How Long Weed Stays in Your System (Urine vs. Saliva vs. Blood)

It’s never a good time for a drug test. You might have been showing up for work on time every day and just smoking in the evenings or at the weekend, but then you find out there’s a drug test coming up. Then, there is one crucial question: how long does weed stay in your system?

If you’re in this situation and are worried about what is going to happen to you, we’ve collected everything you need to know about drug testing for cannabis, and what you should do in the worst case scenario.

How Long Does THC Stay in Your Body?

Testing Method	Screening Test Cutoff	Detection Time (For Regular Users)
Urine	50 ng/mL	Up to 21 days
Saliva	4 ng/mL	2 – 3 days
Blood	1 ng/mL*	2 – 3 days
Hair	1 pg/mg*	Up to 90 days

*These cutoffs are not set by the federal government and may vary.

On average, cannabis use will show up in a standard urine drug screening for around 21 days if you consume regularly, and possibly up to a month. It will likely remain in your system for longer, but at levels that wouldn’t cause you to fail a drug test.

These timescales can vary a lot. In extreme cases – and with certain biological factors – it may take longer than a month. Likewise, it’s technically possible that you’ll pass a test within three days. 

Factors beyond cannabis consumption frequency such as age, biological gender, body mass index (BMI), water and fat distribution, metabolism, and frequency of exercise all play an important role in determining how long THC metabolites stay detectable in your body.

The length of time weed stays in your system also depends on how exactly you’re testing. For example, drugs are detectable in your hair for longer than they are in your saliva. However, most drug testing in the U.S. is in the form of urine tests, so this is the main thing you should consider unless you have reason to believe they’ll use saliva or blood testing.

Drug testing consultant Joe Reilly clarified to us that:

“Current methods for testing include urine, hair, oral fluid, and blood. They are all accurate. The difference is detection times. Oral fluid has the shortest window of detection, then urine and blood and then hair with the longest window of detection.”

Cannabis Is Detectable in Urine of Regular Users for 21 Days or Longer

For regular users, cannabis will be detectable in your urine for between 7 and 21 days on average, and possibly longer in some cases.

It’s unlikely that you’ll fail a standard urine drug test with the federally-recommended 50 ng/mL cutoff for more than 21 days. If it’s the first time you’ve used cannabis, you’ll probably be clear for the test within 3 to 5 days.

However, technology to detect cannabis use in the urine longer than this certainly exists. In fact, with a much smaller cutoff value of 20 ng/mL, regular users can still have detectable cannabis metabolites in their system for over 30 days.

Many things make this complicated. For one, the amount of metabolites in pee doesn’t decrease steadily – it can fluctuate over time as a result of various biological factors.

Dr. Benavides explains, “Overall, the research has also shown that it is possible to test negative then positive, and then negative again. Such variations are due to the fact that cannabinoids and their metabolites are lipophilic, meaning that they attach themselves to fatty tissues and have a hard time letting go even when exposed to lots of water.”

So, putting this all together, when you wonder “how long will cannabis use be detectable in my pee?” there are actually many different questions you could be asking, with different answers. We’ve estimated the answers based on seven studies, summarized in the table below:

• How long until I might pass a standard urine drug screening? You could pass a standard 50 ng/mL drug screening after around 10 days of abstinence.
• How long until it’s unlikely I will fail a standard urine drug screening? It’s unlikely you’ll fail a standard drug screening at 50 ng/mL after 21 days.
• How long until a researcher might conclude that I haven’t used cannabis recently? A lab using a 20 ng/mL cutoff might start to see negative results after around 10 days to two weeks.
• How long until a researcher would definitely conclude that I haven’t used cannabis recently? It would take around 31 days or possibly more before you could be confident a lab-quality test wouldn’t flag your use.  
• What is the absolute worst case scenario? One study from a navy rehab in San Diego (Ellis et. al., 1985) had one participant test positive after 77 days, at 20 ng/mL.
• What is the absolute best case scenario? In all of the research cited, some heavy users passed a urine test (at 20 ng/mL) after just 3 – 4 days.

Study	Cutoff (ng/mL)	Time to first negative/mean	Time to last positive/max	Other
Ellis et. al. (1985)*	20	19.1 days	31.5 days
Smith-Kielland, Skuterud and Morland (1999)	15	5.8 days	14.1 days
Smith-Kielland, Skuterud and Morland (1999)	10.3	8.7 days	19.4 days
Reiter (2001)	20	4.9 days (mean)	18.1 days (max)
Kouri (1999)	20	18.8 days (mean)	>28 days (max)
Schuster et. al. (2019)	50	8.1 days	9.4 days
Lewis et. al. (2015)	15			73% of subjects below cutoff after 14 days.
Swatek (1984)*	20	24.7 days	26.9 days
Swatek (1984)*	50	10.9 days	18.9 days

*Cannabis use suspected or confirmed during study

Myth: Regular Weed Smokers Will Still Fail Drug Tests Over 30 Days Later

Unfortunately, much of the information available online about this topic is misleading at best. It is very common for sources to claim that THC’s metabolites will be detectable in your pee for over 30 days, and even academic sources (Moeller et. al., 2017) make this claim.

However, as Cary (2005) points out, the sources they use don’t hold up to scrutiny. The very high detection times reported are usually:

• A single participant out of a whole study: the averages are more reliable.
• Research conducted in the 1980s, possibly with outdated testing methods. Older tests with lower specificity are more likely to generate false positives (Huestis, Mitchell and Cone, 1994). In other words, if you’re more careful about only counting the right chemical; your final total will be lower.
• Where researchers use a lower cutoff than the standard 50 ng/mL.
• In situations where researchers didn’t know if they’d smoked more during the study, when or how often it occurred before the study, or in some cases (Swatek, 1984) where it was confirmed that they had.

Moeller et. al. (2017), for instance, cites research from Ellis et. al. (1985) to support the claim that cannabis can be detectable for over 30 days in heavy users. But this study has many of the issues listed above:

• It used a lower cutoff than recommended by the federal government for initial marijuana tests (20 ng/mL vs. 50 ng/mL).
• It was conducted in 1985.
• The average time to the first negative result was 16 days.
• The data shows a second “peak” part-way through that was unexplained by the authors. It’s therefore possible, Cary (2005) argues, that some participants used cannabis during the abstinence period.

This isn’t to say it’s impossible for cannabis to still be detectable for more than 30 days. However, it is not as common or likely as most sources and ranges suggest.

For First-Time or Infrequent Users, Your Pee Should Be Clear Within a Week

If you rarely use cannabis, it will only be detectable in your urine for a few days, generally 3-4 days at most. If you smoke sometimes, it could be up to a week.

Marijuana is stored in your body’s fat cells, and in regular users, the build-up leads to longer detection times (Moeller et. al., 2017). For infrequent users, this hasn’t had the chance to build up, so studies which focus on first-use (such as Niedbala et. al., 2001) find maximum detection times of less than three days.

Research on regular but infrequent use (i.e. smoking a couple of times a week) is less common but generally suggests (Smith-Kielland, Skuterud and Morland, 1999) detection times of around five days to a week.

The evidence we’ve used to produce these estimates is summarized below.

Study	Cutoff (ng/mL)	Time to first negative	Time to last positive	Other
Smith-Kielland, Skuterud and Morland (1999)	15	2.8 days	2.6 days
Niedbala et. al. (2001)	50	1.08 days	1.75 days
Niedbala et. al. (2001)	15	1.38 days	2.42 days
Spindle et. al. (2019)	15		5 hours	Vaping was detectable slightly longer than smoking.
Brenneisen et. al. (2010)	15		Of those who tested positive: 12 hours	Only 33% tested positive. With a much lower cutoff, it was a mean of 80 hours until the last detection.

THC Is Detectable in Your Saliva for Around 2 Days

Saliva tests are more useful for detecting recent use than urine tests, and it’s likely to be detectable (at standard cutoff values) for around 2 days, possibly 3.

In most cases, saliva drug tests for cannabis look for THC itself, but they can also look for the major metabolite (11-nor-9-carboxy-THC, a.k.a. THC-COOH). Testing for THC itself (Lee et. al., 2011) only showed positive results for 2 days in everyday smokers, but for THC-COOH, there was a median detection time of 13 days. However, the authors note that using cutoff values of 2 μg/L for THC (in line with federal testing guidelines for the confirmatory test) and 20 ng/L for THC-COOH reduces the detection time to two days in all cases.

However, THC-COOH in saliva isn’t part of the federal testing guidelines, and it’s only recommended (Cone and Huestis, 2007) to substantiate use, not prove it. This is essentially because THC is metabolized inside the body after ingestion, rather than in the mouth itself. So while small quantities of THC-COOH will likely be present, this isn’t the main thing that saliva testing is looking for.

Cannabinoids Will Stay in Your Blood for 2 to 3 Days

Blood testing for cannabis is more useful for catching recent use, like saliva. THC-COOH will only be detectable in the blood for a couple of days after your last use.

Reiter et. al. (2001) also looked at serum concentrations of THC-COOH in the 52 closely-monitored volunteers, and found that THC-COOH was detectable (at 20 ng/mL or more) for a mean of 34 hours, with a maximum value of 74 hours (just over 3 days). 

This study did have a limited number of cannabis users (22, based on urine results), though, so it should be taken with a pinch of salt. Another piece of research with 25 participants (Desrosiers et. al., 2014) found that THC-COOH was still detectable (at 1 ng/mL or more) after 30 hours. However, they did not measure beyond 30 hours to find the last negative result time.

THC Is Detectable in Your Hair for Around 90 Days (If It’s Detectable At All)

THC is detectable for around 90 days in your hair, but in a surprising number of cases, it’s not detectable at all. This is because hair takes time to grow and so fresh use may not be detected in time.

While saliva and blood testing pick up very recent use, hair testing is mainly valuable for detecting drug use from quite some time ago. Each half an inch of hair captures around 30 days’ of drug use (Gryczynski et. al., 2014), so a 1.5 inch sample of hair can cover 90 days of cannabis use. Because of this, it’s more likely to be used for checking abstinence for something like rehab or a scientific study.

Most Evidence Shows Hair Testing for Marijuana Is Inaccurate

However, this is only true if it captures the use at all.

• Gryczynski et. al. (2014) took 275 hair samples to test for marijuana, with a cutoff of 1 pg THC/mg of hair for the screening. When compared to self reports of use, the hair testing only picked up 57.5% of those who said they’d used marijuana.
• Huestis et. al. (2007) took 53 hair specimens from 38 cannabis users, and found that 36% showed no sign of use (either THC or THC-COOH), and another 26% only had THC-COOH in their hair. This means just 38% of users tested positive for THC itself at 1 pg THC/mg of hair, with only 30% of non-daily users being detectable this way and 65% of daily users.
• Taylor et. al. (2016) found an overall correlation of about 52% between reported and detected cannabis use.
• Ledgerwood et. al. (2008) found that while cocaine use was detected more than it is reported, cannabis use is detected in hair less frequently than it is reported. It’s easier to detect cannabis use this way in regular users, but still far from perfect.  

So while it is certainly true that a hair test could pick up some weed smoking from a few months ago, it’s also unavoidably true that there’s about a 50% chance that you could smoke weed an hour or two before the test and still come up negative.

Conclusion

Drug testing might be degrading, unnecessary and ineffective, but until the federal government changes tack on marijuana, it will likely remain a reality, even in legal states. The best thing you can do is use the information you’ve learned here to stay on the right side of the cutoff line, keep your job and not mess up your parole or rehab plans.

Does being able to abstain from marijuana make you a more useful or productive member of society? Not really. But if you can keep it up for a little while, eventually people will stop asking and you can continue to enjoy your life.