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 вЂ” 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, howeverВ вЂ” which can haveВ stronger, longer-lasting effectsВ вЂ” 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 вЂ” 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 bloodВ tests for marijuana work вЂ” 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 вЂ” even though it’s stillВ there in trace amountsВ вЂ”В 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 вЂ” 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 вЂ” 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 вЂ” 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, youвЂ™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 AmericaвЂ™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 mostвЂ¦