Tetrahydrocannabinol, the chemical in marijuana that makes people feel "high," can stay 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 often they consume the drug
- how much someone smokes
- the sensitivity of the drug test
Drugs such as alcohol may completely disappear from the body in just a few hours. In comparison, weed lingers much longer.
Drug tests can detect tetrahydrocannabinol, or 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.
Research on the amount of time a test can detect marijuana shows a wide range of averages. Research from 2017 estimates a detection window for a single marijuana cigarette of about 3 days.
The same study emphasizes that detection windows vary and depend on how often a person smokes.
- For someone smoking marijuana for the first time, tests may detect it for about 3 days.
- In someone who smokes marijuana three or four times per week, the detection window is 5–7 days.
- For people who smoke marijuana 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 marijuana tests are as follows:
- Urine tests can detect marijuana in the urine for approximately 3–30 days after use.
- Saliva tests can detect marijuana for approximately 24 hours after use. Some saliva tests have detected marijuana for up to 72 hours.
- Hair tests are the most sensitive tests, detecting THC for up to 90 days after use. However, these tests are testing the oil in skin that transfers to hair, and 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 3–4 hours.
Drug tests can detect relatively small quantities of THC, and the amount of THC in a given marijuana cigarette varies. However, little research has examined exactly 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 in the journal Clinical Chemistry examines marijuana users smoking a single cigarette with 6.8 percent 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 percent of frequent users and 60–100 percent of infrequent users.
A 2017 study reports on testing where hair samples from 136 marijuana users reporting heavy, light, or no use of marijuana. For the study, researchers cut hair into 1-centimeter sections to test for exposure of up to a month prior.
Some 77 percent of heavy users and 39 percent 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 marijuana, including the following:
More sensitive tests can detect lower doses of marijuana. Tests include blood, urine, hair, and saliva.
Marijuana drug tests look for THC, not marijuana. So the amount of THC that a person consumes is the 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, and 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 marijuana.
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 marijuana, people with higher body fat concentrations may metabolize marijuana 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, increase 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 marijuana 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 14 regular marijuana users assesses the effects of 35 minutes of exercise on a stationary bike. The results conclude that THC concentrations increased by a statistically significant amount, suggesting that exercise right before a drug test may increase the likelihood of a positive test result.
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, and they are decreasing the concentration of THC in the marijuana 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 single most important factor is the time from the last exposure to the time of testing.
There is no way to accurately predict the amount of time it will take an individual to metabolize marijuana and eliminate it from their bodies. Home tests can help people test themselves for the presence of marijuana in their system.
For almost all people, marijuana should disappear or be very low in concentration within 30 days. For infrequent users, it may take 10 days or less for marijuana to leave the body.