You smoked weed one time and now you have a drug test coming up. What does the science say about how long you need to pass?
Many people who get drug tested for THC are regular cannabis users. These people have accumulated THC and its metabolites in their fat and many need several weeks to get clean. However, what about those who rarely use it?
I get this question from people a lot. They smoked or vaped one time, but now they are unlucky enough to face an upcoming drug test. How long do they really need to get clean?
This question is also faced by people who try to “time” their smoking. They know they can get a random test anytime between Monday and Friday, so they smoke only on Friday nights with the hope of being clear by Monday. Is this really a safe practice?
Although there are many calculators to estimate how much time until you are clear (including my own THC detox calculator), these mostly focus on people who are regular cannabis users. Today I will delve into the scientific studies of infrequent users after they toke a single time. You might be surprised at how quickly you can actually pass a drug test after one use!
How Long Until You Start to Test Positive For THC After One Use?
The first thing to know is that your urine levels of THC metabolites peak about 4 to 6 hours after smoking or vaporizing (2019 study). It typically takes 3 to 4 hours after smoking before a urine drug test will produce a positive result. This delay is because it takes time for your body to convert THC to the metabolites that are excreted in the urine and detected by tests.
In theory, you could smoke weed right before a urine drug test, take the test high as fuck, and come out completely clean (NOTE: I am not actually recommending you do this!). Different rules apply if it is a blood or saliva test, because these detect THC itself, and not the metabolites.
How Long Will People Remain Positive For THC After One Use?
Let’s get to the most important question. You smoked cannabis once, and now you want to know how long you should be worried about failing a drug test for. The answer is complicated by a few factors.
There are two cutoff levels for THC metabolites that you should know. Most labs will first do a quick test for THC metabolites at 50 ng/mL. If you fail this, they will then do a second more specific test with a cutoff of 15 ng/mL. If your urine levels are below 50 ng/mL, you will probably be fine. However, there are occasional false positive on the first test. So to be completely in the clear, it is better to have urine levels below 15 ng/mL.
Study 1
In a 1996 study, 6 subjects smoked a low dose (15.8 mg THC) and a high dose (33.8 mg THC) of cannabis and their urine metabolite levels were tracked. The low dose resulted in negative results for all 6 subjects at the 50 ng/mL level within 24 hours. Half of the subjects were also negative at the 15 ng/mL level within 24 hours. The three subjects who needed a little longer were first negative at 31, 36 and 57 hours.
At the high dose in this study, all subjects were negative at the 50 ng/mL level by 48 hours. Half were negative at the 15 ng/mL level by 48 hours, but three subjects needed 54, 80, and 82 hours to be completely clear.
Study 2
In a 2010 study, 12 male subjects smoked cannabis with approximately 45 mg of THC. Only 4 subjects (25%) even hit the lower 15 ng/mL threshold for a positive urine sample. Of these four subjects, their final positive urine samples occurred within 6 to 24 hours.
Study 3
In a 2019 study, 17 subjects both smoked and vaporized two different doses of THC (10 and 25 mg). This study was somewhat limited by the fact that the last urine sample was taken within 8 hours, but it still gives us some useful information.
When subjects smoked, only 24% (low dose) and 47% of subjects (high dose) hit at least 15 ng/mL. With vaporization, 59% (low dose) and 88% (high dose) of subjects hit at least 15 ng/mL. When considering the higher 50 ng/mL cutoff, only 21% of subjects who smoked achieved a positive and 47% of subjects who vaped achieved a positive.
We can’t say from this study how long subjects would have been positive for since the last urine sample was collected within 8 hours after smoking. However, it confirms that a large fraction of people will not be positive at all after smoking a single time.
Many people find this surprising, but it can be explained by the activity of different enzymes in your liver. As I show in the science section, there are multiple metabolic pathways for THC. Only one leads to the metabolite detected by drug tests. If you have relatively low activity of the enzyme for this pathway, you may never fail a drug test.
Summary
Many people will be clear within 24 hours of smoking a low dose. If you smoke a higher dose, you may need 48 hours to be clear at 50 ng/mL. If you want to be extra confident and hit the 15 ng/mL level, you may need longer than 72 hours.
How To Improve Your Chances of Passing After Smoking A Single Time
Keep in mind that the times I estimate above are without implementing any detox strategy. I am confident that most people can pass within 24 hours if they use some basic detox strategies such as urine dilution and activated charcoal, as I outline in the THC Detox Protocols.
If you want to be a little more proactive, taking herbs that are CYP2C9 inhibitors or CYP3A inducers BEFORE you smoke can also help. Just be wary of interactions if you are taking other drugs (check with your doctor or pharmacist first). In my own personal tests, I was never positive for THC more than 12 hours while taking a mixture of a CYP2C9 inhibitor with a CYP3A4 inducer, even when I tested smoking every day!
Remember that your mileage may vary. Always test yourself at home to know where your urine THC metabolite levels are!