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How Long Does Marijuana Stay In Your System? It's Kind Of Complicated

There are a lot of factors that can affect THC and THC-COOH concentrations in one's body.

Opinions expressed by Entrepreneur contributors are their own.

There’s only one reason anyone would really care how long pot remains in the body (unless they’re a researcher or they’re writing an article like this one).

Lothar Drechsel | Getty Images

People wonder how long marijuana stays in their system because they’re concerned about having to take a drug test. And as you undoubtedly know, employers can use several different types of tests.

That’s why the question should really be phrased differently: how long can pot be detected in the blood, in the urine, in the saliva, and in hair?

The answer still isn’t easy, because there are so many variables to consider. How much has someone smoked or consumed, and how strong was the weed? How often do they smoke, and how much do they weigh?

And are you really concerned about how long cannabis stays in the body – or about the length of time that signs of cannabis can be detected?

That’s a lot of questions. Let’s come up with some answers.

RELATED: Study Finds People Who Use Cannabis Get More Exercise

Tracing THC In Your System

The best way to understand how long cannabis stays in your system, and how it’s detected, is to know exactly how the processes work.

When you smoke (or vape) pot, activated THC and other cannabinoids hit the lungs and then enter the bloodstream, where they’re pumped by the heart to all parts of the body. The THC then binds to cannabinoid receptors (and receptor proteins) in the body’s key systems and organs, including the brain. That’s how the psychoactive effects of marijuana are able to take effect.

The process is similar but faster with tinctures, which are absorbed quickly through the lining of the mouth; it’s similar but slower with edibles since they’re first absorbed by the digestive system before they reach the liver and then the bloodstream.

Something else happens to THC as it’s carried in the blood. It passes through the liver multiple times and each time it does, much of it is converted into a number of other substances. The ones that matter most (for our purposes) are called THC-COOH and 11-OH-THC, and they don’t perform any important function. However, those metabolites are stored in the body’s fat, and remain there for a much longer period of time than THC itself stays in the body. That’s what’s important about them.

Now that you have that information, here’s a question. If you wanted to know whether someone had used marijuana in the last month, would you test for THC that can usually be detected in the body for only 1-10 days, or would you test for THC metabolites that are stored in the body’s fat for a month or even longer?

The answer is obvious, and it’s why most tests administered by employers look for THC-COOH or 11-OH-THC. Those metabolites are longer-lasting indicators of whether someone has used pot.

RELATED: Why Do Edibles Make You So Darn High?

Blood, Saliva, Pee, and Hair, Pee, and Hair

If the title of this section makes no sense to you, try singing it to the tune of “head, shoulders knees and toes, knees and toes.” (And no, we’re not wasted at the moment, it just seemed funny.)

As we mentioned at the start, what people really want to know aren’t the details of how long marijuana stays in the system – but how long it can be detected after they’ve used.

You already know that pot’s effects usually wear off after a few hours at most. But the THC remains in your system long beyond that. THC’s “terminal half life” (how long it takes for 50% of a substance to be naturally eliminated by the body) ranges between one and two days for occasional users, and 3-13 days in regular users, depending on how strong the bud is. For THC to be completely eliminated, it can take five times that long.

However, most companies don’t test for THC (that test is expensive), they test for THC-COOH (or sometimes 11-OH-THC). And the amount of time that THC and THC metabolites can be detected differs greatly, depending on what type of drug test is performed.

Urine Testing

This is the most common method employers use to test for pot.

There are two different estimates from very reputable sources on how long THC metabolites will show up on a urine test. The one from the Mayo Clinic is easy to grasp, although a bit simplistic:

  • Occasional users: 3 days
  • Moderate users: 5-7 days
  • Daily users: 10-15 days
  • Heavy users: 30+ days

An earlier study, done at the Toxicology and Drug Monitoring Lab at the University of Missouri and regularly cited as definitive research, looked at two different testing cutoff points.

Most urine tests only register positive at a THC-COOH concentration of at least 50ng/mL, because that’s the level that’s been recommended by the government. The study showed that only heavy users might exceed that cutoff point for more than 30 days, and that just happened occasionally. Most chronic users passed the 50ng/mL test after ten days or fewer; the researchers said it’s unlikely those regular users would have any problem taking a drug test after ten days.

The experts also looked at the more rigorous 20ng/mL cutoff point, most often used by law enforcement and zero-tolerance employers and tougher to pass. At that level, THC metabolites can be detected for up to seven days in one-time users, and for 21 days or longer in chronic users.

So THC and/or metabolites like TCH-COOH are in your system for more than 30 days if you smoke regularly and heavily. But they probably won’t be detected after a month if you are a heavy smoker, and shouldn’t be detected after 10-15 days if you consume more infrequently.

Blood Testing

Fortunately, most employers don’t want to shell out the money for blood testing their staff or job applicants. Even though THC is no longer detectable in blood about five hours after each use, it builds up over time in regular, heavy users.

One study underscores how long that problem can last. The participants were regular smokers, but abstained for seven days – and at the end of that seven day period, about 25% still had detectable levels of THC in their blood, and 100% still had detectable levels of THC-COOH.

In fact, THC-COOH testing is a more difficult problem for everyone. In casual but regular users, it can be detected for as long as seven days. In heavy users, it can remain in the body for an extremely long time, and it can be detected in a blood test 25 days or more after the user’s last smoke session.

The bottom line here is easy to figure out: if you’re an everyday or heavy user, you really don’t want to take a blood test unless it’s for THC, not metabolites. Otherwise, you’re only looking at a window of a week or less before you’ll be considered clean.

Saliva and Hair Testing

Saliva tests are more common than they used to be, while hair testing is usually done to look for hard drug use, not pot. The good news on both is that there are a number of questions about their accuracy and reliability when it comes to marijuana use, so their results aren’t likely to be considered as reliable as those from urine or blood testing.

For casual users, THC will show up in a saliva test for about a day, and the metabolites can only be detected for about three days. Heavy users will have a bigger problem if they’re given a saliva test based on THC metabolites, which can remain detectable for almost a month.

Hair tests won’t show evidence of recent, first-time use, for a reason that makes sense if you think about it: it takes 7-14 days for new hair to grow out of the scalp. But after that, THC-COOH can be detected for as long as three months, as can residual THC in heavy users. It’s unclear whether THC can reliably be detected in the hair of casual users, as studies disagree substantially on that question.

RELATED: Before Mixing Anti-Depressants and Cannabis, Read This

Factors Affecting THC and Metabolite Levels

It should be obvious that the amount you smoke, the potency of what you smoke, and how often you smoke will all affect how much THC you take into your body, and how long the metabolites remaining in your system will last.

Those who aren’t regular smokers and have only taken a couple of tokes shortly before their test probably won’t have much to worry about, nor will those who’ve only taken a couple of bites of an edible. Heavy smokers, daily smokers, or those who favor and can afford the “good stuff” will have a much bigger issue, since they’re putting more THC into their system.

There are other factors that can also affect THC and THC-COOH concentrations in the body.

  • Body fat: TCH metabolites are stored in body fat. So the more fat you have, the longer THC-COOH and 11-OH-THC will remain in your body and be detectable via testing. Skinny users with a low BMI will probably have a shorter testing window to worry about. (Women usually have more body fat than men, so they are more likely to retain the metabolites for a longer period of time.)
  • Metabolism: Some people have higher metabolic rates than others, and their bodies will eliminate THC and its byproducts faster than those with slower metabolisms. This is very different than exercise; exercising right before a test can actually increase the chance that you’ll come up dirty.
  • Hydration: Opinions vary on whether drinking a lot of water right before a urine test will help you pass (it can’t hurt), but research does show that if you don’t regularly drink enough fluids and are dehydrated, your detectable THC and THC-COOH levels will increase.

That’s a very long answer to what seems to be a very simple question. But as is the case with most subjects involving weed, there really are no easy answers – unless the question is “Wanna smoke?”