence that hyper-hydration can affect renal secretion because it is an active process that does not depend on a concentration gradient.

Does hyper-hydration increase the excretion of THC and metabolites?

Here is the basic metabolic pathway for THC (for more details, read the Professor of Pot’s great guide on how THC is metabolized):

THC metabolism

There are two important factors for whether hyper-hydration could possibly increase elimination of THC or any of these metabolites to a significant extent:

Is it reabsorbed? As explained above, hyper-hydration increases renal excretion only for molecules that undergo reabsorption.

Is renal elimination a significant percent of total elimination?: Renal elimination is just one pathway for drug elimination. If other elimination pathways happen much faster than renal elimination, an increase in renal elimination will have little overall effect.

So let’s apply each of these criteria to the four THC molecules:

11-OH-THC and THC-COO-glucuronide do not undergo reabsorption. Therefore, there should not be any effect of hyper-hydration on them.

THC and THC-COOH do undergo reabsorption, so it is at least possible that drinking a lot of water will increase their elimination. However, to what extent? Renal excretion contributes <1% to the total elimination of each. Data from other molecules indicates that hyper-hydration increases renal excretion by about 1/3. Even if we increase renal excretion by 10 times, renal excretion of THC and THC-COOH will still be insignificant relative to other pathways.

Myth busted?

Nobody has yet tested the effect of drinking a lot of water on flushing out THC and metabolites from your body. However, we assessed whether it is even possible using our knowledge of the physiology of the kidney and cannabinoid pharmacology.

Our analysis shows that drinking extra water cannot have a major impact on the elimination of THC or its metabolites. This myth is busted, but you should still stay well hydrated!

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