Although the onset and decline of cannabis effects occur within hours of each other, cannabis can still be detected in the human body for days or even weeks following consumption. That is because THC’s half-life depends on the frequency and quantity of cannabis use. The half-life of THC for an infrequent user is approximately 1.3 days. For heavy users, the half-life can be 5-13 days.

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This means that it takes about 1.3 days for half of the THC metabolites in an infrequent cannabis smoker’s body to be eliminated— depending on the cutoff of the test being used positive specimens were found up to 4, 5 and 12 days after consumption in infrequent users and 17, 22, 27 days in heavy users.

How THC is Metabolized And Eliminated From The Body

Blackboard with the chemical formula of THC iStock / Zerbor
The absorption of THC into the bloodstream depends on the route of consumption. Inhaling cannabis will result in immediate distribution of cannabinoids into the bloodstream. Ingesting edibles can take much longer. THC can be identified in blood plasma seconds after it is inhaled—peak plasma concentration of THC in the blood is reached between 3 and 8 minutes after inhalation. However, when THC is ingested, it can take at least one hour for peak plasma concentration of THC to be reached. There may even be multiple peaks.

THC is a highly lipophilic molecule. This means that it is attracted to fat. Because of this, THC will move from blood plasma to fat and vascularized tissues such as the brain, lung, heart, and liver. It can take anywhere from 8-18 hours for THC to clear the blood entirely and move into deep fat stores. However, it can take as long as 60 hours to remove THC from the bloodstream of a heavy cannabis user. Another factor that may contribute to the length of the time it takes for THC to move out of fat stores is the number of fat cells a person has. Intuitively, an abundance of fat cells gives THC more storage space.

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Over time, THC will then move from these fat stores back into the blood. However, the length of time for this final distribution back into the bloodstream varies. Because THC is so lipophilic, it moves out of fat stores slowly. This may be why cannabis can be detected in the urine tests of chronic cannabis users for a much longer time than infrequent users. THC is finally excreted from the body through feces and urine.

Cannabis can also be detected in hair and sebum, an oily skin secretion. A 2016 Drug and Alcohol Review study by Michelle Taylor tested hair for cannabinoid content and found that the cannabinoid concentration in the blood capillaries, sweat, and sebum surrounding the hair shaft made it possible to detect cannabinoids in the hair of regular cannabis users up to several months after consumption. One of Taylor’s most significant findings was that it was much more likely to detect THC in the hair of participants who reported heavy cannabis use (32.3 spliffs a month) than those who reported light cannabis use.

Cannabis and Drug Testing

Test tires with test chart and urine can iStock / Lothar Drechsel
Cannabis can be detected through blood, saliva, urine, and hair. Because THC moves out of the bloodstream so rapidly, blood tests are not usually used. The irony of this is that blood tests can identify intoxication, likely the most important factor in a drug test. Urine is most commonly used to test for cannabis. The issue with urine testing is that it identifies the concentration of THC metabolites even when the effects of THC have long diminished.

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Drug testing for cannabis is not without controversy. Urine tests detect the presence of THC metabolites in the body, not impairment. That means a positive THC test is only indicative of recent cannabis use, not of the abuse or irresponsible use of cannabis. However, even the idea of irresponsible use is problematic since many cannabis users consume for medical reasons. Since most states that have legalized cannabis have not also included workplace protections in their laws, employers have the right to terminate an employee who showed up to work sober because they legally consumed cannabis on their night off. Medical users also must decide if managing their symptoms with cannabis is worth the risk of losing their jobs.

Beating a Drug Test Isn’t Likely 

Given the way that THC is eliminated from the body, it is very unlikely that a cannabis consumer will beat a drug test through commonly suggested natural routes such as excessive water consumption, exercise, or herbal cleanses. Some suggest drinking large quantities of water and urinating frequently before the drug test. The problem with this method is that it does not account for the slow release of THC metabolites out of fat stores and back into the bloodstream. While the dilution method may slightly impact the concentration of THC in urine, it likely won’t eliminate all of it.

Others suggest intense exercise prior to a drug test. The thought behind this is that burning fat will also remove the THC since fat is where THC is stored. However, a 2014 Basic and Clinical Pharmacology and Toxicology study found that exercise and food deprivation did not create a significant difference in the concentration of cannabinoids detected in serum or urine.

It simply takes time for THC to metabolize, so trying to expedite the process artificially won’t work. If you know your employer is going to require a drug test and you need to keep your job, your best bet is to abstain from cannabis use two weeks to a month prior to the test. There is one other option, and that it is to take your employer to court for wrongful termination if you are laid off for your cannabis use.