Marijuana detection windows
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.
How much do you have to smoke to fail a drug test?
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.
Factors that influence detection
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.
How Long Does It Take to Feel the Effects?
The effects of marijuana can vary from person to person. Some people may feel euphoric and relaxed while others feel anxious and paranoid. In other cases, people report feeling “dopey” and experience a loss of interest in activities or an inability to grasp concepts.
The chemical in marijuana that makes you feel “high” is tetrahydrocannabinol, also called delta-9-THC or simply THC. It enters the body’s bloodstream rapidly after smoking marijuana.
If marijuana is ingested orally rather than smoked, it takes longer to be absorbed into the blood, usually from 20 minutes to an hour and a half, but this can vary based on the amount taken as well as physiological factors such as absorption and rates of metabolism and excretion can influence drug concentrations in circulation.1
Effects can be far-ranging depending on the strain, method of consumption, and amount and can include the following:
- Dry mouth
- Swollen eyelids
- Bloodshot eyes
- Pleasurable body sensations
- Increased appetite (“the munchies”)
- Distorted perception (sights, sounds, time, touch)
- Loss of coordination
- Trouble with thinking, memory, and problem-solving
- Increased heart rate
The short-term effects of marijuana on memory, learning, problem-solving, and coordination last for one to two hours, with some lingering effects for up to 24 hours.2 It’s been shown to impair your driving performance for up to three hours, according to the National Highway Safety Administration.
The effects of marijuana are also influenced by the terpene profiles of a given strain. For instance, citrus terpene profiles tend to be more stimulating, which may be the desired effect, or may contribute to someone feeling anxious.
It is important to know that not all marijuana is created equal. Unlike other prescription drugs, marijuana products aren’t standardized and can vary considerably in quality, makeup, and dosage.
This variance may contribute to how quickly you feel the effects and what those effects are. THC can interact with alcohol, blood thinners, and anti-anxiety medications, so it’s important to discuss your marijuana use with your doctor.
How Long Does Marijuana Last?
The half-life of marijuana is how long it takes for half of the drug to be metabolized and eliminated from the bloodstream. While there are many different cannabinoids, THC is the one most drug tests are looking for.
THC is rapidly broken down and modified into molecules known as metabolites. At least 80 different metabolites are formed from THC and may have their own effects on the body’s endocannabinoid system. These metabolites are stored in body fat and are gradually eliminated from the body through feces and urine.
Some THC metabolites have an elimination half-life of 20 hours whereas others are stored in body fat and have an elimination half-life of 10 to 13 days.
It takes five to six half-lives for a substance to be almost entirely eliminated. This is why you see advice that one-time use is probably not detectable after five to eight days.
Blood and Saliva
Because marijuana stays in the bloodstream for only a short time, blood tests for marijuana are usually not used. The exceptions are in the case of automobile accidents and some roadside sobriety checkpoints.
Blood or saliva tests can show current intoxication. However, unlike blood alcohol concentration tests, they do not indicate a level of intoxication or impairment.
Daily or near-daily cannabis consumption is likely, but not always, detectable by a hair test up to three months later. But, the hair test is not reliably able to detect infrequent cannabis use or determine the amount of cannabis used.
Urine tests for marijuana metabolites also only show recent marijuana use, not current intoxication or impairment. This is because of the time required between use and your body breaking down THC to the metabolites that are eliminated in the urine. Because many employers have a zero-tolerance for drug use, most workplaces use urine tests to detect recent use of drugs.
False Positive Testing
Workplace testing for marijuana might entail first screening the sample with an immunoassay test, known as the EMIT or RIA. If positive results are returned, the sample is again screened with a gas chromatograph-mass spectrometer (GCMS), which is much more accurate and so false positives are rare.
No known substances would cause a marijuana urine test to return a false positive.
At one time, ibuprofen (sold over-the-counter as Advil, Motrin, and Nuprin) would cause false marijuana positives. But today’s tests have been adjusted to eliminate that problem.
In places where marijuana is legal, roadside blood tests have been known to show some false positives in people who had been legally consuming cannabis but were not actively intoxicated at the time of the test. A 2016 report detailed a Belgian policy of testing oral fluid at the roadside that found it decreased these types of false positives.