While a high might only last a few hours, cannabis can linger in your body for days after you consume it, experts say.
But exactly how cannabis works its way through your system is a complicated question. Here’s what we know.
Smoking or vaping cannabis is a “very efficient way to get the drug into your bloodstream,” said Dr. Robert Mann, a senior scientist at the Centre for Addiction and Mental Health.
Studies that he is working on and hopes to publish soon find that people reach a peak level of intoxication within about five minutes of smoking cannabis. “Then THC levels will decline over the next couple of hours very rapidly, then that decline slows off for a while, and then after several hours, I think the levels in the blood are quite low.”
The Canadian guidelines for lower-risk cannabis use recommend that you don’t drive for at least six hours after smoking cannabis, as that’s about how long impairment might last, he said, though obviously if you’re still feeling intoxicated after that, you still shouldn’t drive.
A number of things could affect how intoxicated you get after smoking cannabis, said M-J Milloy, the Canopy Growth professor of cannabis science at the University of British Columbia, and research scientist at the B.C. Centre on Substance Use.
These include the specific type of cannabis — for example, if it’s high in THC, an intoxicating chemical — how much you smoke, and exactly how you consume it. While smoking or vaping cannabis has a very quick effect, he said, it takes longer to feel an effect if you eat it — up to two hours after consuming.
“Cannabis’ effect on individuals as well as its detectability really varies by what people are using,” he said. “And if they are, for example, smoking high-THC joints, that’s a different thing than if they are taking gel caps of high-CBD cannabis.”
There is a lot of variability in the peak THC levels that different people reach, said Mann, and he also attributes that partly to things like “how deeply you might inhale, how long you hold the inhalation for and things like that.”
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THC levels in the blood don’t correlate smoothly to intoxication the way alcohol blood levels do though, Milloy said. “The bottom line is there is no blood test, urine test, etc. for intoxication.”
Although some Canadian police forces use a roadside test to detect the presence of THC, various critics including Milloy say that a positive result wouldn’t necessarily indicate that the person was actually impaired while driving.
While legal tests for cannabis intoxication exist, Milloy said, “I would argue that they are not based on good science.” He expects many cases based on these tests to end up being argued in court.
But once you’ve stopped feeling impaired, that doesn’t mean that the cannabis is totally gone from your body.
“When you use cannabis, the THC gets absorbed into the fat in your body, and so there’s kind of a reservoir there that keeps getting released fairly slowly, so that you will find trace amounts of THC in the blood for a longer period of time, but at very low levels,” Mann said.
Exactly how long it remains, though, is still a matter of debate. An article on urine drug screening by the Mayo Clinic suggests that cannabis can be detected in urine for between three and 30 days. A literature review — admittedly of mostly very small studies — suggested it’s more like one week to 21 days, depending on what kind of THC concentration cutoff you use.
The urine drug test that Milloy uses in his lab is supposed to detect cannabis use within the last 28 days, he said.
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Part of the variation comes from how frequently someone uses cannabis, Mann said. “If you’re a frequent user, there are people who use daily or several times a day, it builds up in your system. It’s in your body at higher levels for a longer period of time just because it’s building up in the body.”
A very occasional user will eliminate THC from their system much faster, the studies show.
There’s still a lot of research to be done on how cannabis is processed, though, Mann said, such as how edibles change the equation, or how long other chemicals found in cannabis like CBD can be detected.
“There’s a lot that we know. It’s not the case that we don’t know anything. But there’s a lot that’s new and a lot that’s still needing investigation.”