Cannabis Hair Follicle Test Detection Time
A hair follicle test can detect cannabis for up to 90 days after last use. The standard sample of 1.5 inches of head hair covers about 90 days of hair growth. Some cases may show a longer window, especially with body hair or heavy long-term use. A hair test does not detect recent use within the past 1 to 2 weeks because it takes time for drug metabolites to become embedded in the hair shaft.
What a Hair Follicle Test Measures
Hair follicle tests look for drug metabolites, including THC-COOH, that travel through the bloodstream and deposit in the hair shaft as it grows. The test does not measure impairment. A positive result shows that cannabis was used at some point during the growth period covered by the hair sample.
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How Long Cannabis Stays in Hair
Head hair grows about 0.5 inch per month. A 1.5-inch sample from the scalp near the back of the head gives a 90-day window. If the lab uses a shorter sample, the window shrinks. If the lab uses body hair, the detection window can extend to 12 months or more because body hair grows slower and can hold metabolites longer. A hair test is different from a urine test, which has a shorter detection window of days to weeks.
Factors That Affect Detection Time
- Use frequency and dose: daily or high-dose use can lead to higher metabolite levels and a longer detectable window.
- Hair type and growth rate: slower hair growth can stretch the window.
- Sample length and location: longer head hair samples cover more time; body hair covers more time than head hair.
- Individual metabolism: body weight, liver function, and genetics can change how THC is processed.
- Product type: concentrates and edibles may leave more metabolites than a single low-dose flower session.
How to Estimate Your Detection Window
Use these steps to make a rough estimate. This is not a guarantee.
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- Find the date of your last cannabis use.
- Measure the length of the hair sample the lab plans to collect, often 1.5 inches for head hair.
- Divide the sample length by 0.5 inch per month to get the number of months covered.
- Compare that time frame to your last use date.
- Adjust for body hair, heavy use, or slow hair growth by adding extra time.
What to Expect from Results
Most labs report a positive or negative result with a cutoff level, often 1 pg/mg for THC-COOH. A result below the cutoff is negative even if some metabolite is present. If you use legal cannabis and face a hair test for work, housing, or legal reasons, ask the testing lab or a lawyer about the specific panel and cutoff. Detection times vary by person and by lab method.