Answer first: the cannabis metabolite that most drug screens target is THC-COOH (11-nor-9-carboxy-THC), and its terminal half-life falls in a range of roughly 3 to 13 days. That is the number to plan around. It is not a countdown clock. Two criteria move it more than anything else: your dosing history and the specimen a lab collects. Body fat, liver enzyme activity, age, and hydration shift it further. A single exposure sits near the short end. Daily use for months sits at the long end and often past it.
What a half-life does and does not tell you
A half-life is the time the body needs to clear half of a given amount of a substance. Repeat that interval and you get a rough picture of how long a compound lingers. About five half-lives is the usual rule for near-complete clearance. Applied to THC-COOH, five half-lives of 3 days already reaches two weeks, and five half-lives of 13 days reaches well past two months. That gap is why two people who both vape once can test negative on very different schedules.
Pros of using half-life as your planning tool
- It gives a defensible range instead of a single false-precision date.
- It explains why detection windows run longer than the high itself.
- It scales in the right direction with frequency and dose.
Cons of using half-life as your planning tool
- It assumes first-order elimination, and cannabis elimination does not follow that curve cleanly.
- It ignores fat tissue, which acts as a slow-release reservoir.
- It cannot predict a specific person's negative test date.
Use case: treat half-life as ballpark planning, not as a clearance calculator. For anything consequential, a lab result is the only real answer.
what affects cannabis half life
The metabolite labs actually measure
THC is metabolized mostly in the liver. The parent compound is first converted to 11-hydroxy-THC, which is active, then to THC-COOH, which is not psychoactive but stays in the body far longer. THC-COOH is conjugated and excreted in urine and bile, and a portion is reabsorbed from the gut. Most urine immunoassays are designed to bind THC-COOH rather than THC itself, which is why the metabolite half-life, not the high, sets the detection window.
Parent THC moves fast: an early-phase plasma half-life of about 1 to 3 hours, with a terminal phase near 20 to 30 hours. THC-COOH is the slow one. Published pharmacokinetic reviews place its terminal half-life at roughly 3 to 13 days, with heavy chronic users at the upper end.
Why daily users clear it more slowly
Cannabinoids are fat soluble. One session leaves a small reservoir in adipose tissue. Months of daily use fill that reservoir, and it keeps releasing metabolite into the blood long after use stops. Total body burden, not the last dose, drives the tail of the curve.
What this changes for you
- Occasional use: expect the short end, days rather than weeks.
- Daily use for a few weeks: expect the middle of the range.
- Heavy daily use for years: expect the long end, and plan for longer than the range suggests.
Use case: if you use daily and face a urine screen, the 3 to 13 day figure is the wrong planning number. Build in a wider buffer.
Detection windows by specimen
Urine
Urine is the most common workplace specimen. Federal guidelines call for an initial immunoassay screen at a 50 ng/mL cutoff for THC-COOH, confirmed by mass spectrometry at 15 ng/mL. Typical windows run 1 to 3 days after a single use, 3 to 7 days for moderate use, and 10 to 30 days or more for chronic heavy use.
- Pros: low cost, standardized cutoffs, long window that catches ongoing use.
- Cons: the long window flags use from weeks ago, and dilute samples can complicate results.
Best for: programs that need a long look-back.
Blood
Blood holds parent THC for about 1 to 2 days in most cases, with THC-COOH detectable somewhat longer, up to about a week in heavy users.
- Pros: the closest link to recent use.
- Cons: short window, invasive collection, and no widely accepted impairment threshold tied to a number.
Best for: recent-use questions, such as post-collision testing.
Saliva
Oral fluid usually shows cannabis for roughly 1 to 3 days, though heavy users can stretch that window.
- Pros: easy collection, hard to substitute, closer to the recent-use window.
- Cons: short detection window, collection variability, and interference from recent food or drink.
Best for: roadside and workplace programs that want a recent-use signal.
Hair
Hair can show cannabis for up to about 90 days, growing roughly 1 cm per month.
- Pros: the longest window of the common tests, useful for documenting a pattern.
- Cons: it cannot date a single use precisely, external contamination is a known issue, and turnaround is slow.
Best for: long-term monitoring rather than a point-in-time answer.
What shifts your personal number
- Frequency and duration of use. This matters most.
- Body fat percentage and body mass.
- Liver enzyme activity and individual metabolism.
- Age and overall health.
- Hydration, which changes urine concentration rather than clearance.
- Test cutoff and specimen type.
How to use this as a legal cannabis consumer
If you buy from a licensed shop, the label tells you potency, which helps you judge your own intake. Shop legally, keep your high at home, and never drive impaired, because a detection window and an impairment window are two different things. If a drug test is on your calendar, treat every number above as an estimate and give yourself the widest buffer you can. Nothing here predicts an individual result, and this is general information rather than medical advice.