Quick answer
Urine tests look for THC-COOH, a metabolite of THC. A single use can be detected for 1 to 3 days. Daily or heavy use can be detected for 30 days or more. Water, vinegar, niacin, and detox drinks do not remove the metabolite. Labs flag diluted samples with creatinine and specific gravity checks.
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How the tests work
SAMHSA guidelines set the urine immunoassay cutoff at 50 ng/mL. A sample above that goes to confirmation by GC-MS or LC-MS/MS at 15 ng/mL. Immunoassays bind to THC-COOH and similar compounds. Ibuprofen, efavirenz, and some other drugs can cross-react and produce a false positive on the screen. Confirmation testing separates those cases.
Detection windows
- Urine, single use: 1 to 3 days.
- Urine, chronic use: up to 30 days, longer in some cases.
- Blood: 1 to 2 days, up to 7 days in heavy users.
- Oral fluid: 24 to 48 hours, up to 72 hours.
- Hair: up to 90 days. Hair grows about 1 cm per month.
Concentrates contain 60% to 90% THC. Higher doses extend the window. Body fat stores THC-COOH and releases it over time.
Myths
Drinking water flushes THC
Water lowers the concentration of THC-COOH in urine. It also lowers creatinine. Labs treat creatinine below 20 mg/dL as a dilute sample. Many employers require a retest. Water does not remove stored metabolite.
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Niacin, vinegar, or bleach
Niacin at high doses can damage the liver. Bleach in a sample is detected and voids the test. Vinegar has no effect on THC-COOH levels.
Detox drinks
Most products work by dilution, not removal. The FTC has warned sellers about unsupported claims. No FDA-approved product masks THC.
CBD cannot cause a positive
Full-spectrum CBD can contain up to 0.3% THC by dry weight. Repeated high doses can accumulate. CBD isolate with no THC does not contain the metabolite and does not trigger a positive in most cases.
Secondhand smoke never registers
Typical secondhand exposure does not reach the 50 ng/mL cutoff. Extreme exposure in a closed space has produced positives in case reports. This is rare.
Poppy seeds cause THC positives
Poppy seeds can affect opioid tests, not THC tests. This claim belongs to a different assay.
What changes results
- Frequency of use
- Dose and THC percentage
- Body fat and metabolism
- Test type and cutoff
- Time since last use
State law
Some states limit pre-employment THC testing. Nevada passed a law in 2019 that took effect in 2020. California AB 2188 takes effect in 2024. Exceptions cover safety-sensitive roles and federal contractors. Federal employees remain subject to testing under the Drug-Free Workplace Act.