The short answer
A standard hair test uses the first 1.5 inches (3.8 cm) of head hair. Head hair grows about 0.5 inch (1.3 cm) per month. That 1.5-inch sample covers about 90 days. Most labs list the detection window for cannabis in hair as up to 90 days, and longer if a longer strand is tested.
Detection is not automatic. Labs look for THC-COOH (11-nor-9-carboxy-THC), a metabolite that stays in hair. The common reporting cutoff is 0.1 picogram per milligram of hair. One use often falls below that cutoff. Heavy daily use over months is more likely to show up.
How cannabis gets into hair
Blood carries THC-COOH to the hair follicle. The follicle feeds the cells that build the hair shaft. Metabolites bind to melanin, the pigment in hair. This is why darker, pigmented hair can hold more metabolite than gray or light hair in some studies. Once the shaft grows past the scalp, the residue is locked in place. Shampoo does not remove it.
Growth rate math
Head hair grows 0.4 to 0.5 inch per month. A 1.5-inch sample equals about three months of growth. A 3-inch sample reaches back about six months. Labs may also use body hair. Body hair grows slower and can reflect a longer, less defined window.
What changes the detection window
- Dose and frequency: daily high-dose use produces more metabolite.
- Potency: high-THC concentrates deliver more THC per session.
- Sample length: longer hair covers a longer period.
- Hair type: body hair has a slower growth cycle and variable results.
- Timing: the 1.5 inches closest to the scalp covers the most recent 90 days.
Cutoffs and lab method
Screening labs run an immunoassay first. A positive screen goes to confirmation by GC-MS/MS or LC-MS/MS. The confirmation step measures THC-COOH at a stated cutoff, often 0.1 pg/mg. A result below the cutoff is reported as negative.
Cleansing products and cosmetic treatment
Shampoos sold to defeat a hair test have no FDA clearance and no published clinical data. Cosmetic treatments are a separate case. Forensic studies report that bleach, dye, and relaxers damage the hair shaft and lower detectable cannabinoid levels. The effect is not reliable. Some samples drop below the cutoff; others still test positive.
Secondhand smoke
Hair absorbs compounds from the environment, including smoke in the air. The amount is small, and most samples stay below reporting cutoffs. A positive result from secondhand exposure alone is uncommon, but labs and courts have argued the point.
Where hair testing is used
Hair testing appears in pre-employment screening, some custody and court cases, and monitoring programs. SAMHSA publishes the Mandatory Guidelines for Federal Workplace Drug Testing Programs, which set collection and testing standards. Employers and labs set their own panels and cutoffs.
What a hair test cannot show
A hair test shows past exposure, not current impairment. THC-COOH is a non-psychoactive metabolite. A positive hair result gives no information about when the person last used, how much they used, or whether they were intoxicated at any point in time.