It probably won’t. Chances are so slim that the answer might as well be a no. There is a persistent urban legend that drinking huge quantities of water before taking a urine test will result in a negative that has been around at least since the emergence of urine testing for cannabis.

With the more widespread societal acceptance of cannabis for both recreational and medical use, fewer and fewer employers today care about workers using cannabis responsibly in their off time.

However, there are still plenty of conservative businesses and organizations that will either bar you from employment or require you to take a rehab program should their drug testing reveal any traces of cannabis in your urine.

This has led to the common question of “how long does weed stay in your urine?”, and “how long does weed stay in your system?”, both valid questions with complicated answers that are surrounded by decades of urban legend.

Here we’ll explain why drinking a lot of water isn’t a sound strategy for avoiding a positive test and what the better course of action may be for avoiding a positive result on a pee test.

Good quality tests will always reveal THC use

Strictly speaking any modern urine test for cannabis that is manufactured to current government standards will reveal cannabis use traces, no matter how much water you previously ingested just before the test.

Important caveats

However — and this is a big “however” — there is an allowed threshold of allowable cannabis traces in most tests. With so many people using cannabis these days, it would be extremely impractical to use extremely sensitive tests, as our businesses would be facing a huge deficit in workers were it to do so.

And because THC (tetrahydrocannabinol) is the active chemical compound that causes the psychoactive effects many employers consider problematic, urine tests usually check for THC, rather than the now a rather popular CBD, another compound of cannabis now widely used for medical use for its proven benefits and lack of a psychoactive effect.

The standard threshold amount of THC in a standard urine test is recommended at 50 ng/mL. This is the amount recommended by both the Federal National Institutes of Health and Substance Abuse and Mental Health Services Administration.

If your THC levels fall under this amount, it will be considered a negative finding, even if THC is detected in amounts below the threshold. This means, in theory, you can drink a lot of water on the day before the test, or on the day itself to reduce the relative amount of THC per milliliter of a urine sample.

Drinking too much water makes testers suspect evasion through over-hydration 

The downside to this, though, is that many types of urine tests can detect over-hydration, or testers themselves may suspect it, and you may be required to re-take the test. This may lead to a positive result upon re-testing.

The reason for this is, THC binds quite easily to your fat cells and it can be quite difficult to eliminate if you have been regularly consuming cannabis. Depending on your metabolism and total consumption, THC can continue to show up in urine tests from a few days to three months.

I smoked up recently. Should I be scared?

Occasional users probably don’t have that much to fear from a marijuana urine test, unless they consumed it extremely recently (one to two weeks prior). For these users, some extra hydration may help, but they shouldn’t count on it.

Regular users will have built up a more significant amount of THC in their fat cells, and it is very little or nothing extra hydration can do to help you get a negative result on a urine test, provided the test is done to recommended standards. You may luck out and be given a defective test, but don’t count on that either.

What you should do

What you may want to do, however, is to buy a home testing kit to test your urine if you are aware that your urine is about to be tested for cannabis. That way you can have a reasonable assurance if you’re safe or if you need to skip the official test and wait longer until your body starts to metabolize and pass more of the THC from your system.