Drug Testing Facts – How does THC operate in the blood stream and what tests can identify it?

(Updated September 5th 2016)

While many countries are pushing to further regulate cannabinoids – the compounds found in cannabis – many workplaces and courts still test for their presence as a condition of parole, employment, and transportation.  This is especially common after an accident has occurred or someone is suspected of operating a vehicle or equipment while under the influence.


Now, depending upon the region, there may be different protocols for testing.

In Australia, police have been known to test drivers’ saliva when they suspect the driver may be under the influence of cannabis.  There is also a specialized breathalyser in development and is seeing active testing in both Australia and the United States.  These tests are meant to approximate a level of cannabis use equating to drunk driving.  The validity of these methods is still largely being argued in the courts but the fact remains that if drugs are present in your system – law enforcement will likely attempt to check for their presence.

In the United States, if a driver is suspected of operating a vehicle or machine while under the influence, police will usually conduct a field sobriety test followed by either a blood test or urinalysis.  Additionally, in court cases where an offender has been paroled, he or she will be required to do regular drug screening panels for the parole hearing.  And, since Reagan signed Executive Order 12564 in 1986, many workplaces are allowed to discriminate against those who test positive for cannabinoids.

As such, it behoves anyone who partakes in marijuana to know how tetrahydrocannibinol (THC) gets into your system and how it’s stored.

What’s so special about THC?

Tetrahydrocannibinol is just one constituent in a class of cannabinoids found in marijuana.  It’s predominant feature is that it’s been identified as one of the more abundant compounds that has psychoactive properties.


Australia sees tetrahydrocannibinols and cannabis in the same legal class as cocaine and methamphetamines (Schedule 8) while the United States has an even stricter characterization of marijuana as a Schedule I substance – a substance that is likely to be abused and has no medicinal value.

Despite continuing studies about the medicinal applications of cannabinoids, court rulings, and even the push for states to legalize – major Western governments still fiercely prosecute those who are found to be in possession.

How Does it Get Inside the Bloodstream?

There are countless ways to ingest or partake in cannabis, hash oil, or straight dronabinol but they all share two common pathways into your system – inhalation or digestion.

Before we explore either of these routes, it’s first important to note that THC does not bind well with water.  As your blood is predominantly water, this means that your blood is not a great carrier of this cannabinoid.  As all blood goes to the liver, we’ll be able to further discuss how your body transports, stores, and excretes this substance.

When processed through the liver, THC forms many metabolites.  These metabolites are what happens when different enzymes and catalysts in the body react with the THC molecule – resulting in slightly different structures that resemble but are not identical to the original.  The main metabolite sought in testing is known as “11-OH-THC”.   Don’t let the numbers and letters scare you – it’s basically describing a hydroxyl group that gets added to the benzo ring of the THC molecule. This typically happens at a specific site in the liver.

This metabolite can further go on to turn into other metabolites (THC-COOH, Carboxylic acid + tetrahydrocannabinol being a prominent one) but we will keep coming back to 11-OH-THC because this is the main metabolite that gets investigated in urinalysis and fecal testing.

When testing for THC in the body, the primary method is through urinalysis where the test will use an immunoassay reagent to look for the presence of 11-OH-THC and THC-COOH.  Most immunoassay metabolite tests in urinalysis are accurate to the nanogram per mililiter (ng/mL) or partsperbillion (ppb) level.

A fecal test will be even more precise in determining the presence of THC as it is the major way the body gets rid of it.  Over half of any consumed THC will eventually pass through this channel.  Less than a quarter will be passed through urine.  And depending upon diet, exercise, and metabolism, up to a quarter can be retained in a person’s adipose tissue (fat) for as long as thirty days.


What is the Threshold or Minimum Detection Limit?

Threshold – or Minimum Detection Limit (MDL) – refers to the bare minimum concentration of THC that will result in patient’s sample coming back “positive” for that substance.

Factors keeping a person’s sample below the MDL vary greatly depending on:

  • Habitual usage
  • Bodily fluid (blood vs. feces vs. urine)
  • Test type (Immunoassay vs. Chromatographic)
  • Metabolism and diet

Habitual Usage

Upon imbibing THC, it can take up to two hours before a person’s urine sample breaks threshold (> 50 ng/mL).  However, for the regular user, it can remain above MDL for over a month — whereas an occasional user falls below threshold typically within a week.

So, depending on your usage, you can formulate an appropriate plan to defeat a standard urinalysis.  There are some additional tips in follow which we’ll discuss in later sections.  We will go into greater detail about how you can safely increase your chances of falling below the MDL during urinalysis if you don’t have the required time to get those metabolites out of your system.

Bodily Fluid

THC is stored in different ways depending on which fluid you’re testing.  Once this cannabinoid enters the blood stream, it is further metabolised by the liver.  From here, metabolites of THC typically pass through the feces of the person.   Some amount is passed through the kidneys and then, subsequently, through urine.  However, over 50% of the amount of THC ingested will go through the bowels while less than 24% will pass through the bladder¹.


Blood, however, is only a good testing substrate if someone is trying to detect cannabis usage within the last 24-48 hours.  After 48 hours, THC levels drop off significantly as it is predominantly converted into its constituent metabolites and no longer detectable in its original form.

Also, as we’ve already discussed, tetrahydrocannibinol is not very water soluble.  Additionally, the liver does a relatively good job of scrubbing THC from the blood system after ingestion.


Saliva is an exploratory option for THC testing as it can be effective in demonstrating THC was consumed within a 2 hour window.  However, for periods greater than two hours it is very inconclusive as the majority of THC that was imbibed has either been broken down by the saliva or passed into the blood stream or digestive tract.


Urine is the predominant fluid that gets tested because it is able to retain trace amounts of THC metabolites for up to a month after usage.  It’s also the easiest method to get a test result – requiring far less processing than feces and better longitudinal accuracy than blood.


And finally, feces is the third most common testing method – as it doesn’t require a large stool sample in order to detect for traces of THC metabolites if they exist in a person’s body.  This test is extremely difficult to adulterate on the part of the test-taker but also requires a more complicit consent.  Most companies, for instance, will never require a stool sample and even the United States’ Department of Transportation (DOT), which regulates the drug panel requirements for commercial drivers, doesn’t normally require a fecal test.  However, without a doubt, if an agency is curious about whether or not a person has been consuming cannabinoids – this method is the least invasive and the most accurate.  There are also very few ways a person can adulterate his or her sample.

Fecal samples will typically demonstrate some level of THC metabolites for up to and past thirty days depending on the regularity of cannabis consumption.

There are other bodily fluids that can be tested, such as semen and cerebral spinal fluid, and these fluids may contain trace metabolites of THC but testing via either of these channels is extremely painful and invasive.


Test Types

In the case of an immunoassay test (EMIT), where they are using a reagent to test for presence of a metabolite (like 11-OH-THC), the resolution (MDL) is typically 50 ng/mL.  However, for members of the US Armed Forces, there have been instances of the MDL being set at 35 ng/mL.

Because THC does not bind well with water, it is not efficiently bound to blood – with it dropping below threshold within 24-48 hours after use once it passes through the liver.  However, it tends to lodge itself in the fatty tissues because it is hydrophobic.  Fats are also hydrophobic – making this a great pairing.  As such, if you’re a habitual user, you may retain traces of THC in your system for up to three months.

However, if you’re not a regular user, the THC is expected to pass below a minimum detection level (MDL) within 30 days of initial use.

If a sample is chromatographically tested in a lab, any presence of THC will be detected.  Depending on the state, region, and country this test is being conducted in — and for what purpose — having any amount whatsoever in your blood stream may be enough to use as some form of evidence.

Especially in cases involving parole violations and multiple felony criminals, any amount of a controlled substance being found in one’s urine or feces will be used as evidence before the judge.

Metabolism and Diet

As we’ve discussed before, THC is not very water soluble.  Thus, it most easily binds into the fatty tissues of a person after it moves into the bloodstream.  Once bound in the fatty tissues, it will remain until those tissues are used for energy.

For those who typically maintain a lower BMI (>8-10% body fat), this cannabinoid passes quickly through the liver and is excreted.  However, if you lead a sedentary lifestyle and have a higher BMI, there is a greater chance of THC binding in your fat cells.  As the body burns fat, the THC is released back into the blood stream and has a second chance to pass into the liver where it will likely be excreted as feces but potentially as urine.

This is precipitously dangerous for testing purposes if you experience anxiety or intermittently engage in physical activities before a urinalysis.  However, if you’re not a regular user and employ precautions, it can be rather simple to defeat most standard urinalyses.  The amount stored in fat is usually not significant enough to bring you above the MDL if you’re outside of the window of 30 days of non-use.  Factors that would affect this would obviously be the amount of fat you maintain and your lifestyle.

Further down, we will discuss some of the methods for getting around the MDL.


THC Intake Methods

Ingestion or inhalation are the prominent methods for cannabinoid consumption.  Intravenous injection is uncommon.

Ingestion, in this case, refers specifically to eating a substance laced with THC.  The market is awash with THC laden lozenges, snacks, and sauces for almost any dietary desire.

However, the main process remains the same.  A small fraction of the THC is broken down by saliva (>less than 1-2%) while the majority is broken down in the stomach.  It is estimated that only 6-20% of the full amount of THC is broken down and processed via digestion whereas 10-35% of the full amount can be processed through inhalation.

This is because when you ingest THC, it undergoes a long process before finally arriving in the liver where it is able to be transported about the blood stream and deliver its psychoactive payload.

Whereas in inhalation, the THC molecules are given near direct access to the blood stream via the alveoli in the lungs.  Alveoli are the small pockets in your lungs that help bring oxygen into your bloodstream and take out carbon dioxide.  When the cannabis smoke or vapor enters into these pockets, the THC is absorbed through the thin cell lining and quickly enters into the blood stream.

The blood then travels down to the liver where the THC is further metabolised into 11-OH-THC and processed by the body for excretion.  This pre-liver phase is where THC is allowed to interact with the central nervous system and bring about its various psychoactive properties.  With digestion, because it is entering the blood stream through the liver, most of it is available to be broken down – thus losing a degree of its efficacy.

However, even in the case of digestion, it is nearly impossible for the liver to process all of the THC immediately.  Plenty gets through and makes it around for another pass through the blood stream.  This is why the effects of cannabis perpetuate long after initial intake.  For digestion, it may take longer for the onset of effects but those effects will undoubtedly take place as long as there is an active dose.


Defeating the Test

“It all winds up in the same place eventually.”

As we discussed, no matter how you imbibe – it’s your liver that will do the processing.  And that is where we can begin to find out how to increase or decrease the processing time of THC metabolites in the system.

Previously noted, feces is the main method of excretion with urine being the secondary.  Obviously, if you’ve consumed tetrahydrocannabinoids within the past thirty days, your feces will contain metabolic evidence of that.  Your urine, however, is much less efficient at processing the metabolites and may paint (no pun intended) a much different picture of your usage.

Your liver is where your body decides where it’s going to put the metabolites – in your adipose tissue (fatty tissues), send it out to the kidneys, or pass it through the bowels.

So, if you want to help your body make that decision, your first step would be to help tell it where to send the metabolites.

Zinc Sulfate – Hijacking the THC car

Zinc sulfate, which can be purchased over the counter in the form of a zinc vitamin supplement, is a large bulky molecule that has been shown to be effective in binding with THC metabolites in the liver.  Adding a bulky subtrate to a THC metabolite forces the body to pass it through feces rather than sending it to the kidneys (and subsequently through your urethra).  Your kidneys are not at all an ideal place for large, bulky molecules and, in fact, kidney stones are an excellent example of what happens when your body makes that mistake.

This method, however, is not by any means a guarantee for passing a urinalysis.

Zinc sulfate is only effective at catching metabolites while its being processed through the liver.  If you imagine being at a four-way stop, Zinc is going west-bound and THC is going south-bound.  Now, if they catch each other at the proverbial four-way intersection stop sign — you’re in luck!  One less THC metabolite being sent to your kidneys.  However, if they don’t cross one another’s path, within a short amount of time the zinc will simply go unaccompanied through your bowels.


Water – Hydrate. Hydrate. Hydrate.

For urinalysis – water is always your best friend.  No matter if it’s a chromatographic analysis or an immunoassay test, it does come down to a simple matter of concentration.  And the only way to bring down the concentration of THC in your urine is by flushing with water.

Now, most urinalysis places will try to get you to give them their sample early in the morning.  This is a devious tactic as your body had been storing all your urine while you slept overnight.  So, your first TWO to THREE urine samples of the day will contain the highest concentration of any accumulated metabolites in your system.

How do you get around that?

Hydrate.  Hydrate.  Hydrate.

Now don’t go putting yourself into a state of enuresis – but you need to be drinking as much water as you can handle.  If you can take a long piss at least twice before giving your sample – you’ve significantly decreased the concentration of THC metabolites in your system.

Mellow Yellow – Fake it ’til you make it!

Now if you’ve followed the previous section’s advice, you’ll notice, “hey wait a minute!  My urine is clear as water!  They’ll think I dilluted it!”


But the worst that happens if you urinate clear is they ask you to do it again.

There are many worse consequences than that for giving them what they asked for.

You’re going to want to take a Vitamin B complex supplement while you’re hydrating.  Vitamin B complex will likely turn your urine a bright yellow because your body excretes excess Vitamin B through your kidneys.  This has the side effect of changing the color of your urine to yellow no matter how hydrated you are.

In the off chance you are asked, you can simply say you take a daily multivitamin — something no one can fault you for.  If they want another sample, for whatever reason, they’ll have to wait at least two more days and if you’ve been hydrating – you should be just that much closer to having absolutely nothing to worry about.

By this time, your blood will usually be completely flushed of any THC metabolites and the only possible place for them to get a good reading would be in a stool sample.

Mr. Clean – Getting tough on grime!

Adulterating your sample with toilet water, bleach, or iodine may be effective if the drug analysis center hasn’t already levelled precautions against those sorts of activities taking place.

Famous examples of this are:

  1. Adding a coloring agent the toilet water to change the color of the urine sample if added as an adulterant.
  2. Placing a seal over the back lid of the toilet which would break in the event of use.  This effectively prevents you from taking water out of the toilet’s reservoir.
  3. Monitoring the bathroom through the use of security cameras or a live person.

NOTE: In the United States, this practice is illegal in some states unless you consent beforehand or you are in State or Federal custody or in the Armed Forces.  Check your state’s laws regarding surveillance and consent for monitoring prior to the drug test.

If your drug testing process DOES NOT have any of these guards (and only if you’re absolutely certain), you may add a small concentration of bleach or iodine to your sample.  Bleach (OH-CL; hydroxyl chloride) is extremely good at breaking down organic metabolites.  It’s basically like throwing those THC metabolites into a blender and hitting PULSE.  Bleach is also extremely hard to detect in low concentrations (a gallon of bleach typically only contains 5% bleach) as it breaks down to its constituent hydroxyl (-OH) and chloride (-CL) ions in solution.

So, if you can manage, add a few (3-4) drops of bleach solution to your urine and it will increase your chances of breaking up those metabolites into an unrecognisable form.

Adding more than three to four drops of 5% bleach solution to the standard 30 mL urine cup is courting danger as higher concentrations of bleach can be detected.

Iodine requires even less – perhaps one to two drops at most.  It will take the sample down to a light brownish yellow hue with the simple addition of two drops.  Simply add the drops and then lightly rotate the sample to let the iodine dilute into the mixture.

While many drug testing centers check for the use of common adulterants, some are simply so deceptive (bleach, iodine) that they could only be detected if large quantities were used.

DO NOT EVER drink bleach or iodine to beat a urinalysis.  It’s never worth it.  Ever, ever, ever.


In conclusion, no matter which way you imbibe THC – it’s going to have to pass through your liver at some point.  And once it’s in your liver, within a week to a month it will be excreted from your system.

If you have to hedge your bets before then, due to urinalysis, your best bet is hydration and the usage of vitamin B.  There are kits which have various degrees of efficacy but overall the most reliable methods have been time, water, and vitamin B.

And lastly, if you do feel compelled to use an adulterant – use one which is easily lost in the background of your urine sample and don’t use more than the recommended addition.  Getting caught for adulterating your sample is about the same as returning with a “positive”.

Be positive.  Stay negative.