The effects of smoking marijuana fade quickly, but the drug can be detected in the body for weeks and sometimes longer. The amount of time the active ingredients and breakdown products of weed remain in the system depends on how often or how much marijuana the user has been smoking or ingesting.
How marijuana affects your system
The active ingredient in marijuana is tetrahydrocannabinol, also called delta-9-THC or simply THC. It enters the body's bloodstream rapidly after smoking marijuana. If marijuana is ingested rather than smoked, it takes longer to be absorbed into the blood, usually from 20 minutes to an hour and a half.
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. It's been shown to impair your driving performance for up to three hours, according to the studies quoted by the National Highway Safety Administration.
THC is detectable in the blood for a short time, about a few hours, because it is rapidly broken down and modified into molecules known as metabolites. At least 80 different metabolites are formed from THC. These metabolites are stored in body fat and are gradually eliminated from the body through feces and urine.
Limitations of drug tests for marijuana
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.
Urine tests for marijuana metabolites can only show recent marijuana use, not intoxication or impairment. This is because of the time required between smoking and your body breaking down THC to the metabolites that are eliminated in the urine. However, because many employers have a zero tolerance for drug use, most workplaces use urine tests for any recent use of drugs.
How long can marijuana be detected?
Some THC metabolites have an elimination half-life of 20 hours. However, some 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.
Most researchers agree that urine tests for marijuana can detect the drug in the body for up to 13 days.
However, there is anecdotal evidence that the length of time that marijuana remains in the body is affected by how often the person smokes, how much he smokes, and how long he has been smoking. Regular smokers have reported positive drug test results after 45 days since last use, and heavy smokers have reported positive tests 90 days after quitting.
The accuracy of marijuana urine tests
Although false positives are common for other substances, they are rare for marijuana due to the sophisticated tests used. The laboratory first screens the sample with an immunoassay test. If positive results are returned, the sample is again screened with a gas chromatograph mass spectrometer (GCMS), which is much more accurate. That's why 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.
Can you fool the test?
Although you will see many tips on how to beat a marijuana drug test, most have proven to be urban legends. Some of these questionable techniques include:
Washing Your System Out: This entails drinking a lot of water or liquids and urinating several times before the test, then taking vitamin B-12 to add color back to the urine. Although this may lower the percentage of THC found in the urine by diluting it, it will not totally eliminate THC metabolites.
Using Drug Screening Agents: Some companies sell various substances and herbal teas that are allegedly capable of ‘cleaning’ the body's system of traces of marijuana. There is little evidence that any of them actually work. The catch is most of them have to be used over an extended period of time, during which the body will naturally eliminate THC from the system anyway.
Tampering With the Test: This involves adding something to the urine to contaminate the sample. There are tales of using Visine, bleach, salt, or detergent to the urine sample, but these items are easily detected by the lab. Several commercial products are sold as urine test adulterants, but none is 100 percent foolproof. All of them can be detected by the laboratory if a separate test is run for them.