How Long Can Marijuana be Detected in Drug Tests?

By Hound Labs

The short answer is, it depends. Several drug testing methodologies exist today, and they all detect past usage of marijuana over different time frames. Before we delve deeper in the specifics, let’s first agree on what we mean by a detection window.

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What is a Drug Detection Window?

A drug detection window is simply the time during which a drug can be detected in a biological sample, such as blood, urine, oral fluids, or breath. What bookends the higher and lower bounds of drug detection times is a cutoff value for the test being performed. The cutoff level is set to optimize the detection of the drug while minimizing false positives. Drug levels below the cutoff mark will appear both before and after the detection window.

Marijuana detection times

The detection window for marijuana varies depending on which type of drug test is used (see Figure 1). Urine is the most common type of drug test given.1 Its detection window for marijuana is up to 30 days.2 This is somewhat dependent on a variety of factors, including frequency of use.  An occasional smoker who uses marijuana a few days a month may have THC remain in his or her body for only a few days, whereas a chronic smoker who smokes daily may have traces of THC in his or her body for up to a month.3

The marijuana detection window using an oral fluid drug test is narrower (up to 24 hours).4 The detection window using breath provides the shortest detection window of up to 2-3 hours after smoking and slightly longer when marijuana is ingested.5 This is an important distinction because while THC or its metabolites may remain in a person’s system for days or weeks, a person is clearly not impaired for that period of time. Only the detection window for breath corresponds closely to the peak impairment window of when someone is under the influence of marijuana.6

What is the best drug testing method?

Once again, this depends. Each drug testing method may suit certain situations based on budget, testing environment, and the goal of the employer or law enforcement officer administering the test. If the goal is to determine recent use and whether someone is likely impaired at a particular time, then a breath test would be the best choice. A breath test has the shortest marijuana detection window and is the only drug testing technology that correlates with recent marijuana use and peak impairment.

However, if there is zero-tolerance for marijuana use, then presumably drug tests that have longer detection windows are desirable.  

The challenge with marijuana

THC, unlike alcohol and many other drugs, is highly fat-soluble, meaning it is stored in fat tissue. After being stored there, it is slowly released back into the rest of the body, and thus takes a long time before it’s eliminated from the body.7 That is why drug tests can detect THC or its metabolites long after someone is actually impaired by the substance.

Depending on the bodily substance examined, THC or its inactive metabolites are eliminated at different rates and for varying durations. Let’s take a closer look at each type of drug test to better understand why this is.

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Breath

Breath provides the shortest marijuana detection time of just a few hours due to how THC enters and then dissipates from the breath.

Inhalation

When THC is smoked, it rapidly enters the bloodstream and travels to the brain where it has an immediate psychoactive effect. Simultaneously, a small amount of active THC is transferred from the bloodstream into the lungs and out through breath. Interestingly, THC that has been circulating in the bloodstream for more than a few hours does not transfer back to the lungs. Thus, THC that can be detected in breath represents only the THC smoked very recently. THC re-enters the breath either through residual material in the lungs from smoking or from material transferred from the blood back into the air during breathing.

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Ingestion

When THC is smoked, it rapidly enters the bloodstream and travels to the brain where it has an immediate psychoactive effect. Simultaneously, a small amount of active THC is transferred from the bloodstream into the lungs and out through breath. Interestingly, THC that has been circulating in the bloodstream for more than a few hours does not transfer back to the lungs. Thus, THC that can be detected in breath represents only the THC smoked very recently. THC re-enters the breath either through residual material in the lungs from smoking or from material transferred from the blood back into the air during breathing.

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Oral Fluids

THC stays in oral fluids for up to 24 hours. The primary way THC enters oral fluids is through direct deposition in the oral cavity during use. When someone smokes or consumes food mixed with marijuana (such as a brownie), THC is sequestered in the oral mucosa or tissue in the mouth during use and released into saliva for up to 24 hours.8

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Blood

THC in blood is measurable for up to 3 weeks. After smoking, THC is rapidly absorbed through the lungs before entering the bloodstream. (In the case of ingesting marijuana, THC first is digested in the stomach before entering the bloodstream, as mentioned above.) THC is then rapidly absorbed by tissues in the body where it remains for a long time. THC gradually is released from these tissues back into the bloodstream. The amount of residual THC found in blood in the days or weeks afterward is small compared to the amount immediately following usage.

Nevertheless, THC levels are still detectable in blood for up to a few weeks,9 and blood tests cannot differentiate between recent and past use because the exact quantity of lingering THC differs due to a number of behavioral and physiological factors.

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Urine

The detection window for urine is even longer than for blood because THC is metabolized into various inactive metabolites before being excreted in urine,10 and it is these inactive metabolites that are tested rather than THC itself. The primary metabolite detected in urine tests is THC-COOH, but it is just one of the more than 80 inactive metabolites that the body produces.11

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Hair

Hair tests generally have the longest detection window — up to 90 days after marijuana use.12 Similar to urine tests, hair tests don’t test for THC but instead test for the non-psychoactive THC metabolite THC-COOH. Unlike other tests, however, it takes 7 days for THC-COOH to show up on hair tests because metabolites first pass from the bloodstream to hair follicles. Starting from the hair shaft, every centimeter of hair length equates to approximately a one-month detection window.13

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