Tetrahydrocannabinol (THC) is the chemical in cannabis that makes people feel “high.” Tests can detect THC in the body for several days or even weeks.
The length of time this chemical stays in the body or continues to show in a drug test depends on many factors. These include:
- how much body fat a person has
- how much and how often they consume the drug
- the sensitivity of the drug test
Drugs such as alcohol may completely disappear from the body in just a few hours. In comparison, cannabis lingers much longer.
Drug tests can detect THC, in urine, blood, and hair for many days after use, while saliva tests can only detect THC for a few hours. This is because of the way the body metabolizes THC.
THC is a
Research on how long a test can detect cannabis shows a wide range of averages. Research from 2017 estimates a detection window for a single cannabis cigarette of about 3 days.
The same study emphasizes that detection windows vary and depend on how often a person smokes.
- For someone smoking cannabis for the first time, tests may detect it for about 3 days.
- In someone who smokes cannabis three or four times per week, the detection window is 5–7 days.
- For people who smoke cannabis once a day or more, tests may detect it in their system for 30 days or longer.
Detection windows also depend on the kind of test a person undertakes. General estimates for various cannabis tests are as follows:
- Urine tests can detect cannabis in the urine for approximately 3–30 days after use.
- Saliva tests can detect cannabis for approximately 24 hours after use. Some saliva tests have detected cannabis for up to 72 hours.
- Hair tests are the most sensitive tests, detecting THC for up to 90 days after use. However, these test the oil in skin that transfers to hair, so they may occasionally show a
false positive. A person who comes into contact with a THC user could, theoretically, test positive on a hair test.
- Blood tests can only detect THC for
Drug tests can detect relatively small quantities of THC, and the amount of THC in a given cannabis cigarette varies. However, little research has examined how much a person must smoke to fail a drug test.
Studies consistently find that frequent weed users are more likely to fail drug tests than infrequent users. A 2012 study examined cannabis users smoking a single cigarette with 6.8% THC.
Urine concentrations of THC were highest 0.6 to 7.4 hours after smoking. Using a highly sensitive urine test, researchers detected THC in the urine of 100% of frequent users and 60–100% of infrequent users.
Some 77% of heavy users and 39% of light users produced positive tests. No non-users had positive test results, suggesting that false positives in hair tests are relatively rare.
Numerous factors influence whether a test detects cannabis, including the following:
More sensitive tests can detect lower doses of cannabis. Tests include blood, urine, hair, and saliva.
Cannabis drug tests look for THC, not cannabis. So the amount of THC that a person consumes is a significant factor.
The effects of THC are cumulative. This means that a person who smokes several times over several days has consumed a higher THC dose than someone who smokes once, so they are more likely to test positive.
The strength of each dose of THC also matters. Without sensitive laboratory equipment, a person cannot reliably determine the strength of their cannabis.
How “high” a person feels is also not a reliable measure, because numerous factors other than THC dose can intensify or weaken this feeling.
Since fat stores cannabis, people with higher body fat concentrations may metabolize cannabis more slowly than a person with less body fat.
Body mass index (BMI) is one way to judge body fat. However, since weight, and therefore BMI, increases with muscle mass, BMI is not a perfect measure of body fat.
Typically, females have more body fat than males. This means that females may metabolize cannabis slightly more slowly.
Dehydration increases concentrations of THC in the body. While drinking lots of water is unlikely to affect a drug test significantly, severe dehydration might.
Exercise will not significantly change the rate at which the body metabolizes THC. Exercising before a drug test, however, might.
A small study of
The researchers believe that exercise may cause fat cells to release THC. In their results, people with higher BMI had more significant increases in THC levels.
For a drug test to be negative, the body must eliminate THC from the system, as well as metabolic chemicals that have links to THC. People with faster metabolisms typically eliminate THC more quickly than those with slower metabolisms.
Ultimately, there are only two strategies that work for this: decreasing the concentration of THC in cannabis and speeding up the metabolism.
Proper hydration can prevent a drug test from showing unusually high THC concentrations. For people whose test results are on the border of positive and negative, this means that being dehydrated may increase the chances of a positive result.
There is no reliable way to speed up the metabolism. Exercise might help the body metabolize more THC but exercising too near to a test may also cause a positive result.
The most important factor is the time from the last exposure to the testing time.
There is no way to accurately predict how long it will take an individual to metabolize cannabis and eliminate it from their bodies. Home tests can help people test themselves for the presence of cannabis in their system.
For almost all people, cannabis should disappear or be very low in concentration within 30 days. For infrequent users, it may take 10 days or fewer for cannabis to leave the body.