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Cannabis - how long does marijuanna stay in your system?

This question has also been asked and answered here: How long does marijuana stay in your system?

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Official Answer by Drugs.com 6 Dec 2010

How long does Marijuana stay in your system for?
It is impossible for anyone to accurately state the time it would take for somebody to test clean. This is because there are so many different variables (see below) that impact on the rate that marijuana is both metabolized and excreted.

We can give an estimate of the time, but the most reliable way is to test yourself twice weekly until your first, morning urine sample tests clean. However, even this is not 100% foolproof because home drug detection kits have a higher upper limit of detection (usually 50 ng/mL) compared to some other medical testing kits. This means you may test negative, but a laboratory test may still show marijuana in your system.

Note that charts, graphs, or computer programs that claim to be able to predict how long it will take you to test drug-free, are essentially useless at doing this, even though they may say otherwise.

What is delta-9 THC?
Marijuana is produced by the cannabis sativa plant. Cannabis sativa contains over 421 chemicals including 61 different cannabinoids, of which delta 9-tetrahydrocannabinol (delta-9 THC) is considered the most psychoactive. Delta-9 THC has a thick, sticky consistency (somewhere between a solid and a liquid) and is easily vaporized. It readily dissolves into lipids and fats, and once in the body gets deposited in fat (adipose) tissue, and in the liver, lungs, and spleen.

Delta-9 THC undergoes metabolism in the liver to another psychoactive compound, 11-OH-THC, and then further metabolism to the inactive THCCOOH. CYP2C9 and CYP3A4 are major enzymes involved in this metabolism and both enzymes show genetic variation – this means that some people will metabolize THC faster than normal, whereas others will metabolize it slower than normal.

What variables impact on the time Marijuana stays in your system for?
Individual variables that play a role in how long marijuana stays in your system for and detection times include:

Body fat: People with more body fat will retain THC for longer. Skinny users have few places to store THC
Fluid intake at the time of the test
Genetics: Fast metabolizers will excrete THC rapidly
How frequently you exercise
Method and frequency of usage: Infrequent users clear THC faster than chronic users
Type of detection test used (blood, hair, saliva, or urine)
Your general health: Many medical conditions impact on how your body retains, stores, and metabolizes marijuana
Other drugs taken: Many drugs interact with CYP2C9 and CYP3A4 enzymes.
In addition, the potency of the marijuana and your tolerance to it can also have an impact.

Table 1: An estimate of marijuana detection time after last usage

Usage Estimate of marijuana detection time
* Occasionally, some chronic users with a high tolerance may eliminate THC as fast as a one-time user.
One time only 4-8 days
2-4 times per week 23-35 days
5-6 times per week 33-48 days
Daily usage 49-70 days after last use*
2-4 times per month 11-18 days
What are the different ways Marijuana can be tested for?
Firstly, it is important to understand the difference between qualitative and quantitative testing.

Qualitative testing tells you if a substance is there or not. It does not tell you how much of the substance is there. There will be a cut off point for detection. For example, the Alere DDs 2 Mobile test system used roadside by law enforcement agencies has a cut off level for Delta-9-THC in saliva of 25 ng/ml. Research has shown that this testing system is 97.5% accurate at this cut off (this means that more than 97 people will accurately test positive out of 100 people who have used marijuana in the past 24 hours).

Quantitative testing measures the actual quantity of a substance. This type of testing is more expensive and time-consuming than qualitative tests and is normally reserved for people who have tested positive in a qualitative test. It needs to be done in a laboratory and is used when a more defined value is needed, say, for prosecution. Quantitative testing can detect minute quantities of a substance, far below the cut off point in a qualitative test. A series of quantitative tests can determine if drug usage is ongoing or has stopped.

The most common ways marijuana can be tested for is in urine, saliva, blood, or hair. Most testing practices look for the presence of THCCOOH which has a much longer half-life (the time it takes for 50% of the substance to be excreted) than delta-9 THC. The half-life of THCCOOH is 20-57 hours in occasional users compared to 3-13 days in regular users.

Table 2: Estimated periods of detection of the marijuana metabolite THCCOOH

Assay Detection period (estimated)
Blood
36 hours
Some reports suggest up to 7 days with chronic use (>3 months)
Hair
Cannot detect first-time drug use in the previous 7-14 days
Every 1cm of hair length represents a one-month window of detection
Detection period depends on hair length but generally 90 days
Saliva
Up to 34 hours
Urine
1-4 days
Some reports suggest up to 70 days with chronic use (>3 months)
A false-negative is when the test result is negative for a substance, but the person has been using the substance. A false-positive is when the test result is positive for a substance, but the person hasn’t been using the substance.

Can second-hand marijuana smoke make you fail a drug test?
For kits that detect at the 50 ng/ml level the circumstances would have to be extreme, for example, if you were in a closed car full of marijuana smokers for a couple of hours and you were drug tested the next day. If you are a non-smoker in a ventilated area where other people are smoking marijuana you are generally safe.

References:
Krotulski AJ, Mohr ALA, Friscia M, Logan BK. Field Detection of Drugs of Abuse in Oral Fluid Using the Alere™ DDS®2 Mobile Test System with Confirmation by Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS). J Anal Toxicol. 2017 Dec 28. doi: 10.1093/jat/bkx105.
Levy S, Siqueira L. Testing for Drugs of Abuse in Children and Adolescents. Clinical Report. Pediatrics 2014;133(6) http://pediatrics.aappublications.org/content/133/6/e1798.long
Sharma P, Murthy P, Bharath MMS. Chemistry, Metabolism, and Toxicology of Cannabis: Clinical Implications. Iranian Journal of Psychiatry. 2012;7(4):149-156.
Windows of detection. Alere Toxicology. 2018 https://www.aleretoxicology.co.uk/en/home/support/testing-explained/windows-of-detection.html
Verstraete AG. Detection times of drugs of abuse in blood, urine, and oral fluid. Ther Drug Monit. 2004 Apr;26(2):200-5.

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