There is no single answer to how long weed stays in your system — it depends entirely on which test is used and how often you use cannabis. Blood and saliva tests typically catch only recent use, measured in hours to a few days. Urine tests, the most common workplace test, can detect THC metabolites for anywhere from about 3 days after a single use to a month or longer with daily, heavy use. Hair tests have the longest window of all, commonly covering roughly the past 90 days.
Quick answer: detection windows by test type
- Blood: Typically a few hours up to 1–2 days after use; narrower than other methods and rarely used outside of accident or DUI investigations.
- Saliva (oral fluid): Generally 24–72 hours, and it’s the only method the federal government has approved specifically for catching very recent use.
- Urine: About 3 days after a single use, 5–7 days for moderate use (a few times a week), and 30 days or more for daily, heavy use. This is the most-used and most-studied test type.
- Hair: Roughly 90 days, based on testing a hair segment close to the scalp; it reflects a longer history of use rather than a single recent session.
Why the range is so wide
Drug tests for cannabis don’t look for active THC itself, except sometimes in blood. Most tests look for THC metabolites, byproducts the body creates as it breaks THC down, and those metabolites behave very differently depending on the person and the test.
Because THC is fat-soluble, it’s stored in body fat and released slowly over time, which is the main reason detection windows stretch out so much further for frequent users. A toxicology review by researcher Paul Cary, published in Drug Court Review, found the average detection window in urine for chronic users was about 14 days at common lab cutoffs, though a minority of heavy users tested positive even after 28 days.
Several other factors shift the window in either direction:
- How often and how much someone uses — the biggest factor by far
- Body fat percentage, since THC is stored in fat tissue
- Metabolic rate and overall health
- The specific test’s sensitivity cutoff (different labs use different thresholds)
- Method of use — smoking or vaping delivers THC quickly, while edibles are processed differently
Urine testing: the most common method
Urine tests look for a metabolite called THC-COOH and are standard for workplace and legal drug screening because they’re inexpensive and have a workable detection window. According to the Drug Policy Alliance, detection time depends heavily on frequency of use: single use tends to clear in roughly a week, while daily or near-daily use can remain detectable for a month or more. The Substance Abuse and Mental Health Services Administration (SAMHSA) sets the cutoff concentrations most labs use to determine a positive result.
Blood testing: the shortest window
Blood tests measure THC more directly, which makes them useful for determining very recent use (for instance, after a crash), but that same directness means the detection window is short. THC typically clears from blood within a day or two, though heavier users have tested positive for a few extra days in some studies. Because blood draws are invasive and the window is narrow, this method is rarely used for routine employment screening.
Saliva testing: built for recent use
Oral fluid testing is the only method SAMHSA has specifically endorsed as reliable for catching same-day use, with a detection window generally covering the last 24 to 72 hours. That makes it increasingly common for roadside and workplace testing, where the goal is identifying recent impairment rather than lifestyle drug use.
Hair testing: the longest look back
Hair testing works differently: THC metabolites travel through small blood vessels into the hair follicle and get deposited as the hair grows. Labs typically test the 1.5-inch segment of hair closest to the scalp, which corresponds to roughly the last 90 days of growth. Because hair grows slowly and steadily, this method can’t detect use from the past few days (hair hasn’t grown enough yet) but does provide the longest overall lookback window.
What this does not tell you
A positive test result confirms cannabis use happened at some point within the detection window. It does not establish how much was used, how recently within that window, or whether someone is currently impaired. The U.S. Department of Justice has noted that a positive metabolite result does not by itself indicate recency, frequency, or amount of use. Several toxicologists, including Johns Hopkins researcher Ryan Vandrey, have pointed out that there is no single, precise detection window that applies to everyone — the ranges above are reasonable estimates, not guarantees.
FAQ
Can secondhand smoke cause a positive test? It’s unlikely at typical exposure levels, though saliva tests are the most sensitive to incidental exposure since THC can enter the mouth directly from the air.
Does drinking more water shorten the detection window? Hydration can dilute urine concentration, which labs usually screen for and may flag as a diluted or invalid sample rather than a clean negative.
Do edibles stay in your system longer than smoking? Method of consumption affects how quickly THC enters the body but the detection window is driven mainly by frequency and amount of use, not by whether it was smoked or eaten.
Are at-home test kits as accurate as lab tests? At-home kits can give a general read but most workplace and legal testing requires lab confirmation, which is more sensitive and harder to dispute.
This article is for informational purposes only and is not medical, legal, or testing advice.
Sources
- Paul L. Cary, “The Marijuana Detection Window,” Drug Court Review (toxicology review)
- Drug Policy Alliance: Workplace Marijuana Testing
- NORML summary of detection-window research
- Leafly: cannabis detection window, including Johns Hopkins and SAMHSA commentary
