BY BEN THOMAS | Original illustration for Ember by Camille Soulat
Whether you use cannabis medically, recreationally, or just as an occasional mood-booster, you’ve probably wondered how long it stays in your body. If you’ve woken up a bit stoned the day after smoking, you might have guessed that it's chilling in your body for at least a few days. Maybe you’ve even read reports that certain drug tests can detect traces of it for up to three months (seriously?!) after ingestion.
The truth, as usual, is somewhere in the middle—and it depends on a wide range of factors. The way you ingest a given dose, the concentration of cannabinoids, your body’s composition and metabolism, and even the frequency with which you use cannabis, can all impact the length of time it remains in your system.
Let’s take a closer look at each of these factors—and find out, once and for all, how long weed can stay in your body.
Cannabis isn’t just one chemical; it contains a whole spectrum of different cannabinoids, which your body processes in a variety of ways. This point is going to be super-important throughout this whole article, so say it with me one more time: Cannabis is not a single chemical.
The wide range of cannabinoids in any given dose, and the different ways your body metabolizes those compounds, are the main reasons why there’s no single answer about how long weed stays in your system. To see why, let’s break down a quick example.
A bowl of flower doesn’t actually contain much active THC or CBD. What its resin does contain is delta-9-tetrahydrocannabinolic acid (THC-A) and its CBD equivalent, CBD-A. The process of heating—via smoking, vaping or baking an edible—converts those precursor acids into pharmacologically active chemicals like delta-9-THC.
But while delta-9-THC gets eliminated from your body in just two to five hours, it’s only one small part of the cannabinoid kaleidoscope. If you eat an edible, for example, your liver converts most of the THC-A into a super-psychoactive metabolite called 11-Hydroxy-THC—resulting in the notorious delayed-reaction “brownie high” that seems to come out of nowhere, rocket you into the stratosphere for hours, then vanish as mysteriously as it came.
The whole time these THC variants are having fun and hanging out in your brain’s endocannabinoid system, your body is hard at work breaking them down into a chemical called—get ready for it — 11-Nor-9-carboxy-delta-9-tetrahydrocannabinol (THC-COOH). And while THC-COOH itself isn’t psychoactive, traces of it can hang around in your body for as long as 30 days, providing a trail of evidence pointing back to the original THC.
So how long does it take your body to process and eliminate THC-COOH? The answer depends on how the amount of THC in the dose, how it was imbibed, how frequently you use cannabis, and how often you exercise.
Let’s start with the basics. Obviously, the more THC you put into your system overall, the more THC-COOH your body will produce as a result, and the longer it’ll take your system to break it down and get rid of it. Daily cannabis use lengthens that breakdown period further, because it constantly adds to the “backlog” of THC-COOH that still remains from previous sessions.
Fat and exercise also play major roles in your body’s ability to process residual cannabinoids. Because THC-COOH is highly fat-soluble, it’s easily absorbed into fat cells, where it can be stored for weeks. By the same token, fat-burning exercise breaks down fatty tissue that stores cannabinoids, helping your body eliminate THC-COOH more quickly.
It’s also important to note that cannabinoids clear out of different body systems at different rates—and those rates can vary wildly from one person to the next. For example, a 2009 study led by Erin L Karschner, a forensic toxicologist at the National Institute on Drug Abuse, found that a full 36 percent of cannabis users had no detectable THC-COOH in their blood plasma after just a week of abstaining.
Still, that study’s estimate was on the low end—and Karschner’s team focused exclusively on blood plasma tests. By contrast, a 2014 survey led by Karen E. Moeller, clinical professor in pharmacy practice at the University of Kansas, found that cannabis metabolites could only be detected in the urine of “occasional users” for three days—but that in “chronic users” (no pun intended, but I’ll take it!) traces of THC-COOH stuck around for up to 30 days.
One piece of good news: A comprehensive 2005 review by the National Drug Court Institute found that traces of cannabis are rarely detectable in blood, saliva or urine after 29 days. And while some researchers claim to have detected THC residue in hair samples as long as 90 days after the fact, other experts have challenged those claims, pointing out that THC metabolites can easily be transferred into non-users’ hair follicles by contact with other people’s hands, sweat, or smoke.
Ultimately, though, “There is no conclusive ‘end date’ after which THC is no longer detected,” says Tory R. Spindle, instructor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine. “The dose, route of administration, and frequency of use all play a role in how long THC or its metabolites are detectable.” For example, after a single cannabis smoking session, THC might be detected in blood for less than two hours, while THC metabolites may be detected in urine for a few days—or even longer, depending on the person.
So how long does weed stay in your system? A few hours, a few days, or up to a month—depending on exactly what you’re asking.
Ben Thomas is a journalist and novelist who's lived in 40+ countries. He specializes in telling stories from the frontiers of science, history, culture and the cosmos—and the points where all these fields intersect.