How Long Does Weed Stay In Your System?

It’s probably the most asked question amongst cannabis users: How long is THC detectable in the human body after you smoke weed? The question has been tackled numerous times in both popular and scientific literature. The short answer is that it’s complicated. Inter-individual differences mean that THC hangs around longer for some people than for others. Further, some tests can detect THC and its metabolites for a greater length of time than others. The purpose of this article is to address these issues, providing a description of testing methods, what each test detects, and the variables that affect THC retention.

Though many states have legalized cannabis for either medical or recreational use, it is still federally illegal. For the medical cannabis user, this situation makes understanding the nature of these tests vitally important. For example, suppose a person is involved in a car crash. Responding police officers blood testing at the scene, and the results indicate that the driver had THC metabolites in their system. This could result in a DUI charge; however, it’s possible that the last use of cannabis was days or even weeks prior, and had nothing to do with the accident. If this accident takes place in a neighboring state, where medical cannabis is still illegal, those marijuana laws may pose an even bigger legal risk. Understanding what these tests can and cannot detect will help give medical cannabis users a better feel for the potential risks taken when using their medication.


Unfortunately, there is no simple answer to this question. Many variables affect how long it is present in the body, and the ability to detect THC is dependent on the type of test being used. This section will discuss some of these variables, and give approximate detection windows for each test.

A combination of behavioral and physiological variables determines how long THC remains in the body (and for how long you will test positive). Frequency of use and dosage strength are examples of behavioral determinants. An individual who uses more often and takes in higher amounts of THC per use will accumulate more THC than an occasional user. Research performed at the National Institute on Drug Abuse has led to tests that can distinguish between occasional and chronic use. This is determined by the relative amounts of THC metabolites detected, and the rates of elimination. Not only do chronic users start with higher concentrations of THC, but they drop more slowly. In essence, their “reservoirs” are full, while occasional users’ are empty.

Physiological variables include rates of THC metabolism, storage, and excretion. As mentioned previously, THC is absorbed into the body’s fat reserves. A fast metabolizer will store less than a slow metabolizer, and because it spends less time in the bloodstream. The amount of exercise a person gets also makes a difference, as exercise causes the body to access its fat stores. This can stimulate release of accumulated THC into the bloodstream.

Given the complex nature of THC accumulation, it’s hard to give definitive detection windows for each type of test. However, it is possible to estimate timeframes that will cover most individuals. With the exception of the hair tests, these are all capable of detecting recent use. Here’s a list of guidelines for each testing type:

  • Urine testing: 7 days for occasional users, 30 days for regular users
  • Blood testing: 14 days for occasional users, 30 days for regular users
  • Saliva testing: 72 hours for both occasional and regular users
  • Sweat patch testing: 10 days for both
  • Hair follicle testing: 90 days or more for both

How Do THC Tests Work?

The cannabis plant produces a wide array of chemicals, which are primarily divided into two different groups: cannabinoids, and terpenes. Cannabinoids are named for their initial discovery in the cannabis plant. There are nine different types of cannabinoid, and each one comes in multiple different subtypes. This means that any given strain, or chemovar, of cannabis may contain >100 different cannabinoids. However, most research attention has been paid to two in particular, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD).

THC is the major psychoactive ingredient in cannabis, while CBD is responsible for most of its medicinal properties. Terpenes are much simpler molecules that give the plant many of its aromatic properties. Terpenes are not specific to cannabis; in fact, many of them are named for the plant in which they were first discovered. For example, α-pinene is prevalent in pine trees, and can give certain cannabis chemovars a piney scent. While some data suggests certain terpenes may have medicinal properties, very little research has been done in this area.

In general, any tests for illicit substances will focus on the presence of THC. The main reason for this is that THC has non-linear pharmacokinetics, meaning that the drug “stays in your system” long after its effects have diminished. This is in contrast to alcohol, for example, where the amount of alcohol in the blood determines the effects felt. The difference has to do with the way THC is metabolized. THC is fat-soluble, meaning that it is absorbed into the body’s fat tissue after ingestion. It is then slowly released back into the bloodstream and metabolized. THC itself actually has a fairly short half-life in the body, but its metabolites can be detected for much longer. Therefore, all testing methods look for the metabolites, as well. While some tests may look for CBD, minor cannabinoids like CBG and CBN, and/or terpenes, these tests are much rarer and are not often used.

Much attention has been paid in recent years to finding markers of recent use. In this case, “recent” means a time-span of hours, not days or weeks. In general, this comes down to detecting THC itself, and not just its metabolites, but the presence of minor cannabinoids can indicate recent use as well. This research is still in its early days, but with the increasing legalization efforts in many states and the potential ramifications for cannabis users, it is proceeding quite rapidly.

What Are The Different Types of Tests?

There are many different ways to detect cannabis use. Generally, the tests attempt to test for THC in the body, including urine samples, blood, mouth swabs, hair tests and sweat. Hair is also routinely tested as well. Each of these tests have different detection thresholds and limitations, which are discussed below.

Urine Tests

Urine analysis is the most commonly used test for cannabis use. Two different strategies are commonly used for urine analysis. A lab may choose to begin with a simple qualitative test that detects the presence or absence of THC. This type of test does not determine how much is present, but it is faster and less expensive. If a sample is positive for active THC, the lab will then perform a second test to determine how much THC is present. They may also ask for a second urine sample in order to determine the rate of elimination. While urine analysis is widely used, it may not be the best method to detect cannabis use. According to a recent review on the subject, only 13% of ingested THC is detectable in urine, and the majority of this takes place within the first five days (ref. 2). Therefore, occasional users may easily escape detection using this testing method.

There are a lot of very popular videos on Youtube showing how to pass a urine test trying to detect THC. Your results may vary with these methods. The best advice, if you need to pass a drug test, is not to smoke in the first place.

a picture of vials of bloodBlood Tests

Blood testing is a much more sensitive and accurate measure of cannabis use. However, it is not routinely undertaken, as collection of blood samples is invasive and requires extraction by trained professionals. It also requires more processing following collection, as red blood cells do not absorb THC, and their presence in the sample can complicate the analysis. THC itself can be reliably detected for up to 6 hours, indicating recent use. Further, blood plasma samples may contain a wider array of metabolites and minor cannabinoids. However, chronic cannabis use can be detected by blood analysis for up to 30 days; in some individuals, this can extend even farther.

Saliva Tests

Oral fluid tests via mouth swab is rapidly becoming a more popular testing matrix. It is much easier to collect than blood or urine. It is also more difficult to fake, as it can be readily collected in the presence of a testing official. According to one review on the subject, oral fluid more closely correlates with blood THC than does urine, making it a more accurate reflection of cannabis use (ref. 3). Oral fluid tests are capable of reliably detecting recent use within a 24 hour window; some metabolites can be detected for up to 72 hours. Currently, the federal testing guidelines recommend the use of urine analysis for federal employees, but oral fluid tests are in the final stages of regulatory approval and may soon become the test of choice.

Sweat Tests

Sweat patches are also becoming a more popular testing method. Each patch is applied for a week; at the end of that time, it is removed and tested for the presence of THC and its metabolites. The detection window for sweat patches is ten days, the seven days it is worn and the three days prior. These are becoming an attractive choice for monitoring parolees, as new patches can be applied at weekly meetings.

Hair Follicle Tests

Hair follicle analysis allows the longest detection windows, up to several months, and can be detected in incredibly small concentrations (1 part per billion!). Somewhat paradoxically, THC is actually poorly incorporated into hair due to its acidic nature. As a counter-example, traces of cocaine, a base, were detected in hair samples from South American mummies dating back to 2000 BC. Hair analysis is less accurate when detecting occasional use. In one controlled study, occasional users escaped detection 52% of the time, while chronic users were identified in 85% of the tests (ref. 1).Hair samples can be contaminated by secondhand smoke, meaning that an individual can test positive even if they haven’t personally consumed cannabis. However, it may be possible to tell the difference between passive exposure and active cannabis use; current research offers conflicting results on this point.

More Studies Are Needed

So, how long does THC stay in the body?

It’s complicated, but hopefully this review of testing methods and established timelines will help to make sense of it all. It’s important to note that this is an area of active research, and new test capabilities are being developed. In an unfortunate twist, the classification of cannabis as a Schedule I drug actually hampers this process. Researchers need to go through an extensive regulatory process to gain approval for research on such a highly controlled drug. Nevertheless, progress is still being made. As our nation pursues outright legal cannabis it will be more important than ever for the public to understand these testing methods and their limitations.


  1. Huestis and Smith. Cannabinoid Markers in Biological Fluids and Tissues: Revealing Intake. Trends in Mol. Med. Feb. 2018.
  2. Leghissa, Hildenbrand, and Schug. A review of methods for the chemical characterization of cannabis natural products. Journal of Sep. Sci. Jan. 2018.
  3. Lee and Huestis. Current knowledge on cannabinoids in oral fluid. Drug Testing and Analysis. Aug. 2013. 

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