Short answer
Urine tests look for 11-nor-9-carboxy-THC, a THC metabolite. The result depends on test type, frequency of use, body fat, and liver enzymes. No product sold in the U.S. is approved by the FDA to hide THC from a test.
The test measures a metabolite, not THC
Immunoassay screens use an antibody that binds THC-COOH. The federal urine screening cutoff is 50 ng/mL. A gas chromatography mass spectrometry confirmation uses 15 ng/mL. Blood tests measure parent THC instead. That difference explains why urine and blood results disagree on the same person.
Detection windows by specimen
Urine
One use: 1 to 3 days. Daily use: up to 30 days or more. Body fat and dose shift the number.
Blood
Peak concentration arrives 3 to 10 minutes after smoking. Parent THC clears in 1 to 2 days for casual use and up to 7 days for heavy use.
marijuana drug test misinformation
Saliva
Oral fluid shows THC for 24 to 48 hours. The federal oral fluid screening cutoff is 4 ng/mL. The confirmation cutoff is 2 ng/mL.
Hair
Hair holds metabolites for up to 90 days. The federal hair cutoff is 0.1 ng/mg for screening and confirmation. Hair grows about 0.5 inches per month, so segment testing can point to a time range.
Myth: drinking water beats a urine test
Water dilutes THC-COOH per milliliter. It also dilutes creatinine. Labs measure creatinine in every federal specimen. A result below 20 mg/dL is reported as dilute. Below 2 mg/dL, the specimen is invalid. Dilution flags a test. It does not clear one.
Myth: exercise flushes stored THC
THC-COOH sits in fat tissue. Exercise mobilizes fat. Studies on chronic users found that exercise raised plasma THC-COOH levels. The release adds metabolite to blood and urine.
Myth: CBD cannot trigger a positive
Full-spectrum CBD oil can carry THC. FDA lab testing found CBD products with THC content above the label claim, and products that listed no THC but contained it. CBD isolate contains no THC. The label is the weak point.
Myth: secondhand smoke never causes a positive
Most passive exposure cases land below the cutoff. Cases above 50 ng/mL exist. They involve hours in a closed, unventilated space, such as a small car.
Myth: a positive test proves impairment
THC-COOH stays in urine for days after the high ends. A positive urine result shows past exposure. It does not show dose, time of use, or impairment. NIDA notes there is no accepted threshold that maps a THC level to impairment.
Myth: detox drinks and niacin are proven
FDA has approved no product that masks drugs in a test. High-dose niacin carries a risk of liver injury. Zinc and vinegar have no supporting trial data.
Facts that hold up
- Prescription dronabinol and nabilone produce a positive THC result.
- Hemp seed oil holds little to no THC. Hemp-derived CBD may.
- Poppy seeds cause opiate positives, not THC positives.
- Urine cutoffs: 50 ng/mL screen, 15 ng/mL confirmation.
- Hair cutoffs: 0.1 ng/mg screen and confirmation.
- Oral fluid cutoffs: 4 ng/mL screen, 2 ng/mL confirmation.
What a THC test cannot report
A test reports presence and concentration. It cannot report dose, frequency, route, or the last time of use. Employers and labs set their own cutoffs, so two labs can return different results from the same sample.