Short answer: a hair test looks for THC metabolites, mostly 11-nor-9-carboxy-THC (THC-COOH), locked inside the hair shaft as it grows. Scalp hair grows about half an inch per month, so the standard 1.5-inch sample covers roughly 90 days of use. Unlike urine, hair does not clear in a few days. Once a metabolite is in that strand, it stays until the hair is cut or sheds.
Cannabis Metabolite Test Accuracy Guide
What the test actually measures
The name is a bit of a misnomer. The follicle sits under the skin and keeps growing. Labs test the shaft above the scalp, usually clipped close to the head and cut into segments.
THC COOH Metabolite Drug Test Cutoff Guide
The primary target is THC-COOH, the same inactive metabolite urine screens look for. Some panels also test for the parent THC molecule, cannabinol, and occasionally CBD markers. Typical screening cutoffs sit around 1 picogram per milligram of hair with confirmation near 0.1 pg/mg. Those numbers live in a different unit than urine cutoffs, so comparing them directly does not work.
How metabolites get into hair
- Blood flow to the follicle during active growth
- Sebum and sweat coating the scalp
- External contamination from smoke, resin, or handling plant material
That third route is the messy one. Sitting in a hotboxed car, touching a grinder, or living with a heavy smoker can deposit THC on the outside of the hair. Labs run multiple solvent washes before analysis to strip surface residue, and the math around how well that works is still debated in the literature.
The 90-day window, and why it is not exact
Half an inch a month means 1.5 inches equals about three months of history. Body hair grows slower, so it can stretch the window even further if a lab switches to it. What hair testing cannot do is tell you when you used. It answers yes or no across the whole window, not a timeline.
Occasional users sometimes test below the cutoff. Frequent users almost never do. Dose, potency, hair color, and the specific lab all move the needle.
What does and does not change results
- Shaving your head: collectors pull body hair instead. Same idea, longer window.
- Bleaching or dyeing: can drop levels, sometimes under the cutoff, but the effect is unpredictable and labs flag chemically damaged samples.
- Detox shampoos: no credible evidence they clear embedded metabolites.
- Melanin: THC binds to it, so darker, coarser hair tends to hold higher concentrations. This has driven real fairness and civil rights complaints about the technology.
Where hair testing stands in the US
Hair is not part of the federally regulated workplace panels run under SAMHSA authority, and it has not been approved for DOT testing. Private employers, family courts, and some probation offices still use it. Legality of cannabis in your state does not protect you from a private employer's policy, and a medical card usually will not either.
Questions I get a lot
Can one joint show up?
Possibly. It depends on the dose, the potency, your hair type, and the lab's cutoff. One session is far less likely to register than daily use, but it is not a zero.
How long until my hair is clean?
About 90 days for scalp hair from the time of last use, longer for body hair. Nothing you drink, take, or rinse with speeds that up.
Does CBD matter?
Full-spectrum products carry small amounts of THC. Daily use of those can accumulate. Isolate products are a smaller risk, though labeling is not always accurate.
If a test result affects a job, a custody case, or a court order, talk to an attorney and ask the lab for its cutoff values and wash procedure. Those two numbers shape more outcomes than anything else.