Ask anybody who is worried about drug testing and they’ll probably tell you that drug tests can detect cannabis for up to 6 months from ingestion. But how true is this? Is it just a common myth, or is there a thread of truth?
To start off answering this question, we can actually use an article that Digamma Consulting published with us: “Smoking vs. Eating Cannabis: The Effects on Patient Health.” The article covers several of the primary metabolites involved in smoking/ingesting cannabis. These metabolites include:
- THC - The primary psychoactive ingredient in cannabis
- OH-THC - A metabolite produced by the human liver that is even more psychoactive than THC
- THC-COOH - A THC metabolite that is not psychoactive, and lingers in the body.
The metabolite most relevant to drug testing is THC-COOH. The other cannabinoids, THC and OH-THC, don’t persist in the human body for longer than a few hours, making them undetectable when you’re no longer under their effects. In any jurisdiction where cannabis is legal, the presence of THC-COOH is not considered indicative of impairment.
The Persistence of THC-COOH
So how long does THC-COOH persist in the body? Since this is the substance that many drug tests look for, it’s important to know if you’re worried about a drug test at your place of employment. After all, even in states where cannabis is legal, your employer can still fire you for cannabis use, as it cannabis use is not protected by law.
If we look at detection levels in some drug tests, we find that the minimum threshold for detection of cannabinoids in the body is around 20 ng/mL of blood, or 20 nanograms. 20 nanograms is 0.02 micrograms.
One particular study published on the NCBI states the following:
- “THC-COOH elimination half-lives were 5.2 (0.08) and 6.2 (6.7) days in frequent and infrequent users, respectively (21). After 5-mg intravenous THC-COOH to 10 non–cannabis users, terminal serum THC-COOH elimination half-life was 17.6 (5.5) h; however, monitoring was for only 4 days, potentially underestimating this parameter (34).”
This establishes that the half-life of THC-COOH in the human body is around 5.2 to 6.2 days. Every 6.2 days, the human body should have removed at least half of what remains. The CDC provides a more liberal estimate, suggesting that over 90% of THC-COOH is eliminated from the body in the same time period, but for chronic users the THC-COOH can be stored in fat tissues for longer periods.
Now we can do a napkin calculation with the numbers we have. The NCBI study delivered 20 milligrams of THC to users daily, so we’ll start with that number. The study reports that 24 hours after ingestion, blood THC-COOH levels were around 21 micrograms per liter. If we convert that to milliliters, that’s 0.021 micrograms per milliliter. Holy cow! In one day, we’re already at the minimum threshold for the drug screening test we described above. If we wait another week, we’ll be well below the threshold. A single half-gram joint could pass a drug screen within a week.
But this makes a lot of assumptions:
- The NCBI study looked at blood levels of THC-COOH, not urine levels. Most drug tests require you to submit a urine sample, not a blood sample, and the THC-COOH likely was excreted via #1 or #2, to put it politely.
- The NCBI study did not examine fatty tissue in their subjects. In a one-time user, it’s unlikely that many cannabinoids would be stored in these fatty tissues, but they’re much more likely to persist in chronic users long after consumption. If you’re a regular user, your blood and urine levels could be much higher much longer.
- The absorption and metabolism rate of THC will vary by ingestion method and the individual. Some ingestion methods tend to persist longer (edibles), some are more efficient at THC delivery (vape). Individuals with slower metabolisms will likely take longer to process the THC as well.
A very conservative “safe” estimate is that the THC-COOH from a single joint should be undetectable within two weeks for most people. However, if you’re a chronic user (pun intended), it will take significantly longer for your levels to drop below the threshold.
What about CBD?
The more astute among you probably noticed we have been almost exclusively talking about THC and THC-COOH. Good news, there’s studies on the metabolism of CBD as well.
However, you may be missing the forest for the trees if you’re looking at the metabolism of CBD. The FDA recently announced that CBD is federally legal. In other words, should a drug screening detect CBD metabolites, there may not be a legal basis for firing. That said, some states allow firing for any reason.
So let’s take a brief look at the metabolism of CBD and figure out what we can decipher. The first thing to look at is CBD itself. According to a study which gave subjects 10 mg of CBD per kilogram body weight per day for a week, blood plasma levels of CBD peaked at 11.2 nanograms per mL, well below the detection threshold we discussed above (20 nanograms/mL). The only study that shows CBD persisting longer than a day was a direct intravenous injection of 20mg of CBD, and that quickly dropped below detectable ranges within a few days.
Additionally, we have to look at metabolites of CBD. Unlike THC which rather neatly metabolizes into only a few compounds, CBD can metabolize into many different compounds:
You’ll notice that there’s about 12 different metabolites from CBD in the image above that showed concentrations greater than 1% in the NCBI study. The study identified at least 40 total different metabolites. However, the most abundant metabolite was 7-COOH-CBD and its derivatives.
What I could decipher from the study is that these metabolites will generally exhibit at roughly the same rate as CBD (13.3% to 12.1%), meaning that these metabolites should generally be undetectable within a few days as well.
If you’re smoking a single joint with THC in it, you may be able to pass a drug test within a few weeks of use. However, if you’re a chronic chronic user (that’s not a typo), the secondary metabolites of cannabis may persist in your fat cells for a very long time, lending some credence to the “six months” idea.
If you use exclusively CBD products, you should be able to pass most drug tests within a few days. CBD and its metabolites don’t persist in the body at high enough concentrations to reach the minimum thresholds for drug tests.
So, did we get anything wrong? Did we forget something? Let us know in the comments or survey below!
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About the Author
Hunter Wilson is a community builder with Growers Network. He graduated from the University of Arizona in 2011 with a Masters in Teaching and in 2007 with a Bachelors in Biology.