The effects of smoking marijuana fade quickly, but the drug, known affectionately as "weed," can be detected in the body for weeks and sometimes longer. How long it remains in the system depends on how often or how much marijuana the user has been smoking.
How Marijuana Is MetabolizedThe active ingredient in marijuana -- Tetrahydrocannabinol or delta-9-THC or simply THC -- 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.
But THC is detectable in the blood for a short time, usually a few hours, because it is rapidly metabolized 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 TestsBecause marijuana stays in the bloodstream for a short time, blood tests for marijuana are usually not used, except in the case of automobile accidents and some roadside sobriety check points. Blood or saliva tests can show current intoxication. However, unlike blood alcohol concentration tests, they do not indicate a level of intoxication or impairment.
On the other hand, urine tests for marijuana metabolites can only show recent marijuana use, not intoxication or impairment, because of the time required between smoking and the metabolites being 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 a elimination half-life of 10 to 13 days. Most researchers agree that urine tests for marijuana can detect the presence of 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.
How Effective Are Marijuana Urine Tests?Although false positives are common for other substances, they are rare for marijuana due to the sophistication of today's tests. The laboratory first screens the sample with a 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. 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, Nuprin and others -- would cause false marijuana positives. But today's tests have been adjusted to eliminate that problem.
Can You Fool the Test?Although many tips are published on websites and magazines of marijuana decriminalization advocates on how to beat a marijuana drug test, most have proven to be urban legends. Some techniques include:
Washing Yourself Out - This entails drinking a lot of water or liquids and urinating several times before the test, then taking vitamin B-2 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 urine, but these items are easily detected by the lab. Several commercial products are used as urine test adulterants, but none are 100% foolproof. And all of them can be detected by the laboratory, if a separate test is run for them.
The Bottom LineIf you are required to take a urine test on short notice for employment or other purposes and you have recently smoked marijuana, you are probably going to fail the test. This is particularly true if you are a regular or heavy pot smoker.
Employees that abuse alcohol and drug pose serious problems for today's employers. Many are taking steps to develop a workplace drug policy that includes random drug testing for current employees and routine testing for all new job applicants.
The only reliable way of passing the test is to stop smoking marijuana.
Do you experience withdrawals when you quit smoking weed?
Take the Marijuana Withdrawal Symptom Quiz.
Drew, M., et al, "The Metabolism of Marijuana." Achieve Solutions 1999.
National Institute on Drug Abuse "Marijuana: Facts for Teens." Revised March 2008.
National Institutes of Health "Workshop on the Medical Utility of Marijuana." February 1997.
National Highway Traffic Safety Administration "Cannabis / Marijuana" Accessed November 2008.
NORML "The ABCs of Marijuana and Drug Testing." Apr 01, 2004.