It has been widely reported that you should not exercise prior to a drug test for marijuana to avoid burning fat and releasing THC into your bloodstream. Is there any scientific evidence for this?


Today I will examine a claim surrounding exercise and drug testing:

You shouldn’t exercise prior to a drug test for marijuana or you will burn fat and release THC that will cause you to fail the test.

Recommendations range from not exercising immediately before the test all the way to stopping exercise a week before the test. The lack of specific recommendations around this already has a voice in my head whispering bullshit.

Let’s examine the science behind this claim. But first, we need to understand some basic concepts of THC metabolism and drug testing.

THC metabolism and pharmacokinetics

You should first familiarize yourself with the basics of THC metabolism and how urine drug tests for marijuana work. Here is a quick summary though:

When you smoke marijuana (or consume via any other route), THC distributes from your blood into fatty tissues. If you are a regular marijuana user, the levels in your fat will build up over time, making it take longer to clear out of your system. The more fat you have, the larger will be your pool of fat-stored THC.


THC metabolism urine excretion basic


However, it is not THC that urine marijuana tests detect, but metabolites of THC. One of the major metabolites of THC is called THC-COOH. This metabolite also distributes to the fat to a large extent. THC-COOH is converted to the THC-COO-glucuronide metabolite, which is excreted into the urine.


Studies on exercise and drug testing

Two scientific studies have looked at the effect of exercise on THC and metabolite levels. In fact, the first study has been widely cited as “proving” that you should not exercise immediately prior to a drug test.

Let’s see what conclusions the authors of the first study draw:

 “Overall, these results suggest that exercise may elevate blood THC levels by releasing dormant THC from fat stores.”

Sounds promising, but let’s see what the authors of the second study conclude:

“We conclude that exercise…is unlikely to cause sufficient cannabinoid concentration changes to hamper correct interpretations in drug testing programmes.”

Oh shit! Looks like a scientific throwdown is about to happen. The voice in my head whispering this claim is bullshit is getting a little louder. I guess we’ll have to look at the details and draw our own conclusions.

In the first study, 14 regular marijuana users performed moderately-intensity exercised for 35 minutes. In the second study, 6 regular marijuana users performed moderate-intensity exercise for 45 minutes.

Investigators took blood samples prior to exercise, immediately after exercise, and (in the first study only) 2 hours after exercise. THC and THC-COOH levels were measured in the blood samples.

Effect of exercise on blood levels of THC and THC-COOH 

In the first study, THC levels in the blood increased an average of 12% after exercise (although they returned back to baseline levels within 2 hours). In the second study, blood THC levels increased by an average of 25% after exercise. 

Given that a similar increase in blood THC levels was reported by two independent studies, I believe that this is real. However, does a small (and short-lived) increase in blood THC levels matter in the context of a urine drug test that measures metabolites of THC and not THC itself? 

Prior to exercise, study subject’s blood levels of THC-COOH were 25 times times higher than THC. THC does not get into the urine for the most part except through the THC-COOH pathway. If levels of THC-COOH do not change with exercise, what happens to THC levels is not that important.

So what happened to blood levels of THC-COOH after exercise? Neither study showed any change in blood levels of THC-COOH after exercise. The lack of change of blood THC-COOH indicates that exercise will not have any immediate effect on a urine drug test.


Effect of exercise on urine levels of THC-COOH

The blood results are pretty convincing, but it is even better to look directly at THC metabolites in the urine after exercise. Although the first study did not collect urine, the second study did collect urine before and after exercise. As is typical in many drug testing labs, they converted all THC-COO-glucuronide to THC-COOH prior to testing.

Urine THC-COOH levels were on average 23% lower after exercise. Given the small number of subjects in the study, the decrease could be by chance, but it does not seem likely that exercise is going to increase urine THC-COOH.

Myth busted?

We can consider this myth busted. If you are taking a typical urine drug test for marijuana, exercising right before the test will likely not have any effect on whether you pass.

Although it is true that exercise will cause a small increase in blood THC levels (which lasts less than 2 hours), this does not lead to an increase in THC metabolite levels in either the blood or urine.

Looks like my bullshit meter is well calibrated.


[Featured image credit: Flickr/ Nathan Rupert]
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Last modified: April 23, 2017

2 Responses to " Exercise Releases THC From Fat Cells – Drug Testing Myth? "

  1. Tom says:

    So I have a drug test coming up soon. I have gone over 90 days and I have just recently passed an at home drug test. I had the time to quit since I knew my graduation date was coming (yay). However I am 100lbs overweight and i’ve decided to start taking better care of myself. No better time than when youre 30 right.

    I just want to make sure that its safe to do some exercise and start working at it before hand. Would you risk it?

    • Yes, you should go for it. The increase in blood THC levels after exercise is very small and transient. It lasts less than 2 hours and urine levels of THC metabolites don’t even seem to be affected. There is no reason you shouldn’t exercise for fear that it will cause a positive on the drug test.

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