Short answer
Yes. A standard head hair test covers about 90 days. Labs cut a 3.9 cm (1.5 inch) segment from the scalp, close to the skin. Head hair grows about 1.27 cm (0.5 inch) per month. That segment holds roughly three months of growth.
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Why the window is 90 days
Hair forms in the follicle and traps compounds from blood, sweat, and sebum. As the strand grows, each section carries a record of what was in the body at that time. One month of growth equals about 1.27 cm. A 1.5 inch segment equals about 90 days. Longer hair tested end to end reaches further back.
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What labs look for
Most labs test for THC-COOH, a metabolite of THC, and for parent THC. Common cutoffs are 1 pg/mg for screening and 0.1 pg/mg for confirmation. Cutoffs differ by lab. A result under the cutoff is reported as negative.
One-time use compared with frequent use
A single use often lands under the cutoff. One dose produces low metabolite levels in the follicle. Frequent use raises the amount deposited. Daily or near-daily users show positive results across the full 90 day segment. Edibles and concentrates can raise levels more than a small amount of smoked flower.
Body hair and other samples
Body hair grows about 0.5 cm per month, roughly half the rate of head hair. Labs treat body hair as a longer and less precise window, often up to 12 months. Body hair cannot pin down a date of use. Nail tests cover a similar or longer span.
Things that change the result
- Length of sample. A 6 inch hair sample tested full length covers about 12 months, not 90 days.
- Bleaching and dyeing. These strip some drug from the strand. Results are uneven. Some labs reject treated hair.
- Hair color. Melanin binds THC. Dark hair takes up more than gray or light hair in lab studies.
- External contamination. Smoke, residue, and handling can deposit THC on the outside of the strand. Labs use wash steps to limit this.
What hair tests cannot show
A hair test shows exposure in a window. It does not show impairment, dose, or a specific time of use. A positive result does not prove a person was high at work or behind the wheel. Hair testing is not part of the federal workplace drug testing panel, which covers urine and oral fluid. Private employers and some courts use it.
Bottom line
Head hair holds THC and THC-COOH for about 90 days in a standard 1.5 inch test. Body hair and long hair samples reach further back. Cutoffs, use frequency, and hair treatment all change the result.