Short Answer

Detection windows depend on the test and the person. After one use: urine 1 to 3 days, blood 1 to 2 days, saliva 24 to 48 hours, hair up to 90 days. After months of daily use: urine 10 to 30 days or longer, blood up to 7 days.

Window by Test Type

Urine

Urine tests measure THC-COOH, a metabolite made in the liver. That metabolite leaves the body in urine over days to weeks. Fat tissue stores it and releases it as fat burns.

  • One use: 1 to 3 days.
  • Use 2 to 4 times per week: 5 to 7 days.
  • Daily use: 10 to 30 days.
  • Heavy daily use over years: 30 to 60 days in published case reports.

Blood

Blood tests measure THC itself, not just the metabolite. THC clears from blood in hours. Window: 1 to 2 days after one use, up to 7 days with heavy daily use. THC-COOH stays in blood longer than THC.

Saliva

Oral fluid tests pick up THC from recent smoke or vapor. Window: 24 to 48 hours. Some tests detect THC at 72 hours.

Hair

Hair tests measure THC and THC-COOH bound in the hair shaft. Head hair grows about 0.5 inch per month. A 1.5 inch sample covers about 90 days. Body hair grows slower and covers a longer window.

Factors That Change the Timeline

  • Frequency and dose. Daily high-dose use extends every window.
  • Potency. Concentrates and high-THC flower carry more THC per dose.
  • Route. Edibles and oral products clear slower than inhaled smoke or vapor.
  • Body fat. THC is fat soluble, so more fat stores more metabolite.
  • Liver function and metabolism. Faster metabolism clears THC faster.
  • Age and hydration. Low water intake slows excretion.
  • Exercise and fat loss. Fat loss can push stored THC-COOH into blood and urine.

Cutoff Levels

Federal workplace urine testing uses a 50 ng/mL immunoassay screen. A sample above that level goes to a confirmatory test, GC-MS or LC-MS/MS, with a 15 ng/mL cutoff. Detection windows in studies shift with the cutoff. At a 20 ng/mL cutoff, more samples read positive and windows stretch out.

CBD, THC, and Drug Tests

CBD isolate holds no THC. Full-spectrum hemp extract can hold THC at 0.3% or less by dry weight under US law. Daily high-dose use of a full-spectrum product can build THC-COOH above a test cutoff. A certificate of analysis lists the THC content of a batch. Check the COA for total THC before you buy. A product with no COA gives you no number to check.

False Positives

Some drugs cross-react with urine immunoassays: efavirenz, some NSAIDs, proton pump inhibitors, and dronabinol. A positive screen is not a positive test. Confirmatory mass spectrometry rules out these cases.

Secondhand Smoke

Studies of non-smokers in closed rooms with heavy smoke found urine THC-COOH below 50 ng/mL in most cases. At a 20 ng/mL cutoff, some samples read positive.

What Is Not Known

No formula predicts one person's clearance date. Fat percentage, dose history, and cutoff interact. A lab test on your own sample is the same step a lab takes.