Over several years of writing about THC detox, there is one question that I received more than any other:How many days will it take until I can pass a urine drug test?
Information on this topic is notoriously vague. There are many guides available, but they are not very personalized. It is not helpful to know a super wide range (e.g. you will be clear sometime between 2 and 30 days) – you need to know the time to detox THC that is specific to you.
I developed this calculator based on research of which individual characteristics actually predict time to clear THC and which ones are useless (you might be surprised!)
I’ve seen a lot of guides about the time to detox THC and pass a drug test, but they almost never say what data they are based on. That makes me suspicious. So I searched all over the interweb for the best dataset to do my own analysis.
I found the right dataset in a2008 studythat took 60 potheads and (with informed consent) kept them locked in a research unit, collecting urine samples for up to 30 days. Here are 4 important things that I discovered in this dataset that were used to make the detox calculator:
1. Test results could be negative, but then later be positive
The time between thefirst negative resultand thelast positive resultcould be really long. The record was 28 days! In fact, it was rare to have a negative result without at least one later positive result. So this is the rule, not the exception.
[Read more about the issue of negative test results turning to positives.]
This also presented a snag for my analysis…do I consider the “time to clear THC” as the time tofirst negative resultor the time tolast positive result? I decided to split the difference and take the average of the two. I figured this puts people in the range to consistently pass if they properlydilute their urine.
2. Initial urine metabolite levels were correlated with time to clear THC
This finding should come as no surprise. The higher your urine THC metabolite levels are to start, the longer it will take to clear. But note the variability….two people started around 200 ng/mL, but it took one person 3 days to clear and another person over 3 weeks!
Also note that log-transforming the urine metabolite levels gave a better prediction and explained 39% of the variability in time to clear.
3. The amount of cannabis smoked did NOT correlate with time to clear THC
This was probably the most surprising finding of my analysis. How could the amount smoked not correlate with time to clear? To explore further, I looked at the relationship between the amount smoked and initial urine metabolite levels. After all, the starting urine metabolite levels should be determined how much you smoke.
As this plot shows, there was a relationship between the amount of cannabis smoked and initial urine metabolite levels, but I was surprised by howincredibly weakit actually was!
It is astounding that someone smoking 1 joint a week can have higher urine THC metabolite levels than someone smoking 30 joints a week. Or that two people can each smoke 1 joint per day, but one person will start with urine metabolite levels 6 times higher than the other. Yet, this is what the data show.
Sure, some people might be smoking more dank weed or roll fatter joints than others. But I don’t think this alone can explain these differences. I believe there is also a strong effect of drug metabolizing enzyme activity, including due togenetic differences.
Most guides to THC clearance separate people into low, medium, and heavy tokers. My advice? Don’t bother with this…just skip straight tomeasuring your urine metabolite levels.
4. Body fat percentage was correlated with time to clear THC
I have written before about the relationship betweenbody fat and THC pharmaco*kinetics. Basically, the more body fat you have, the more THC (and metabolites) can accumulate in your fat tissue. The slower your THC elimination will be, since all that accumulated THC will keep your levels high as it leaves your fat.
My current analysis confirms this. Percent body fat correlated significantly better with time to clear THC than height, weight, or body mass index (BMI). In fact, body fat explained 27% of the variability in time to clear THC.
Body fat percentage was not highly correlated with initial urine levels, so these two factors are both independent predictors of time to clear THC.
How can we put the above discoveries to use? By building a mathematical model that predicts time to clear THC using the two main predictors: urine metabolite levels and body fat percentage. Together, these two predictors explained 57% of the variability in time to clear THC metabolites. That’s actually pretty damn good!
After developing the model using multiple linear regression, this was the final equation to predict time to clear:
-0.66 -0.93*(body fat %) + 0.333*(body fat %)*ln(urine metabolite level in ng/mL)
[Note:For 10% body fat, you should just use the value of 10 (not 10% or 0.1). Ln is the natural log function.]
Of course, it is easier just to use the calculator above!
