Short answer
A standard hair drug test detects cannabis use from about 90 days back. The test uses the first 1.5 inches (3.8 cm) of hair measured from the scalp. Hair grows about 0.5 inch per month, so 1.5 inches covers about three months.
how far back can a hair drug test detect cannabis
Most labs screen for THC-COOH, a metabolite the body makes after cannabis use. The common cutoff is 1 picogram per milligram (pg/mg) of hair.
How the 90-day window works
Hair length and growth
- 1.5 inches (3.8 cm) of scalp hair covers about 90 days.
- 0.5 inch (1.3 cm) covers about 30 days.
- 3 inches (7.6 cm) covers about 180 days, if the lab tests the full length.
Some labs test longer segments for workplace or legal cases. Results depend on the lab protocol and the sample collected.
does hair drug test show how long you used cannabis
Body hair
Body hair grows slower than scalp hair. A test on body hair can reach back up to 12 months in some labs. Collection site matters. Armpit, chest, and leg hair all differ.
Recent use
Hair tests miss use from the last 7 to 10 days. New hair has to grow out of the scalp before a lab can collect it. A hair test will not show use from yesterday.
What changes the result
- Dose and frequency. Heavy daily use leaves more THC-COOH in hair. One-time use often falls below the cutoff.
- Hair color and texture. Dark, coarse hair binds more THC-COOH than light, fine hair in published studies. Labs do not correct for this.
- Chemical treatment. Bleach, dye, and relaxers can lower THC-COOH levels. Results vary. No method removes all residue.
- Secondhand smoke. Exposure can raise levels. Labs and courts treat these claims case by case.
- Lab cutoff. A 0.1 pg/mg cutoff finds more positives than a 1 pg/mg cutoff.
What a hair test cannot tell you
A hair test shows that cannabis entered the body in a window. It does not show dose, impairment, or time of last use. It does not prove a person was high at work or behind the wheel.
Hair tests also do not measure CBD directly. CBD products can contain small amounts of THC. A user who takes full-spectrum CBD daily may test positive in rare cases. Read the certificate of analysis for THC content.