can doctors find weed in your blood

How long does marijuana stay in your system? How THC blood tests work, and what you should know

Even after a marijuanaВ highВ wears off, traces of the drugВ remain in your blood. The tell-tale molecule is THC,В weed’sВ main psychoactive chemical.В В В В

ButВ exactlyВ how long does this stuff hang around? ThatВ depends on a fewВ different factors, theВ most important beingВ how you consume marijuana — and how often.В

How THC gets into your blood

When you inhale weed smoke,В THC has a direct line into your bloodstream via the lungs. The 2012 paper “Chemistry, Metabolism and Toxicology of Cannabis: Clinical Implications,” published in the Iranian Journal of Psychiatry, describes how THC isВ “rapidly absorbed through lungs after inhalation” and “quickly reaches high concentration in blood,” which travels through the lungs to be oxygenated by the air you breathe.В

Weed consumedВ in edible form, however — which can haveВ stronger, longer-lasting effects — isВ largelyВ metabolized by the liver, greatlyВ reducing the overall amount ofВ THCВ that enters theВ bloodstream.В “Ingesting edibles introduces only 10% to 20%В of THC and other cannabinoids to the blood plasma, whereas inhaled cannabis falls closer to 50% or 60%,” according toВ Leafly.

Where it gets interesting, however, is how THC sticks around after you’ve enjoyed its immediate effects.

How THC stays in your blood

BecauseВ THC is fat-soluble (as opposed to, say, alcohol, which is water-soluble), it penetrates your fatty tissuesВ asВ it gradually disappears from the blood. The result is that youВ can retain active THC in your fat deposits for about threeВ weeks. And asВ your body metabolizes thatВ fat, it slowly redistributes theВ THC back intoВ yourВ bloodstream. The more one uses marijuana, the more THC gets stored for later release intoВ blood.В

You can retain active THC in your fat deposits for about three weeks.

ButВ THC concentration in blood plasmaВ peaksВ three to 10В minutes after inhalation, orВ one to twoВ hours after oral ingestion. The amount of THC found in your blood in the days and weeks afterward is residual by comparison, and certainly doesn’t indicate that you’re stoned theВ whole time:В The high from inhalation, by comparison,В lastsВ justВ twoВ hours, and an edible high can typically last about four to sixВ hours. In other words,В trace amounts of THC don’t reveal a whole lot about how much weed you’ve consumed, or when — onlyВ that you did soВ in the not-too-distant past.В

Why does that matter? Because THC isВ what shows up onВ aВ blood test.В В В

How blood tests for marijuana work — and why they fall short

Blood-based drug testsВ screen for a wide variety of illicit substances and haveВ short detectionВ windows for most, meaning they’re typically used to establish very recent drug use.В THC can be undetectable byВ blood test as soon asВ sixВ hoursВ after it enters your system — even though it’s stillВ there in trace amounts — and heavy smokers may test positive for up to 24 hours.

THC can be undetectable byВ blood test as soon asВ sixВ hoursВ after it enters your system.В

This relatively small window is oneВ majorВ reason employersВ tend to prefer simpleВ urineВ and hairВ tests, which mayВ revealВ a single instance ofВ marijuana use within the past week andВ habitual use over several months, respectively. Blood testsВ are consideredВ better-suited for, say, DUI investigations, when law enforcement is trying to gauge a suspect’s current level of intoxication.В

But whileВ THCВ may show up inВ someone’sВ bloodstream, there’sВ no scientific wayВ to judge levels ofВ impairment on this measurement. Also, as mentioned above, a potent edible won’t necessarily do much to alter the THC content of a user’s blood: Inhalation, while it produces a shorter high, deliversВ “a hundredfold difference” in the amount of THC that makes it into the blood.В В В В

All thisВ makes blood-basedВ legal limits for drivers problematic.В As theВ Chicago Tribune reported,В citingВ a study by theВ AAAВ Foundation for Traffic Safety:В

Marijuana is not metabolized in the system in the same way as alcohol. So while a person with a blood-alcohol level of .08 or higher is considered too drunk to drive, it’s not possible to say the same thing absent other evidence about a person testing at fiveВ nanograms per milliliter of blood of THC — the level used to find impairment by Colorado, Montana and Washington, the study found.

DUI attorneys have noted that the fiveВ nanogram thresholds represents anВ amount of THC thatВ chronic users could have in their bodiesВ at all times, regardless of whether they’d consumed cannabis within the last couple hours.

CanВ THCВ stay in your blood forever?

Smoking weed once doesn’t mean there will always be THCВ in your blood — in fact, it won’t be there very long at all.В But researchВ showsВ habitual users retain “substantial whole blood THC concentrations” for several days after going cold turkey. In effect, this means that people who continue to consume marijuana frequently will always have some THC in their blood.

Research shows habitual users retain “substantial whole blood THC concentrations” for several days after going cold turkey.

YetВ even a chronic, heavy smokerВ can fully detox by abstaining for a long enough period, and theВ neurocognitive effects ofВ constantВ low blood THC concentrations aren’t well understood. Preliminary findings are more focused on how ongoing marijuana usage mayВ alter brain chemistry, and scientists have established that THC can be measured in the brain even when it’s no longer traceable in blood. All in all, then, the ability toВ quantifyВ blood THC content is maybe less useful than it sounds at first.

Still, we know it’s thereВ a good while after the buzz has faded — sort of like that friend you invited over for a joint who now can’t seem to get off the couch.В

More marijuana news and guides

Keep up to date with the ever-changing landscape of legal marijuana. From fluctuating weed prices to the health benefits of vaping, the science of drug tests to hot new trends like microdosing, you’ll always know the key facts in a confusing culture where misinformation is all too common. Stay abreast of states looking to reform their laws, learn the terminology, discover groundbreaking advances in cannabis treatments and find out what America’s favorite semi-illicit substance can do for you.

This article was originally published on Feb. 21, 2017

Even after a marijuanaВ highВ wears off, traces of the drugВ remain in your blood. The tell-tale molecule is THC,В weed’sВ main psychoactive chemical.В В В В ButВ exactlyВ how long does this stuff hang around? ThatВ depends on a fewВ different factors, theВ most…

How Long Does Marijuana Stay in Your System?

Marijuana in Your Blood, Urine, Hair, & Saliva

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Steven Gans, MD is board-certified in psychiatry and is an active supervisor, teacher, and mentor at Massachusetts General Hospital.

The effects of 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 can range from a few hours to 90 days, depending on how often or how much marijuana the person has been using.

Although a number of states in the U.S. have active medical marijuana laws and recreational use of marijuana for adults over the age of 21 is legal in 11 states, Washington, DC, the Northern Mariana Islands, and Guam, the federal government continues to classify marijuana as a Schedule I controlled substance.

According to the Substance Abuse and Mental Health Services Administration (SAMHSA), roughly 1 in 10 people who use marijuana will become addicted—and these rates rise to 1 in 6 if they start using the drug prior to age 18.

The FDA has not yet approved medical marijuana for any medical indication, but it is often prescribed for chronic pain, nausea, HIV, multiple sclerosis, glaucoma, and irritable bowel syndrome (IBS).

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.  

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”)
  • Relaxation
  • Stimulation
  • 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.   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.  

Factors That Affect Detection Time

The length of time marijuana remains in your body depends on many different factors, including frequency of use, body mass, metabolism, sex, and hydration levels.

Frequency of Use

There is some evidence that the length of time that marijuana remains in the body is affected by how often the person uses marijuana, how much they use, and how long they have been using.

People who use marijuana regularly have reported positive drug test results after 45 days since last use, and people who use more heavily have reported positive tests up to 90 days after quitting.

In a 2017 study of 136 people who use cannabis subjected to hair tests, the presence of five cannabinoids, THC, THC-OH, THC-COOH, cannabinol, and cannabidiol, were present in 77% of the heavy users and 39% of the light users.  

Women often metabolize THC at a slightly slower rate since they tend to have higher levels of body fat than male counterparts.


The faster your metabolism, which can be impacted by age, physical activity, and certain health conditions, the faster marijuana will exit your body.

Body Mass Index (BMI)

THC metabolites are often stored in the fat cells in your body, so the higher your body fat (or BMI), the slower you’ll likely be able to metabolize and excrete marijuana.


When you’re dehydrated, you’ll have more concentrations of THC in the body. Flooding yourself with water won’t make you pass a drug test, however. Instead, it will dilute it and you’ll likely need to retake the test.

Smoking vs. Vaping vs. Edibles

The method of use also impacts the detection time. If marijuana is smoked or vaped, the THC levels in the body will drop faster than if you ingest it. Edibles take longer to break down in the body and leave your system.

How to Get Marijuana Out of Your System

Many employers have a workplace drug policy that includes random drug testing for current employees and routine testing for all new job applicants.

If 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 your use is regular or heavy.

You can be fired for failing a drug test even in states where the recreational use of marijuana has been legalized.

The only completely reliable way of passing the test is to stop smoking or ingesting marijuana or cannabis products.

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 the following.

Washing Your System Out

This method 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.

Some people will also try to exercise before the test, but that can actually backfire, depending on the test, as it can release stored THC from fat into the blood, according to one study in the journal Drug and Alcohol Dependence.  

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 that most of them have to be used over an extended period of time, during which the body will naturally eliminate THC 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 are 100% foolproof. All of them can be detected by the laboratory if a separate test is run for them.

Symptoms of Overdose

It is very difficult to physically overdose on marijuana because the lethal dose is so much higher than the effective dose. Very few marijuana overdoses have ever been reported. If someone you know has taken too much marijuana, and that is the only thing they have taken, an overdose is highly unlikely, but that doesn’t mean that marijuana is not harmful.  

Psychological distress is possible as is impairment of judgment, both of which can lead to hazardous behaviors that can harm yourself and someone else. Although rare, people can experience THC toxicity when using marijuana in high doses, especially in the form of edibles. Symptoms can include:

  • Heart arrhythmias
  • Psychosis or paranoia (or exacerbation of pre-existing mental health conditions)
  • Seizures
  • Uncontrollable vomiting

If you or someone you love has a family history of mental illness, it is beneficial for you to consult your doctor before using marijuana.

The concept of “set and setting” is also important. Since people who have taken too much marijuana can experience sensory overload, minimizing overstimulating inputs in the environment can help them to relax. Some people are also more affected by marijuana than others.

Getting Help

You may have a prescription for medical marijuana, or you may want to partake of weed or marijuana edibles in states where it is now legal for recreational use at the state level. There is a common perception that you cannot develop a physical dependence on marijuana, but this is not the case. Psychological dependence is also a consideration.

If you discontinue marijuana after regular or heavy use, you may experience symptoms of withdrawal. Signs of marijuana withdrawal include:

  • Appetite change
  • Cramps or nausea after eating
  • Cravings
  • Digestion problems
  • Headaches
  • Mood swings
  • Sleep disruption
  • Weight gain
  • Weight loss

If you find that you can’t handle symptoms of withdrawal without relapsing, you may be at risk for a substance use disorder. Do not be afraid to seek out professional support from a mental health professional.

There are therapists who can help you understand your reasons for using. They also can help you determine how cannabis use is impacting your life, and how to develop a plan to stop using if that ends up being the best choice for you.

If you or a loved one are struggling with substance use or addiction, contact the Substance Abuse and Mental Health Services Administration (SAMHSA) National Helpline at 1-800-662-4357 for information on support and treatment facilities in your area.

For more mental health resources, see our National Helpline Database.

Marijuana, known as weed or pot, can last anywhere from a few hours to 90 days, depending on several factors including the type of drug test used. ]]>