How long does THC stay in your system, and why does the answer swing so far? This covers how THC is stored in fat, what a urine test actually looks for, the cut-offs labs use, what research-ward studies found for urine, blood, saliva and hair, whether edibles stay longer, whether exercise or fasting releases THC from fat, and why a positive test on its own can't say whether someone is high now.
The full answer
How long does THC stay in your system?
How long does THC stay in your system? After one joint, a urine test usually stops finding it within about one to four days, but someone who smokes every day can stay positive for weeks, sometimes more than a month. I'm Justin Hartfield, and I've been selling cannabis for twenty-five years. The numbers swing that far because THC gets stored in your fat, and a urine test isn't even looking for THC.
How long does THC stay in your body?
How long does THC stay in your body? Far longer than the high, because THC soaks into your fat and seeps back into your blood slowly. In 1989, a team in Sweden still found THC in fat taken from heavy users four weeks after they last smoked, and in eight heavy users who smoked THC tagged so it could be tracked, its half-life in the blood averaged about four days. Your liver turns part of the THC into 11-hydroxy-THC, the second high from my edibles answer, and then turns that into THC-COOH, which doesn't get you high at all. That's what a urine test looks for.
How long does THC stay in your urine?
How long does THC stay in your urine? It depends on how often you use and where the lab draws the line. US federal workplace tests screen at fifty nanograms of THC-COOH per milliliter and confirm at fifteen. In 1996, six men at the US National Institute on Drug Abuse smoked single joints on a research ward, and at the fifteen cut-off their last positive came, on average, about a day and a half after a weaker joint and nearly four days after a stronger one. In 2008, sixty users lived on a closed research unit for up to a month, and at the fifty cut-off their last positive came after four to fifteen days on average, depending on how much they started with. In a 1985 study of eighty-six heavy users, the longest stayed positive at a cut-off of twenty for forty-six days in a row, and most went negative for a few days and then positive again. My guess is that's stored THC leaking back out of fat.
How long does THC stay in your blood?
How long does THC stay in your blood? For someone who rarely uses, not long. In 2014 at the same institute, after one joint, blood THC fell below five nanograms per milliliter after a median of one hour in occasional smokers and three and a half hours in frequent ones. In daily users it lingers. In 2009, eighteen heavy users spent a week on a research unit without cannabis, and sixteen of them had at least one day when their blood THC went up instead of down. In 2013, of thirty daily smokers kept on a research unit for up to a month, two of the five still there at day thirty had THC in their blood, about a third of a nanogram per milliliter.
How long does THC stay in your saliva?
How long does THC stay in your saliva? Right after you use, a saliva test mostly picks up THC left in your mouth, so levels peak then. In 2014, twenty-four people smoked one joint and were tested with an on-site saliva kit, which stopped reading positive after a median of twelve hours in occasional smokers and twenty-one in frequent ones.
How long does THC stay in your hair?
How long does THC stay in your hair? Researchers count each centimeter of hair as about a month. In 2016, a team in Bristol tested the hair of a hundred and thirty-six people and found THC in seventy-seven percent of the daily or near-daily users, thirty-nine percent of the light users and none of the non-users, and they concluded hair can flag heavy use over about three months, but not light use. THC can also land on hair from outside. In 2015 in Freiburg, ten people who didn't use cannabis rolled a joint on five days, and all of them ended up with THC and THCA, the raw plant form, in their hair.
How long do edibles stay in your system?
How long do edibles stay in your system? Probably longer in urine than the same THC smoked, though that comes from two separate studies. In 2018 at Johns Hopkins, eighteen adults ate THC brownies, and in the six who had twenty-five milligrams, the THC-COOH in their urine peaked at an average of three hundred and thirty-five nanograms per milliliter. In the 1996 study, the peak after a joint with about thirty-four milligrams averaged about a hundred and fifty. Those were different people, tested twenty-two years apart, so it's a rough comparison. My guess is the liver, which gets the first pass at swallowed THC.
Does exercise or fasting release THC from fat?
Does exercise or fasting release THC from fat? In rats, fasting does. In 2009, researchers in Sydney gave rats THC for ten days, and a day without food raised the THC and THC-COOH in their blood. In people, I found one small study. In 2013 the same team put fourteen regular users on an exercise bike for thirty-five minutes, and the THC in their blood rose slightly, more in people with a higher body mass index, while an overnight fast made no clear difference. I found no study showing it makes anyone feel high again.
Does a positive drug test mean you're high?
Does a positive drug test mean you're high? No. From smoking, the high fades over two or three hours, and from an edible it can last up to twelve. A 2021 review of eighty studies estimated that for people who don't use regularly, most driving-related skills come back within about five hours of inhaling twenty milligrams of THC, and nearly all within seven. A urine test keeps reading THC-COOH for days or weeks after that, so on its own a positive result can't say whether someone is high now.
The whole film is on my site, linked below. Next: Can you get a contact high?
What the studies show
Every claim in this answer, the study behind it, who was studied, and who paid for the research. “None listed” means the paper names no funder.
