How long does a hair drug test detect cannabis?
A hair drug test has a detection window of about 90 days for cannabis. Labs take the 1.5 inches (3.8 cm) of scalp hair closest to the scalp, and scalp hair grows about 0.5 inch (1.27 cm) per month, so that segment records the past three months of use.
The 90-day figure is a standard, not a promise. Detection depends on how much THC a person used, how often, and the cutoff the lab applies. A light user may test negative well before day 90.
thc in hair from secondhand smoke
How does hair testing work?
Hair growth sets the window
Drugs enter the bloodstream, reach the follicle, and get locked into the hair shaft as it grows. The shaft keeps that record. Washing or treating the surface does not remove what sits inside.
hair drug test detection window 90 days
Growth is steady and slow, so each inch of hair equals about two months of history. A 1.5-inch sample covers 90 days, and a 3-inch sample reaches back six months. Most labs cap the sample at 1.5 inches so results stay comparable between people.
Detecting THC in Hair from Secondhand Smoke: A Comprehensive Buying Guide
What labs look for, and at what levels
For cannabis, labs test for 11-nor-9-carboxy-THC (carboxy-THC), the main THC metabolite. The parent molecule rarely appears in hair.
Common cutoffs are 1 picogram per milligram (pg/mg) for the first screen and 0.1 pg/mg for confirmation. Cutoffs vary by lab, and a lower cutoff raises the number of positives.
What changes the 90-day detection window?
- Hair length tested: longer samples reach back further in time.
- Body hair: grows slower, so it can hold a record for up to 12 months.
- Use pattern: daily use builds higher metabolite levels than weekend use.
- Potency: concentrates and vape oils carry more THC per dose than flower.
- Hair treatments: bleach and relaxers damage the shaft and can lower readings or ruin a sample.
- Metabolism: body weight, liver function, and genetics shift how much metabolite reaches the follicle.
Hair vs urine vs blood: detection windows side by side
| Method | Detection window | Notes |
|---|---|---|
| Hair | Up to 90 days | 1.5-inch scalp sample |
| Urine | 1 to 3 days after one use; up to 30 days with heavy daily use | Most common workplace test |
| Blood | 1 to 2 days | Shows active THC, not past use |
| Oral fluid | Up to 24 hours | Used in roadside and some workplace programs |
Hair holds the longest record of the four methods. That reach is why courts and some employers pay more for it.
How accurate is a hair drug test?
Labs report accuracy above 90 percent when a positive screen is confirmed with GC-MS or LC-MS/MS. The confirmation step matters, because a first screen can flag other compounds.
Two limits stand out. A hair test cannot show when inside the 90 days a person used, only that the drug is present. Dark, coarse hair also binds more drug than fine or light hair, so two people with the same habit can post different numbers.
Does convenience in cannabis change the detection window?
No. Format does not change how the body stores THC. Vapes, gummies, drinks, and delivery orders all end up as the same metabolite in the follicle.
What convenience changes is frequency. A product that takes seconds to use, with no smell and no setup, fits into more days of the week. More use days means more metabolite in the hair shaft, so a hair test that reads clean for an occasional user can read positive for a daily one.
High-potency formats add to this. One strong vape hit can deliver more THC than a joint, and repeated hits push hair levels higher.
What does not clear THC from hair?
- Detox shampoos: no shampoo has been shown to pull drug from inside the shaft.
- Bleaching or relaxing: may drop readings, but often damages the sample and triggers an invalid result.
- Shaving: labs switch to body hair, which holds a longer record.
- Water, exercise, sauna: these flush nothing from hair.
Labs also wash each sample with solvents before testing. That wash step exists to strip surface contamination, so products that coat the outside of the hair do little.
Who orders hair tests?
Hair testing shows up in pre-employment screening, custody and family cases, probation, and treatment programs. It is less common than urine in routine workplace testing because it costs more and takes longer to process.
The federal workplace testing panel, run under SAMHSA rules, does not include hair. Federal agencies use urine and oral fluid.
Frequently asked questions
Can a hair test detect a single use?
At standard cutoffs, most single-use cases stay below the line. One session may leave too little carboxy-THC in the shaft to cross 1 pg/mg. A lab using a 0.1 pg/mg cutoff has a better chance of catching it.
How soon after use does THC show up in hair?
About 5 to 10 days. The drug has to travel through the bloodstream and become part of the growing shaft, which takes time. That lag is why a hair test misses use from the past week.
Do hair dyes or relaxers hide cannabis use?
They can lower readings, and that is the problem. A damaged sample may come back as invalid, and the lab can ask for a new collection or report the result as inconclusive.
How much hair does a lab need?
About 100 to 120 strands, or a lock the width of a pencil, cut close to the scalp. If scalp hair is too short, a lab can take body hair instead.
Is 90 days the longest a hair test can reach?
For a 1.5-inch scalp sample, yes. Longer samples and body hair extend the record, but most labs stick to 1.5 inches for consistent results.