How long can a hair follicle test detect THC?
A hair follicle test can detect THC for up to 90 days in head hair, and up to 12 months in body hair. The 90-day figure comes from the sample itself: a standard test cuts roughly 3.9 cm (1.5 inches) of hair closest to the scalp, and head hair grows about 1.3 cm (0.5 inch) per month. That sample represents about the last three months of use. A single session is often missed. Repeated or heavy use is far more likely to appear.
Why hair holds THC longer than urine
Your body converts THC into carboxy-THC (THC-COOH). That metabolite travels through the bloodstream, reaches the follicle at the root, and gets locked into the keratin as the shaft grows. Once it is inside the shaft, it does not get metabolized or cleared the way urine does. Urine is a snapshot that empties in days. Hair is a growing record.
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One consequence: recent use may not show up yet. THC generally needs about 7 to 10 days to enter the follicle and grow far enough out of the scalp to be captured in a cut sample.
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Detection windows by sample type
- Head hair, standard 1.5-inch sample: up to 90 days.
- Head hair, longer or segmented sample: up to 6 months or more, since each segment maps to a stretch of past growth.
- Body hair (arm, leg, chest): 6 to 12 months. Growth is slower and shedding is uneven, so old deposits stay in place longer.
- Use within the past week: usually below the detection point until the hair grows out.
Cutoff levels
Most labs screen with an immunoassay at a cutoff near 1 pg/mg of THC-COOH, then confirm positives with LC-MS/MS or GC-MS/MS at a lower cutoff near 0.1 pg/mg. An occasional user with light exposure can land under the screen cutoff and test negative. A daily user with high-potency concentrates usually clears both cutoffs with room to spare. Cutoffs are not universal. Hair testing is not part of the federal mandatory guidelines for regulated workplace testing, so labs and employers set their own numbers.
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What shifts the window
- Dose, frequency, and potency, including concentrates and high-milligram edibles.
- Body fat and metabolic rate, which change how much metabolite circulates.
- Hair color. Melanin binds cannabinoids, so darker hair tends to hold more than light hair.
- Bleaching, dyeing, and repeated chemical straightening, which can lower concentration but also trigger a lab to flag cosmetic treatment.
- Sample length and the number of segments tested.
- Secondhand smoke exposure, which is a low but real source of contamination in heavy indoor smoke.
How a hair sample is collected
Prerequisites: clean, dry hair, no gels, oils, or sprays applied that day, and a photo ID. The collector wears gloves and works with single-use scissors.
- Confirm identity and open a chain-of-custody form for the sample.
- Pick the collection site at the crown of the head, toward the back, where hair grows at the most consistent rate.
- Cut about 1.5 inches of hair at scalp level, keeping the root end oriented together.
- Wrap the strands at the cut end and seal them in the collection envelope.
- Record the collection time, sign the form, and ship the sealed kit to the lab.
- Wait for the lab to wash the sample, run the screen, and confirm any positive with mass spectrometry.
Limits of the test
A hair result shows that THC entered the body during a broad window. It cannot show impairment, cannot pin use to a specific day, cannot prove how much was used, and cannot separate a single heavy session from steady use. A positive result also does not distinguish medical use from recreational use. If the result matters for a job, a court case, or a treatment plan, ask the lab for the cutoff it used and whether a second, confirmatory method was run on the same sample.