How Long Does Marijuana Stay in Your System?

How Long Marijuana in System
Marijuana presence in the human body can be detected through blood, saliva, urine, and hair tests, with urine analysis being the most common method. THC, the active compound in marijuana, is absorbed into the body's fatty tissues and metabolized in the liver into metabolites, which can be detected for varying periods depending on the frequency of use. For occasional users, it typically takes 3 days to test negative, while for chronic heavy users, it can take more than 30 days. Various factors such as body mass index, hydration, and exercise can influence the rate of THC elimination from the body.
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Legal reforms have given Floridians access to medical marijuana. But access is not always the same as acceptance. Federal regulations dating from the Regan era prohibit U.S. government employees from marijuana use of any type. Various state and local governments maintain similar “zero tolerance” policies, and some private-sector employers have followed suit.

While a number of states have passed laws that prohibit employers from discriminating against workers who legally use marijuana (medically or recreationally), Florida is currently not among these states. This creates a gray area for some of the more than 700,000 patients who currently qualify for medical marijuana therapies in Florida, but are working in an environment that is not marijuana friendly. For users who may be subjected to testing, understanding how marijuana is processed by the body, and how its prior use is detected, may be more than academic.

How Is Marijuana Detected?

The presence of marijuana in the human body can be detected by testing blood, saliva, urine, and hair. Blood tests are rarely used, however, because THC moves out of the bloodstream fairly quickly. Research volunteers who smoked a single marijuana cigarette with a 1.75 or 3.5% THC concentration exhibited measurable levels of THC in the bloodstream in less than a minute. THC levels peaked less than 10 minutes after the cessation of smoking, declined rapidly, and were generally below common methods of detection within two hours.

But this rapid drop does not mean that THC and other cannabinoids found in marijuana exit the body rapidly. Unlike alcohol, which is water-soluble and largely eliminated from the body within 12 to 24 hours, THC is absorbed into the body’s fatty tissues, and is then released over time and metabolized in the liver into derivative forms known as metabolites, which can be detected in saliva, urine, feces and hair samples. Urine analysis is the most common method used to test for marijuana, and most tests are designed to look for the major THC metabolite known as THC-COOH. Tests look for traces of this compound measured in nanograms per milliliter (ng/ml).

THC-COOH metabolite levels measured by urinalysis typically peak one to four days after the last use of marijuana. THC-COOH levels in the urine of habitual marijuana users may reach concentrations of 300 ng/ml with continued use. After the cessation of use, these levels tend to drop into the 50 ng/ml range within three to five days, but remain at detectable levels for as long as one month. At the other end of the spectrum, THC-COOH levels in the urine of occasional or one-time users of marijuana typically drop to levels of 10 ng/ml or less within four to five days of the cessation of smoking.

Testing Methodology & Thresholds

Urinalysis testing for THC metabolites is typically conducted in a two-phase process. Initial screenings are performed using enzyme immunoassay, which is a relatively simple and inexpensive diagnostic technique. If marijuana metabolites are detected, a secondary test is performed using gas chromatography/mass spectrometry (GC/MS), which is a more complex and precise process used to confirm the initial positive reading. Research indicates that initial enzyme immunoassay testing has an accuracy in the 95 to 97% range, while GC/MS confirmatory testing is virtually 100% accurate.

While there is some variation in the the minimum threshold levels that would trigger a positive result on a urinalysis test for THC-COOH, the most widely used thresholds adopted by the U.S. government and many other entities are 50 ng/ml or greater for the initial screening and 15 ng/ml or greater for confirmatory tests. On an interesting side note, the National Collegiate Association of Athletics (NCAA) recently relaxed its guidelines for student athletes, raising the minimum threshold for positive indication of marijuana use by urinalysis from 30 ng/ml to 150 ng/ml.

How Long Does It Take To Test Negative?

Moving beyond the science, the question most medical marijuana users want to know is how long it might take to receive a negative test result for a urinalysis screening. As will be explained in the next section, there are a number of variables that might influence the outcome, but there are also some generally-accepted timelines that can serve as a reference point. Reporting from the Mayo Clinic suggests that negative results can be expected as follows:

* Occasional users (marijuana use up to three times a week): Within 3 days

* Moderate users (marijuana use four times a week): Within 5 to 7 days

* Chronic users (marijuana use daily): Within 10 to 15 days

* Chronic heavy users (marijuana use multiple times a day): More than 30 days

More specific insights come from a paper published by the National Drug Court Institute, in which the authors challenge what they describe as “the common assumption that cannabinoids will remain detectable in urine for 30 days or longer following the use of marijuana.” The report states:

* Based upon recent scientific evidence, at the 50 ng/ml cutoff concentration for the detection of cannabinoids in urine,  it would be unlikely for a chronic user to produce a positive urine drug test result for longer than 10 days after the last smoking episode.

* At the 20 ng/ml cutoff concentration for the detection of cannabinoids in urine, it would be uncommon for a chronic marijuana smoker to produce a positive urine drug test result longer than 21 days after the last smoking episode.

* For occasional or single-event marijuana use, at the 50 ng/ml cutoff level, it would be unusual for the detection of cannabinoids in urine to extend beyond 3-4 days following the smoking episode.

* At the 20 ng/ml cutoff for cannabinoids, positive urine drug test results for the single event marijuana use would not be expected to be longer than 7 days.

Testing For Marijuana Use With Hair

A range of cannabinoids and metabolites that includes THC and THC-COOH can be detected in human hair. But while these tests have the advantage of being minimally invasive, they are not widely used due to limitations in specificity. Hair testing can identify prior marijuana use by heavy users for periods of up to 90 days, but has proven less reliable for identifying occasional use. This conclusion was summarized in a 2017 research report in which the authors stated:

Our study has identified hair cannabinoids to be a useful test to detect heavy cannabis use; however, this testing method is unreliable when applied to low- to moderate-frequency cannabis users. Furthermore, we were unable to use hair testing to determine the quantity of cannabis used by an individual in a specified time frame. As a result, the real-life application of hair testing for cannabis use is likely to be limited.

An additional concern with hair testing methods is the chance of false-positive results based on environmental exposure to cannabis. A 2015 research report  (15) titled, “Finding cannabinoids in hair does not prove cannabis consumption,” noted measurable levels of THC-COOH in hair samples taken from individuals with no history of marijuana consumption. The authors postulated that these concentrations were the result of either physical transfer from a cannabis user through sweat and physical contact, or from exposure to second-hand cannabis smoke.

Variables Influencing Elimination Rates

Studies have suggested a number of variables that can influence the rate at which different people eliminate THC-COOH. Among the most obvious are the duration of marijuana use and the quantity consumed. Additionally, there are indications that persons with a higher body mass index (BMI) may retain THC metabolites longer, as these substances are fat soluble. Evidence for this hypothesis comes from a study published in the Journal of Analytical Toxicology that reported “a significant correlation between BMI and time until last positive urine cannabinoid test.” The authors also note that:

The release of THC from adipose tissue into the blood is highly variable, possibly based on differences in activity, diet, enzymatic activity and other undetermined factors. This release causes fluctuations in blood concentrations that in turn lead to variability in urinary cannabinoid concentrations.

Hydration has also been shown to play a role in the rate at which THC and THC-COOH are eliminated from the body. Dehydration can cause hypotonic urinary output, which may lead to increased levels of THC-COOH in urine samples. Based on this information, individuals seeking to reduce THC metabolite levels in urine would be well-served to maintain an adequate water intake.

But while dehydration can raise THC metabolite levels in urine, hyper-hydration does not necessarily result in a reduction in these levels. The belief that drinking large quantities of water will flush marijuana from the boy more rapidly has been largely negated by studies that reveal that drinking more water than the body requires did not result in an increased rate of the elimination of substances such as THC metabolites.

Exercise has been shown to have some effects on the rate at which marijuana metabolites are released from the body’s fat cells, but the effect may be relatively small and transient. The peer-reviewed journal Drug and Alcohol Dependence reported on a study involving 14 regular cannabis users who spent 25 minutes on a stationary bike, either having eaten recently or having fasted overnight. Their results indicated that

* Exercise induced a small but statistically significant increase in plasma THC levels.

* Increases were greater in persons with a higher BMI.

* There was no difference in the level of elevation between the non-fasting and short-duration fasting participants.

The authors concluded by stating:

These results suggest that exercise may elevate blood THC levels by releasing dormant THC from fat stores. The data suggest the interpretation of blood THC levels in roadside and workplace tests might be complicated by recent exercise.

But similar results were not confirmed by a different study involving six regular marijuana users who undertook 45 minutes of moderate-intensity exercise following a 24-hour period of fasting. After compiling data from both blood and urine samples, the authors concluded:

Neither exercise at moderate intensity nor 24-hr food deprivation caused significant elevations in blood or urine cannabinoid levels in our six human subjects. Our results are in accordance with data from a similar study where only slight and transient THC plasma elevations were noted during exercise, and none during fasting.

We conclude that exercise and fasting in regular cannabis users are unlikely to cause sufficient concentration changes to hamper interpretation in drug testing programs.

Given the numerous variables involved in the elimination of cannabis from the human body, it is not currently possible to determine an exact point in time when a negative test for THC metabolites can be assured. Based on currently available information, however, medical marijuana users may be able to create a reasonable expectation for a personal negative outcome based on their circumstances and patterns of use.

Updated: May 23, 2024

Pierce Hoover

Pierce Hoover is a career journalist with more than three decades of experience in print, broadcast and online writing, editing and reporting, with more than 5,000 articles published in national and international print media and online. His focus on medical marijuana therapies mirrors his broader interest in science-based alternative medical practices.


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